Hypoxia-Directed Treatment of Human Papillomavirus-Related Oropharyngeal Carcinoma

PURPOSE

Standard curative-intent chemoradiotherapy for human papillomavirus (HPV)-related oropharyngeal carcinoma results in significant toxicity. Since hypoxic tumors are radioresistant, we posited that the aerobic state of a tumor could identify patients eligible for de-escalation of chemoradiotherapy while maintaining treatment efficacy.

METHODS

We enrolled patients with HPV-related oropharyngeal carcinoma to receive de-escalated definitive chemoradiotherapy in a phase II study (ClinicalTrials.gov identifier: NCT03323463). Patients first underwent surgical removal of disease at their primary site, but not of gross disease in the neck. A baseline 18F-fluoromisonidazole positron emission tomography scan was used to measure tumor hypoxia and was repeated 1-2 weeks intratreatment. Patients with nonhypoxic tumors received 30 Gy (3 weeks) with chemotherapy, whereas those with hypoxic tumors received standard chemoradiotherapy to 70 Gy (7 weeks). The primary objective was achieving a 2-year locoregional control (LRC) of 95% with a 7% noninferiority margin.

RESULTS

One hundred fifty-eight patients with T0-2/N1-N2c were enrolled, of which 152 patients were eligible for analyses. Of these, 128 patients met criteria for 30 Gy and 24 patients received 70 Gy. The 2-year LRC was 94.7% (95% CI, 89.8 to 97.7), meeting our primary objective. With a median follow-up time of 38.3 (range, 22.1-58.4) months, the 2-year progression-free survival (PFS) and overall survival (OS) rates were 94% and 100%, respectively, for the 30-Gy cohort. The 70-Gy cohort had similar 2-year PFS and OS rates at 96% and 96%, respectively. Acute grade 3-4 adverse events were more common in 70 Gy versus 30 Gy (58.3% v 32%; P = .02). Late grade 3-4 adverse events only occurred in the 70-Gy cohort, in which 4.5% complained of late dysphagia.

CONCLUSION

Tumor hypoxia is a promising approach to direct dosing of curative-intent chemoradiotherapy for HPV-related carcinomas with preserved efficacy and substantially reduced toxicity that requires further investigation.

Lesion Dosimetry for [177Lu]Lu-PSMA-617 Radiopharmaceutical Therapy Combined with Stereotactic Body Radiotherapy in Patients with Oligometastatic Castration-Sensitive Prostate Cancer

A single-institution prospective pilot clinical trial was performed to demonstrate the feasibility of combining [177Lu]Lu-PSMA-617 radiopharmaceutical therapy (RPT) with stereotactic body radiotherapy (SBRT) for the treatment of oligometastatic castration-sensitive prostate cancer. Methods: Six patients with 9 prostate-specific membrane antigen (PSMA)-positive oligometastases received 2 cycles of [177Lu]Lu-PSMA-617 RPT followed by SBRT. After the first intravenous infusion of [177Lu]Lu-PSMA-617 (7.46 ± 0.15 GBq), patients underwent SPECT/CT at 3.2 ± 0.5, 23.9 ± 0.4, and 87.4 ± 12.0 h. Voxel-based dosimetry was performed with calibration factors (11.7 counts per second/MBq) and recovery coefficients derived from in-house phantom experiments. Lesions were segmented on baseline PSMA PET/CT (50% SUVmax). After a second cycle of [177Lu]Lu-PSMA-617 (44 ± 3 d; 7.50 ± 0.10 GBq) and an interim PSMA PET/CT scan, SBRT (27 Gy in 3 fractions) was delivered to all PSMA-avid oligometastatic sites, followed by post-PSMA PET/CT. RPT and SBRT voxelwise dose maps were scaled (α/β = 3 Gy; repair half-time, 1.5 h) to calculate the biologically effective dose (BED). Results: All patients completed the combination therapy without complications. No grade 3+ toxicities were noted. The median of the lesion SUVmax as measured on PSMA PET was 16.8 (interquartile range [IQR], 11.6) (baseline), 6.2 (IQR, 2.7) (interim), and 2.9 (IQR, 1.4) (post). PET-derived lesion volumes were 0.4-1.7 cm3 The median lesion-absorbed dose (AD) from the first cycle of [177Lu]Lu-PSMA-617 RPT (ADRPT) was 27.7 Gy (range, 8.3-58.2 Gy; corresponding to 3.7 Gy/GBq, range, 1.1-7.7 Gy/GBq), whereas the median lesion AD from SBRT was 28.1 Gy (range, 26.7-28.8 Gy). Spearman rank correlation, ρ, was 0.90 between the baseline lesion PET SUVmax and SPECT SUVmax (P = 0.005), 0.74 (P = 0.046) between the baseline PET SUVmax and the lesion ADRPT, and -0.81 (P = 0.022) between the lesion ADRPT and the percent change in PET SUVmax (baseline to interim). The median for the lesion BED from RPT and SBRT was 159 Gy (range, 124-219 Gy). ρ between the BED from RPT and SBRT and the percent change in PET SUVmax (baseline to post) was -0.88 (P = 0.007). Two cycles of [177Lu]Lu-PSMA-617 RPT contributed approximately 40% to the maximum BED from RPT and SBRT. Conclusion: Lesional dosimetry in patients with oligometastatic castration-sensitive prostate cancer undergoing [177Lu]Lu-PSMA-617 RPT followed by SBRT is feasible. Combined RPT and SBRT may provide an efficient method to maximize the delivery of meaningful doses to oligometastatic disease while addressing potential microscopic disease reservoirs and limiting the dose exposure to normal tissues.

Yttrium-90 Activity Quantification in PET/CT-Guided Biopsy Specimens from Colorectal Hepatic Metastases Immediately after Transarterial Radioembolization Using Micro-CT and Autoradiography

PURPOSE

To evaluate the yttrium-90 (90Y) activity distribution in biopsy tissue samples of the treated liver to quantify the dose with higher spatial resolution than positron emission tomography (PET) for accurate investigation of correlations with microscopic biological effects and to evaluate the radiation safety of this procedure.

MATERIALS AND METHODS

Eighty-six core biopsy specimens were obtained from 18 colorectal liver metastases (CLMs) immediately after 90Y transarterial radioembolization (TARE) with either resin or glass microspheres using real-time 90Y PET/CT guidance in 17 patients. A high-resolution micro-computed tomography (micro-CT) scanner was used to image the microspheres in part of the specimens and allow quantification of 90Y activity directly or by calibrating autoradiography (ARG) images. The mean doses to the specimens were derived from the measured specimens' activity concentrations and from the PET/CT scan at the location of the biopsy needle tip for all cases. Staff exposures were monitored.

RESULTS

The mean measured 90Y activity concentration in the CLM specimens at time of infusion was 2.4 ± 4.0 MBq/mL. The biopsies revealed higher activity heterogeneity than PET. Radiation exposure to the interventional radiologists during post-TARE biopsy procedures was minimal.

CONCLUSIONS

Counting the microspheres and measuring the activity in biopsy specimens obtained after TARE are safe and feasible and can be used to determine the administered activity and its distribution in the treated and biopsied liver tissue with high spatial resolution. Complementing 90Y PET/CT imaging with this approach promises to yield more accurate direct correlation of histopathological changes and absorbed dose in the examined specimens.

The use of single-timepoint images to link administered radioiodine activity (MBq) to a prescribed lesion radiation-absorbed dose (cGy): a regression-based prediction interval tool for the management of well-differentiated thyroid cancer patients

PURPOSE

To introduce a biomarker-based dosimetry method for the rational selection of a treatment activity for patients undergoing radioactive iodine 131I therapy (RAI) for metastatic differentiated thyroid cancer (mDTC) based on single-timepoint imaging of individual lesion uptake by 124I PET.

METHODS

Patients referred for RAI therapy of mDTC were enrolled in institutionally approved protocols. A total of 208 mDTC lesions (in 21 patients) with SUVmax > 1 underwent quantitative PET scans at 24, 48, 72, and 120 h post-administration of 222 MBq of theranostic NaI-124I to determine the individual lesion radiation-absorbed dose. Using a general estimating equation, a prediction curve for biomarker development was generated in the form of a best-fit regression line and 95% prediction interval, correlating individual predicted lesion radiation dose metrics, with candidate biomarkers ("predictors") such as SUVmax and activity in microcurie per gram, from a single imaging timepoint.

RESULTS

In the 169 lesions (in 15 patients) that received 131I therapy, individual lesion cGy varied over 3 logs with a median of 22,000 cGy, confirming wide heterogeneity of lesion radiation dose. Initial findings from the prediction curve on all 208 lesions confirmed that a 48-h SUVmax was the best predictor of lesion radiation dose and permitted calculation of the 131I activity required to achieve a lesional threshold radiation dose (2000 cGy) within defined confidence intervals.

CONCLUSIONS

Based on MIRD lesion-absorbed dose estimates and regression statistics, we report on the feasibility of a new single-timepoint 124I-PET-based dosimetry biomarker for RAI in patients with mDTC. The approach provides clinicians with a tool to select personalized (precision) therapeutic administration of radioactivity (MBq) to achieve a desired target lesion-absorbed dose (cGy) for selected index lesions based on a single 48-h measurement 124I-PET image, provided the selected activity does not exceed the maximum tolerated activity (MTA) of < 2 Gy to blood, as is standard of care at Memorial Sloan Kettering Cancer Center.

TRIAL REGISTRATION

NCT04462471, Registered July 8, 2020. NCT03647358, Registered Aug 27, 2018.

Radioimmunoscintigraphy and Pretreatment Dosimetry of 131I-Omburtamab for Planning Treatment of Leptomeningeal Disease

Radiolabeled antibody treatment with 131I-omburtamab, administered intraventricularly into the cerebrospinal fluid (CSF) space, can deliver therapeutic absorbed doses to sites of leptomeningeal disease. Assessment of distribution and radiation dosimetry is a key element in optimizing such treatments. Using a theranostic approach, we performed pretreatment 131I-omburtamab imaging and dosimetric analysis in patients before therapy. Methods: Whole-body planar images were acquired 3 ± 1, 23 ± 2, and 47 ± 2 h after intracranioventricular administration of 75 ± 5 MBq of 131I-omburtamab via an Ommaya reservoir. Multiple blood samples were also obtained for kinetic analysis. Separate regions of interest (ROIs) were manually drawn to include the lateral ventricles, entire spinal canal CSF space, and over the whole body. Count data in the ROIs were corrected for background and physical decay, converted to activity, and subsequently fitted to an exponential clearance function. The radiation absorbed dose was estimated to the CSF, separately to the spinal column and ventricles, and to the whole body and blood. Biodistribution of the injected radiolabeled antibody was assessed for all patients. Results: Ninety-five patients were included in the analysis. Biodistribution showed prompt localization in the ventricles and spinal CSF space with low systemic distribution, noted primarily as hepatic, renal, and bladder activity after the first day. Using ROI analysis, the effective half-lives were 13 ± 11 h (range, 5-75 h) for CSF in the spinal column, 8 ± 3 h (range, 3-17 h) for ventricles, and 41 ± 11 (range, 23-81 h) for the whole body. Mean absorbed doses were 0.63 ± 0.38 cGy/MBq (range, 0.24-2.25 cGy/MBq) for CSF in the spinal column, 1.03 ± 0.69 cGy/MBq (range, 0.27-5.15 cGy/MBq) for the ventricular CSF, and 0.45 ± 0.32 mGy/MBq (range, 0.05-1.43 mGy/MBq) for the whole body. Conclusion: Pretherapeutic imaging with 131I-omburtamab allows assessment of biodistribution and dosimetry before the administration of therapeutic activity. Absorbed doses to the CSF compartments and whole body derived from the widely applicable serial 131I-omburtamab planar images had acceptable agreement with previously reported data determined from serial 124I-omburtamab PET scans.

F-18 meta-fluorobenzylguanidine PET imaging of myocardial sympathetic innervation

BACKGROUND

I-123 meta-iodobenzylguanidine (MIBG) imaging has long been employed to noninvasively assess the integrity of human norepinephrine transporter-1 and, hence, myocardial sympathetic innervation. Positron-emitting F-18 meta-fluorobenzylguanidine (MFBG) has recently been developed for potentially superior quantitative characterization. We assessed the feasibility of MFBG imaging of myocardial sympathetic innervation.

METHODS

16 patients were imaged with MFBG PET (30-minute dynamic imaging of chest, followed by 3 whole-body acquisitions between 30 minutes and 4-hour post-injection). Blood kinetics were assessed from multiple samples. Pharmacokinetic modeling with reversible 1- and 2-compartment models was performed. Kinetic rate constants were re-calculated from truncated datasets. All patients underwent concurrent MIBG SPECT.

RESULTS

MFBG myocardial uptake was rapid and sustained; the mean standardized uptake value (SUV (mean ± standard deviation)) was 5.1 ± 2.2 and 3.4 ± 1.9 at 1 hour and 3-4-hour post-injection, respectively. The mean K1 and distribution volume (VT) were 1.1 ± 0.6 mL/min/g and 34 ± 22 mL/cm3, respectively. Both were reproducible when re-calculated from truncated 1-hour datasets (Intraclass Correlation Coefficient of 0.99 and 0.91, respectively). Spearman's ϱ = 0.86 between MFBG SUV and VT and 0.80 between MFBG PET-derived VT and MIBG SPECT-derived heart-to-mediastinum activity concentration ratio.

CONCLUSION

MFBG is a promising PET radiotracer for the assessment of myocardial sympathetic innervation.

TriDFusion (3DF) image viewer

Background

An open-source, extensible medical viewing platform is described, called the TriDFusion image viewer (3DF). The 3DF addresses many broad unmet needs in nuclear medicine research; it provides a viewer with several tools not available in commercial nuclear medicine workstations, yet invaluable for imaging in research studies.

Results

The 3DF includes an image integration platform to register images from multiple imaging modalities together with delineated volumes of interest (VOIs), structures and dose distributions. It can process images from different vendors’ systems and is therefore vendor neutral. The 3DF also provides a convenient tool for performing multi-modality image analysis and fusion. The functional components currently being distributed is open-source code that includes: (1) a high quality viewer that can display axial, coronal, and sagittal tomographic images, maximum intensity projection images, structure contours, and isointensity contour lines or dose colorwash, (2) multi-image fusion allowing multiple images to be fused with VOI and dose distributions, (3) a suite of segmentation tools to edit and/or create tumor and organ VOIs, (4) dosimetry tools for several radioisotopes, (5) clinical tools for correcting acquisition errors, including patient orientation, and (6) the ability to save the resultant image and VOI as DICOM files or to export the numerical results as comma separated values files. Because the code is written in MATLAB™, it is highly readable and is easier for the coder to make changes compared to languages such as C or C++. In what follows, we describe the content of the new TriDFusion (3DF) image viewer software platform using examples of a number of clinical research workflows. Such examples vary in complexity but illustrate the main attributes of the software.

Conclusions

In summary, 3DF provides a powerful, convenient, easy-to-use suite of open-source imaging research tools for the nuclear medicine community that allows physicians, medical physicists, and academic researchers to display, manipulate, and analyze images.

Optimizing 90Y Particle Density Improves Outcomes After Radioembolization

PURPOSE

To determine how particle density affects dose distribution and outcomes after lobar radioembolization.

METHODS

Matched pairs of patients, treated with glass versus resin microspheres, were selected by propensity score matching (114 patients), in this single-institution retrospective study. For each patient, tumor and liver particle density (particles/cm³) and dose (Gy) were determined. Tumor-to-normal ratio was measured on both ^99mTc-MAA SPECT/CT and post-⁹⁰Y bremsstrahlung SPECT/CT. Microdosimetry simulations were used to calculate first percentile dose, which is the dose in the cold spots between microspheres. Local progression-free survival (LPFS) and overall survival were analyzed.

RESULTS

As more particles were delivered, doses on ⁹⁰Y SPECT/CT became more uniform throughout the treatment volume: tumor and liver doses became more similar (p = 0.04), and microscopic cold spots between particles disappeared. For hypervascular tumors (tumor-to-normal ratio ≥ 2.6 on MAA scan), delivering fewer particles (< 6000 particles/cm³ treatment volume) was associated with better LPFS (p = 0.03). For less vascular tumors (tumor-to-normal ratio < 2.6), delivering more particles (≥ 6000 particles/cm³) was associated with better LPFS (p = 0.02). In matched pairs of patients, using the optimal particle density resulted in improved overall survival (11.5 vs. 6.8 months, p = 0.047), compared to using suboptimal particle density. Microdosimetry resulted in better predictions of LPFS (p = 0.03), and overall survival (p = 0.02), compared to conventional dosimetry.

CONCLUSION

The number of particles delivered can be chosen to maximize the tumor dose and minimize the liver dose, based on tumor vascularity. Optimizing the particle density resulted in improved LPFS and overall survival.

Biodistribution and Radiation Dosimetry of Intraperitoneally Administered 124I-Omburtamab in Patients with Desmoplastic Small Round Cell Tumors

The aim of this study was to assess the pharmacokinetics, biodistribution, and radiation dosimetry of ¹²⁴I-omburtamab administered intraperitoneally in patients with desmoplastic small round cell tumor. Methods: Eligible patients diagnosed with desmoplastic small round cell tumor with peritoneal involvement were enrolled in a phase I trial of intraperitoneal radioimmunotherapy with ¹³¹I-omburtamab. After thyroid blockade and before radioimmunotherapy, patients received approximately 74 MBq of ¹²⁴I-omburtamab intraperitoneally. Five serial PET/CT scans were obtained up to 144 h after injection. Multiple blood samples were obtained up to 120 h after injection. Organ-absorbed doses were calculated with OLINDA/EXM. Results: Thirty-one patients were studied. Blood pharmacokinetics exhibited a biphasic pattern consisting of an initial rising phase with a median half-time (±SD) of 23 ± 15 h and a subsequent falling phase with a median half-time of 56 ± 34 h. Peritoneal distribution was heterogeneous and diffuse in most patients. Self-dose to the peritoneal cavity was 0.58 ± 0.19 mGy/MBq. Systemic distribution and activity in major organs were low. The median absorbed doses were 0.72 ± 0.23 mGy/MBq for liver, 0.48 ± 0.17 mGy/MBq for spleen, and 0.57 ± 0.12 mGy/MBq for kidneys. The mean effective dose was 0.31 ± 0.10 mSv/MBq. Whole-body and peritoneal cavity biologic half-times were 45 ± 9 and 24 ± 5 h, respectively. Conclusion: PET/CT imaging with intraperitoneally administered ¹²⁴I-omburtamab enables assessment of intraperitoneal distribution and estimation of absorbed dose to peritoneal space and normal organs before therapy.

SUVfdg: A standard-uptake-value (SUV) body habitus normalizer specific to fluorodeoxyglucose (FDG) in humans

Purpose

To devise a new body-habitus normalizer to be used in the calculation of an SUV that is specific to the PET tracer 18F-FDG.

Methods

A cohort of 481-patients was selected for analysis of 18F-FDG uptake into tissues unaffected by their disease. Among these, 65-patients had only brain concentrations measured and the remaining 416 were randomly divided into an 86-patient test set and a 330-patient training set. Within the test set, normal liver, spleen and blood measures were made. In the training set, only normal liver concentrations were measured. Using data from the training set, a simple polynomial function of height and weight was selected and optimized in a fitting procedure to predict each patient’s mean liver %ID/ml. This function, when used as a normalizer, defines a new SUV metric (SUVfdg) which we compared to SUV metrics normalized by body weight (SUVbw), lean-body mass (SUVlbm) and body surface-area (SUVbsa) in a five-fold cross-validation. SUVfdg was also evaluated in the independent brain-only and whole-body test sets.

Results

For patients of all sizes including pediatric patients, the normal range of liver 18F-FDG uptake at 60 minutes post injection in units of SUVfdg is 1.0 ± 0.16. Liver, blood, and spleen SUVfdg in all comparisons had lower coefficients of variation compared to SUVbw SUVlbm and SUVbsa. Blood had a mean SUVfdg of 0.8 ± 0.11 and showed no correlation with age, height, or weight. Brain SUVfdg measures were significantly higher (P<0.01) in pediatric patients (4.7 ± 0.9) compared to adults (3.1 ± 0.6).

Conclusion

A new SUV metric, SUVfdg, is proposed. It is hoped that SUVfdg will prove to be better at classifying tumor lesions compared to SUV metrics in current use. Other tracers may benefit from similarly tracer-specific body habitus normalizers.

Analysis of capecitabine metabolites in conjunction with digital autoradiography in a murine model of pancreatic cancer suggests extensive drug penetration through the tumor

Previously published digital autoradiography of 3 H-labeled capecitabine reveals a near-uniform distribution of activity throughout a murine pancreatic model. This is in contrast both to 14 C-labeled gemcitabine, and established expectations, as the dense stroma of pancreatic cancer is understood to inhibit drug penetration. Capecitabine is a pro-drug for 5 FU. The positioning of the radiolabel on capecitabine leaves open the possibility that much of the autoradiographic signal is generated by nontoxic compounds. Studies were performed on tumors derived via organoid culture from a murine KPC tumor. As before, we performed autoradiography comparing 3 H capecitabine to the gemcitabine analog 18 F-FAC. The metabolism of capecitabine in this model was studied through LC-MS of tumor tissue. The autoradiographs confirmed that the 3 H label from capecitabine was much more uniformly distributed through the tumor than the 18 F from the gemcitabine analog. LC-MS revealed that approximately 75% of the molar mass of capecitabine had been converted into 5 FU or pre-5 FU compounds. The remainder had been converted into nontoxic species. Therapeutically relevant capecitabine metabolites achieve a relatively even distribution in this pancreatic cancer model, in contrast to the gemcitabine analog 18 F-FAC. In a human xenograft model, (BxPC3), the 3 H label from capecitabine was also uniformly spread across the tumor autoradiographs. However, at 2 h post-administration the metabolism of capecitabine had proceeded further and the bulk of the agent was in the form of nontoxic species.

Application of Community Detection Algorithm to Investigate the Correlation between Imaging Biomarkers of Tumor Metabolism, Hypoxia, Cellularity, and Perfusion for Precision Radiotherapy in Head and Neck Squamous Cell Carcinomas

The present study aimed to investigate the correlation at pre-treatment (TX) between quantitative metrics derived from multimodality imaging (MMI), including 18F-FDG-PET/CT, 18F-FMISO-PET/CT, DW- and DCE-MRI, using a community detection algorithm (CDA) in head and neck squamous cell carcinoma (HNSCC) patients. Twenty-three HNSCC patients with 27 metastatic lymph nodes underwent a total of 69 MMI exams at pre-TX. Correlations among quantitative metrics derived from FDG-PET/CT (SUL), FMSIO-PET/CT (K1, k3, TBR, and DV), DW-MRI (ADC, IVIM [D, D*, and f]), and FXR DCE-MRI [Ktrans, ve, and τi]) were investigated using the CDA based on a "spin-glass model" coupled with the Spearman's rank, ρ, analysis. Mean MRI T2 weighted tumor volumes and SULmean values were moderately positively correlated (ρ = 0.48, p = 0.01). ADC and D exhibited a moderate negative correlation with SULmean (ρ ≤ -0.42, p < 0.03 for both). K1 and Ktrans were positively correlated (ρ = 0.48, p = 0.01). In contrast, Ktrans and k3max were negatively correlated (ρ = -0.41, p = 0.03). CDA revealed four communities for 16 metrics interconnected with 33 edges in the network. DV, Ktrans, and K1 had 8, 7, and 6 edges in the network, respectively. After validation in a larger population, the CDA approach may aid in identifying useful biomarkers for developing individual patient care in HNSCC.

Optimizing reconstruction parameters for quantitative 124I-PET in the presence of therapeutic doses of 131I

Background

The goal of this work was to determine the quantitative accuracy and optimal reconstruction parameters for ¹²⁴I-PET imaging in the presence of therapeutic levels of ¹³¹I. In this effort, images were acquired on a GE D710 PET/CT scanner using a NEMA IEC phantom with spheres containing ¹²⁴I and increasing amounts of ¹³¹I activity in the background. At each activity level, two scans were acquired, one with the phantom centered in the field of view (FOV) and one 11.2 cm off-center. Reconstructions used an ordered subset expectation maximization algorithm with up to 100 iterations of 16 subsets, with and without time-of-flight (TOF) information. Results were evaluated visually and by comparing the ¹²⁴I activity relative to the scan performed in the absence of ¹³¹I.

Results

¹³¹I within the FOV added to the randoms rate, to dead time, and to pile-up within the detectors. Using our standard clinical reconstruction parameters, the image quality and quantitative accuracy suffered at ¹³¹I activities above 1.4 GBq. Convergence rates slowed progressively in the presence of increasing amounts of ¹³¹I for both TOF and nonTOF reconstructions. TOF reconstructions converged more quickly than nonTOF but often towards erroneous concentrations. Iterating nonTOF reconstructions to convergence produced quantitatively accurate images except for the off-center phantom at the very highest level of background ¹³¹I tested.

Conclusions

This study shows that quantitative PET is feasible in the presence of large amounts of ¹³¹I. The high randoms fractions resulted in slow reconstruction convergence and negatively impacted TOF corrections and/or the accuracy of TOF information. Therefore, increased iterations and nonTOF reconstructions are recommended.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-021-00398-z.

Radiation segmentectomy of hepatic metastases with Y-90 glass microspheres

PURPOSE

To evaluate safety and efficacy of radiation segmentectomy (RS) with ⁹⁰Y glass microspheres in patients with limited metastatic liver disease not amenable to resection or percutaneous ablation.

METHODS

Patients with ≤ 3 tumors treated with RS from 6/2015 to 12/2017 were included. Target tumor radiation dose was > 190 Gy based on medical internal radiation dose (MIRD) dosimetry. Tumor response, local tumor progression (LTP), LTP-free survival (LTPFS) and disease progression rate in the treated segment were defined using Choi and RECIST 1.1 criteria. Toxicities were evaluated using modified SIR criteria.

RESULTS

Ten patients with 14 tumors underwent 12 RS. Median tumor size was 3 cm (range 1.4-5.6). Median follow-up was 17.8 months (range 1.6-37.3). Response rates per Choi and RECIST 1.1 criteria were 8/8 (100%) and 4/9 (44%), respectively. Overall LTP rate was 3/14 (21%) during the study period. One-, two- and three-year LTPFS was 83%, 83% and 69%, respectively. Median LTPFS was not reached. Disease progression rate in the treated segment was 6/18 (33%). Median overall survival was 41.5 months (IQR 16.7-41.5). Median delivered tumor radiation dose was 293 Gy (range 163-1303). One major complication was recorded in a patient post-Whipple procedure who suffered anaphylactic reaction to prophylactic cefotetan and liver abscess in RS region 6.5 months post-RS. All patients were alive on last follow-up.

CONCLUSION

RS of ≤ 3 hepatic segments can safely provide a 2-year local tumor control rate of 83% in selected patients with limited metastatic liver disease and limited treatment options. Optimal dosimetry methodology requires further investigation.

Abstract 1663: Activation of STING in response to partial-tumor radiation exposure

Purpose: To determine whether STING activation is involved in partial volume radiation therapy.

Background: The potential of radiotherapy (RT) to induce immune recognition of cancer cells is a growing topic of research. It has been suggested that partial volume irradiation used in GRID therapy/Lattice RT, a type of RT in which radiation does not cover the entire tumor intentionally but rather is spatially fractionated, sometimes induces an immune response. We previously showed that a single dose of radiation delivered to half of the tumor (50% RT) activated an anti-tumor immune response comparable to the response in a fully-irradiated tumor in the immunogenic 67NR murine tumor model (breast carcinoma) and in the less immunogenic and more radioresistant Lewis lung carcinoma (LLC) tumor model. We have also demonstrated that this immune response was due to the infiltration of CD8+ T cells along with an increased expression of ICAM adhesion molecules. Treatment with either anti-CD8 or anti-ICAM antibodies abrogated the hemi-RT response. Furthermore, a significant abscopal effect was observed after partial irradiation with a single dose of 10Gy in a bilateral 67NR tumors model. It has been shown that ionizing radiation can mediate antitumor immunity via the activation of the cytosolic DNA sensor cGAS/STING pathway. Therefore, in this study, we tested whether the hemi-irradiation-mediated immune response involves the cGAS/STING canonical pathway, or a non-canonical activation of STING, in the 67NR or LLC tumor models. It has been reported that STING can be activated, independently of cGAS, via non-canonical activation of STING, involving ATM and TRAF6, among other factors.

Brief methods: We investigated 67NR murine orthotopic breast tumors in Balb/c mice and LLC cells injected in the flank of C57Bl/6, cGAS or STING KO mice. RT was delivered to 50% or 100% of the tumor volume using a 2X2 cm collimator on a microirradiator allowing precise irradiation. Tumors were collected at different time points post-RT and assessed for different measurements.

Results: There is significant activation of the cGAS/STING pathway in the hemi-irradiated tumors as compared to control and to 100% exposed 67NR tumors. Interestingly, the increased expression of the cGAS/STING pathway was found in the hemi-irradiated tumor but, also in the non-irradiated part of the tumor. In the LLC model, a non-canonical activation of STING is involved. Using both cGAS and STING KO mice, we demonstrated that the partial exposure RT-mediated immune response is dependent on STING activation in the host while cGAS is dispensable.

Conclusions: Identifying the upstream pathways responsible for STING activation in the partial RT-mediated immune response in different tumor types would improve this therapy and its potential combination with immune checkpoint blockade.

Citation Format: Mickael Mathieu, Prerna R. Nepali, Sadna Budhu, Simon N. Powell, John L. Humm, Joseph O. Deasy, Adriana Haimovitz-Friedman. Activation of STING in response to partial-tumor radiation exposure [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1663.

A phase I trial of sorafenib with whole brain radiotherapy (WBRT) in breast cancer patients with brain metastases and a correlative study of FLT-PET brain imaging

PURPOSE

Sorafenib has demonstrated anti-tumor efficacy and radiosensitizing activity preclinically and in breast cancer. We examined sorafenib in combination with whole brain radiotherapy (WBRT) and explored the [18F] 3'deoxy-3'-fluorothymidine (FLT)-PET as a novel brain imaging modality in breast cancer brain metastases.

METHODS

A phase I trial of WBRT + sorafenib was conducted using a 3 + 3 design with safety-expansion cohort. Sorafenib was given daily at the start of WBRT for 21 days. The primary endpoints were to determine a maximum tolerated dose (MTD) and to evaluate safety and toxicity. The secondary endpoint was CNS progression-free survival (CNS-PFS). MacDonald Criteria were used for response assessment with a correlative serial FLT-PET imaging study.

RESULTS

13 pts were evaluable for dose-limiting toxicity (DLT). DLTs were grade 4 increased lipase at 200 mg (n = 1) and grade 3 rash at 400 mg (n = 3). The MTD was 200 mg. The overall response rate was 71%. Median CNS-PFS was 12.8 months (95%CI: 6.7-NR). A total of 15 pts (10 WBRT + sorafenib and 5 WBRT) were enrolled in the FLT-PET study: baseline (n = 15), 7-10 days post WBRT (FU1, n = 14), and an additional 12 week (n = 9). A decline in average SUV_max of ≥ 25% was seen in 9/10 (90%) of WBRT + sorafenib patients and 2/4 (50%) of WBRT only patients.

CONCLUSIONS

Concurrent WBRT and sorafenib appear safe at 200 mg daily dose with clinical activity. CNS response was favorable compared to historical controls. This combination should be considered for further efficacy evaluation. FLT-PET may be useful as an early response imaging tool for brain metastases.

TRIAL AND CLINICAL REGISTRY

Trial registration numbers and dates: NCT01724606 (November 12, 2012) and NCT01621906 (June 18, 2012).

Comparison of FDG and FMISO uptakes and distributions in head and neck squamous cell cancer tumors

Purpose

Glycolysis is increased by hypoxia, suggesting a possible correlation between the accumulation of 2-[18F]fluoro-2-deoxy-D-glucose (FDG) in malignant tumors and regional hypoxia defined by 1H-1-(3-[18F]fluoro-2-hydroxypropyl)-2-nitroimidazole (FMISO) PET. The aim of this study is to investigate the intra-tumoral spatial distribution and quantitative relationship between FDG and FMISO in a cohort of head and neck squamous cell cancer (HNSCC) patients.

Methods

Twenty HNSCC patients with 20 primary tumors and 19 metastatic lymph nodes (LNs) underwent FDG and FMISO PET within 1 week. The metabolic target volume (MTV) was defined on the FDG PET images using a region growing algorithm. The hypoxic volume (HV) was defined by the volume of voxels in an FMISO image within the MTV that satisfy a tumor-to-blood ratio (T/B) greater than 1.2. FDG and FMISO lesions were co-registered, and a voxel-by-voxel correlation between the two datasets was performed. FDG and FMISO TVs’ SUVs were also compared as well as the intra-tumoral homogeneity of the two radiotracers. Separate analysis was performed for the primary tumors and LNs.

Results

Twenty-six percent of the primary tumors and 15% of LNs showed a strong correlation (R > 0.7) between FDG and FMISO intra-tumor distributions when considering the MTV. For the HV, only 19% of primary tumors and 12% of LN were strongly correlated. A weak and moderate correlation existed between the two markers SUV_avg, and SUV_max in the case of the primary tumors, respectively. However, this was not the case for the LNs. Good concordances were also observed between the primary tumor’s and LNs HV SUV_avgs as well as between the corresponding hypoxic fractions (HF’s).

Conclusions

A moderate correlation between FDG and hypoxia radiotracer distribution, as measured by FMISO, seems to exist for primary tumors. However, discordant results were found in the case of LNs. Hypoxia appears to be the dominant driver of high FDG uptake in selected tumors only, and therefore FDG PET images cannot be used as a universal surrogate to identify or predict intra-tumor hypoxia.

Precision Radiotherapy: Reduction in Radiation for Oropharyngeal Cancer in the 30 ROC Trial

BACKGROUND

Patients with human papillomavirus-related oropharyngeal cancers have excellent outcomes but experience clinically significant toxicities when treated with standard chemoradiotherapy (70 Gy). We hypothesized that functional imaging could identify patients who could be safely deescalated to 30 Gy of radiotherapy.

METHODS

In 19 patients, pre- and intratreatment dynamic fluorine-18-labeled fluoromisonidazole positron emission tomography (PET) was used to assess tumor hypoxia. Patients without hypoxia at baseline or intratreatment received 30 Gy; patients with persistent hypoxia received 70 Gy. Neck dissection was performed at 4 months in deescalated patients to assess pathologic response. Magnetic resonance imaging (weekly), circulating plasma cell-free DNA, RNA-sequencing, and whole-genome sequencing (WGS) were performed to identify potential molecular determinants of response. Samples from an independent prospective study were obtained to reproduce molecular findings. All statistical tests were 2-sided.

RESULTS

Fifteen of 19 patients had no hypoxia on baseline PET or resolution on intratreatment PET and were deescalated to 30 Gy. Of these 15 patients, 11 had a pathologic complete response. Two-year locoregional control and overall survival were 94.4% (95% confidence interval = 84.4% to 100%) and 94.7% (95% confidence interval = 85.2% to 100%), respectively. No acute grade 3 radiation-related toxicities were observed. Microenvironmental features on serial imaging correlated better with pathologic response than tumor burden metrics or circulating plasma cell-free DNA. A WGS-based DNA repair defect was associated with response (P = .02) and was reproduced in an independent cohort (P = .03).

CONCLUSIONS

Deescalation of radiotherapy to 30 Gy on the basis of intratreatment hypoxia imaging was feasible, safe, and associated with minimal toxicity. A DNA repair defect identified by WGS was predictive of response. Intratherapy personalization of chemoradiotherapy may facilitate marked deescalation of radiotherapy.

KRAS mutation effects on the 2-[18F]FDG PET uptake of colorectal adenocarcinoma metastases in the liver

Background

Deriving individual tumor genomic characteristics from patient imaging analysis is desirable. We explore the predictive value of 2-[18F]FDG uptake with regard to the KRAS mutational status of colorectal adenocarcinoma liver metastases (CLM).

Methods

2-[18F]FDG PET/CT images, surgical pathology and molecular diagnostic reports of 37 patients who underwent PET/CT-guided biopsy of CLM were reviewed under an IRB-approved retrospective research protocol. Sixty CLM in 39 interventional PET scans of the 37 patients were segmented using two different auto-segmentation tools implemented in different commercially available software packages. PET standard uptake values (SUV) were corrected for: (1) partial volume effect (PVE) using cold wall-corrected contrast recovery coefficients derived from phantom spheres with variable diameter and (2) variability of arterial tracer supply and variability of uptake time after injection until start of PET scan derived from the tumor-to-blood standard uptake ratio (SUR) approach. The correlations between the KRAS mutational status and the mean, peak and maximum SUV were investigated using Student’s t test, Wilcoxon rank sum test with continuity correction, logistic regression and receiver operation characteristic (ROC) analysis. These correlation analyses were also performed for the ratios of the mean, peak and maximum tumor uptake to the mean blood activity concentration at the time of scan: SUR_MEAN, SUR_PEAK and SUR_MAX, respectively.

Results

Fifteen patients harbored KRAS missense mutations (KRAS+), while another 3 harbored KRAS gene amplification. For 31 lesions, the mutational status was derived from the PET/CT-guided biopsy. The Student’s t test p values for separating KRAS mutant cases decreased after applying PVE correction to all uptake metrics of each lesion and when applying correction for uptake time variability to the SUR metrics. The observed correlations were strongest when both corrections were applied to SUR_MAX and when the patients harboring gene amplification were grouped with the wild type: p ≤ 0.001; ROC area under the curve = 0.77 and 0.75 for the two different segmentations, respectively, with a mean specificity of 0.69 and sensitivity of 0.85.

Conclusion

The correlations observed after applying the described corrections show potential for assigning probabilities for the KRAS missense mutation status in CLM using 2-[18F]FDG PET images.

Non-invasive imaging prediction of tumor hypoxia: A novel developed and externally validated CT and FDG-PET-based radiomic signatures

BACKGROUND

Tumor hypoxia increases resistance to radiotherapy and systemic therapy. Our aim was to develop and validate a disease-agnostic and disease-specific CT (+FDG-PET) based radiomics hypoxia classification signature.

MATERIAL AND METHODS

A total of 808 patients with imaging data were included: N = 100 training/N = 183 external validation cases for a disease-agnostic CT hypoxia classification signature, N = 76 training/N = 39 validation cases for the H&N CT signature and N = 62 training/N = 36 validation cases for the Lung CT signature. The primary gross tumor volumes (GTV) were manually defined by experts on CT. In order to dichotomize between hypoxic/well-oxygenated tumors a threshold of 20% was used for the [¹⁸F]-HX4-derived hypoxic fractions (HF). A random forest (RF)-based machine-learning classifier/regressor was trained to classify patients as hypoxia-positive/ negative based on radiomic features.

RESULTS

A 11 feature "disease-agnostic CT model" reached AUC's of respectively 0.78 (95% confidence interval [CI], 0.62-0.94), 0.82 (95% CI, 0.67-0.96) and 0.78 (95% CI, 0.67-0.89) in three external validation datasets. A "disease-agnostic FDG-PET model" reached an AUC of 0.73 (0.95% CI, 0.49-0.97) in validation by combining 5 features. The highest "lung-specific CT model" reached an AUC of 0.80 (0.95% CI, 0.65-0.95) in validation with 4 CT features, while the "H&N-specific CT model" reached an AUC of 0.84 (0.95% CI, 0.64-1.00) in validation with 15 CT features. A tumor volume-alone model was unable to significantly classify patients as hypoxia-positive/ negative. A significant survival split (P = 0.037) was found between CT-classified hypoxia strata in an external H&N cohort (n = 517), while 117 significant hypoxia gene-CT signature feature associations were found in an external lung cohort (n = 80).

CONCLUSION

The disease-specific radiomics signatures perform better than the disease agnostic ones. By identifying hypoxic patients our signatures have the potential to enrich interventional hypoxia-targeting trials.

Predicting Gemcitabine Delivery by 18F-FAC PET in Murine Models of Pancreatic Cancer

¹⁸F-FAC (2'-deoxy-2'-¹⁸F-fluoro-β-d-arabinofuranosylcytosine) has close structural similarity to gemcitabine and thus offers the potential to image drug delivery to tumors. We compared tumor ¹⁸F-FAC PET images with ¹⁴C-gemcitabine levels, established ex vivo, in 3 mouse models of pancreatic cancer. We further modified tumor gemcitabine levels with injectable PEGylated recombinant human hyaluronidase (PEGPH20) to test whether changes in gemcitabine would be tracked by ¹⁸F-FAC. Methods: ¹⁸F-FAC was synthesized as described previously. Three patient-derived xenograft (PDX) models were grown in the flanks of NSG mice. Mice were given PEGPH20 or vehicle intravenously 24 h before coinjection of ¹⁸F-FAC and ¹⁴C-gemcitabine. Animals were euthanized and imaged 1 h after tracer administration. Tumor and muscle uptake of both ¹⁸F-FAC and ¹⁴C-gemcitabine was obtained ex vivo. The efficacy of PEPGPH20 was validated through staining with hyaluronic acid binding protein. Additionally, an organoid culture, initiated from a KPC (Pdx-1 Cre LSL-Kras^G12D LSL-p53^R172H) tumor, was used to generate orthotopically growing tumors in C57BL/6J mice, and these tumors were then serially transplanted. Animals were injected with PEGPH20 and ¹⁴C-gemcitabine as described above to validate increased drug uptake by ex vivo assay. PET/MR images were obtained using a PET insert on a 7-T MR scanner. Animals were imaged immediately before injection with PEGPH20 and again 24 h later. Results: Tumor-to-muscle ratios of ¹⁴C-gemcitabine and ¹⁸F-FAC correlated well across all PDX models and treatments (R ² = 0.78). There was a significant increase in the tumor PET signal in PEGPH20-treated PDX animals, and this signal was matched in ex vivo counts for 2 of 3 models. In KPC-derived tumors, PEGPH20 raised ¹⁴C-gemcitabine levels (tumor-to-muscle ratio of 1.9 vs. 2.4, control vs. treated, P = 0.013). PET/MR ¹⁸F-FAC images showed a 12% increase in tumor ¹⁸F-FAC uptake after PEGPH20 treatment (P = 0.023). PEGPH20-treated animals uniformly displayed clear reductions in hyaluronic acid staining. Conclusion: ¹⁸F-FAC PET was shown to be a good surrogate for gemcitabine uptake and, when combined with MR, to successfully determine drug uptake in tumors growing in the pancreas. PEGPH20 had moderate effects on tumor uptake of gemcitabine.

Comparing the intra-tumoral distribution of Gemcitabine, 5-Fluorouracil, and Capecitabine in a murine model of pancreatic ductal adenocarcinoma

PURPOSE

To develop a technique to compare the intra-tumoral distribution of the drug gemcitabine, its surrogate [18F]-fluoroarabinocytosine ([18F]-FAC) and related chemotherapeutics 5-FU and capecitabine in a pre-clinical model of pancreatic ductal adenocarcinoma (PDAC).

EXPERIMENTAL DESIGN

Using a KPC-organoid derived model of PDAC, we obtained autoradiographic images of the tumor distribution of, [14C]-gemcitabine, [14C]-5-FU, [3H]-capecitabine. These were compared indirectly by co-administering [18F]-FAC, a close analog of gemcitabine with a proven equivalent intra-tumor distribution. The short half-life of 18F allows for clean separation of 3H/14C labeled drugs in specimens by dual isotope digital autoradiography. Autoradiographic images of [14C]-gemcitabine, [3H]-capecitabine and [14C]-5-FU were each correlated to [18F]-FAC on a pixel-by-pixel basis. The tumor drug penetration was compared using cumulative histograms.

RESULTS

Gemcitabine distribution correlated strongly with FAC as expected. 5-FU also gave a similar microdistribution to that of FAC, whereas no correlation was found between capecitabine or its metabolic products and FAC distribution. Accumulation of Gemcitabine and 5-FU was lower in hypoxic regions of the tumor, whereas no such correlation was observed for capecitabine and its metabolites.

CONCLUSIONS

Gemcitabine and 5-FU target the same regions of the tumor, leaving hypoxic cells untreated. Capecitabine metabolites penetrate further into the tumor but it is yet to be determined whether these metabolites are the active form of the drug.

Targeted Brain Tumor Radiotherapy Using an Auger Emitter

PURPOSE

Glioblastoma multiforme is a highly aggressive form of brain cancer whose location, tendency to infiltrate healthy surrounding tissue, and heterogeneity significantly limit survival, with scant progress having been made in recent decades.

EXPERIMENTAL DESIGN

¹²³I-MAPi (Iodine-123 Meitner-Auger PARP1 inhibitor) is a precise therapeutic tool composed of a PARP1 inhibitor radiolabeled with an Auger- and gamma-emitting iodine isotope. Here, the PARP inhibitor, which binds to the DNA repair enzyme PARP1, specifically targets cancer cells, sparing healthy tissue, and carries a radioactive payload within reach of the cancer cells' DNA.

RESULTS

The high relative biological efficacy of Auger electrons within their short range of action is leveraged to inflict DNA damage and cell death with high precision. The gamma ray emission of ¹²³I-MAPi allows for the imaging of tumor progression and therapy response, and for patient dosimetry calculation. Here we demonstrated the efficacy and specificity of this small-molecule radiotheranostic in a complex preclinical model. In vitro and in vivo studies demonstrate high tumor uptake and a prolonged survival in mice treated with ¹²³I-MAPi when compared with vehicle controls. Different methods of drug delivery were investigated to develop this technology for clinical applications, including convection enhanced delivery and intrathecal injection.

CONCLUSIONS

Taken together, these results represent the first full characterization of an Auger-emitting PARP inhibitor which demonstrate a survival benefit in mouse models of GBM and confirm the high potential of ¹²³I-MAPi for clinical translation.

11C-Choline PET/CT in Recurrent Prostate Cancer: Retrospective Analysis in a Large U.S. Patient Series

Our purpose was to evaluate the performance of ¹¹C-choline PET/CT in detecting biochemically recurrent prostate cancer (PCa) in a large non-European cohort (in the context of emerging evidence for prostate-specific membrane antigen PET in this setting) and to map patterns of PCa recurrence. Methods: We retrospectively analyzed ¹¹C-choline PET/CT scans from 287 patients who were enrolled in an imaging protocol based on rising prostate-specific antigen (PSA) levels (mean, 3.43 ng/mL; median, 0.94 ng/mL; range, 0.15-89.91 ng/mL) and suspected recurrent PCa. A total of 187 patients had undergone primary radical prostatectomy (RP) (79/187 had secondary radiotherapy), 30 had undergone primary radiotherapy, and 70 had a persistent PSA elevation after receiving initial treatment (69 after RP, 1 after radiotherapy). The level of suspicion for recurrence on ¹¹C-choline PET/CT was scored (0, negative; 1, equivocal; 2, positive) by 2 readers. The correlation between ¹¹C-choline PET/CT positivity and initial treatment, Gleason score, National Comprehensive Cancer Network stage, PSA level, PSA doubling time, PSA velocity, and time between initial treatment and PET imaging was evaluated. Prostate Cancer Molecular Imaging Standardized Evaluation (PROMISE) criteria were used to map ¹¹C-choline recurrence patterns. Results: Considering scores 1 and 2 as positives, consensus between the 2 readers deemed 66% of the ¹¹C-choline PET/CT scans as positive. When sorted by PSA level, 45% of patients with a PSA of less than 0.5 ng/mL, 56% of patients with a PSA of 0.5-0.99 ng/mL, 70% of patients with a PSA of 1.0-1.99 ng/mL, and 90% of patients with a PSA of at least 2.0 ng/mL scored either 1 or 2 on ¹¹C-choline PET/CT scans. When considering scores of 2 only, ¹¹C-choline PET/CT positivity was 54% (28%, 46%, 62%, and 81%, respectively, for patients with PSA < 0.5 ng/mL, 0.5-0.99 ng/mL, 1.0-1.99 ng/mL, and ≥ 2.0 ng/mL). In multivariate analysis, only PSA level was significantly associated with scan positivity. Pattern analysis showed that pelvic lymph nodes were the most common site of recurrence, and 28% of patients had ¹¹C-choline-positive suspected recurrences outside the initial treatment field. Conclusion: ¹¹C-choline PET/CT can detect PCa recurrence even among patients with low PSA levels when interpretation accounts for the clinical context, providing a certain pretest probability. Until prostate-specific membrane antigen agents are fully approved for PCa, choline PET/CT may provide clinical utility.

Imaging Patients with Metastatic Castration-Resistant Prostate Cancer Using 89Zr-DFO-MSTP2109A Anti-STEAP1 Antibody

Six-transmembrane epithelial antigen of prostate-1 (STEAP1) is a relatively newly identified target in prostate cancer. We evaluated the ability of PET/CT with 89Zr-DFO-MSTP2109A, an antibody that recognizes STEAP1, to detect lesions in patients with metastatic castration-resistant prostate cancer (mCRPC). Methods: Nineteen mCRPC patients were prospectively imaged using approximately 185 MBq/10 mg of 89Zr-DFO-MSTP2109A. 89Zr-DFO-MSTP2109A PET/CT images obtained 4-7 d after injection were compared with bone and CT scans. Uptake in lesions was measured. Fifteen patients were treated with an antibody-drug conjugate (ADC) based on MSTP2109A; ADC treatment-related data were correlated with tumor uptake by PET imaging. Bone or soft-tissue biopsy samples were evaluated. Results: No significant toxicity occurred. Excellent uptake was observed in bone and soft-tissue disease. Median SUVmax was 20.6 in bone and 16.8 in soft tissue. Sixteen of 17 lesions biopsied were positive on 89Zr-DFO-MSTP2109A, and all sites were histologically positive (1 on repeat biopsy). Bayesian analysis resulted in a best estimate of 86% of histologically positive lesions being true-positive on imaging (95% confidence interval, 75%-100%). There was no correlation between SUVmax tumor uptake and STEAP1 immunohistochemistry, survival after ADC treatment, number of ADC treatment cycles, or change in prostate-specific antigen level. Conclusion:89Zr-DFO-MSTP2109A is well tolerated and shows localization in mCRPC sites in bone and soft tissue. Given the high SUV in tumor and localization of a large number of lesions, this reagent warrants further exploration as a companion diagnostic in patients undergoing STEAP1-directed therapy.

Pharmacokinetic Assessment of 18F-(2S,4R)-4-Fluoroglutamine in Patients with Cancer

¹⁸F-(2S,4R)-4-fluoroglutamine (¹⁸F-FGln) is an investigational PET radiotracer for imaging tumor glutamine flux and metabolism. The aim of this study was to investigate its pharmacokinetic properties in patients with cancer. Methods: Fifty lesions from 41 patients (21 men and 20 women, aged 54 ± 14 y) were analyzed. Thirty-minute dynamic PET scans were performed concurrently with a rapid intravenous bolus injection of 232 ± 82 MBq of ¹⁸F-FGln, followed by 2 static PET scans at 97 ± 14 and 190 ± 12 min after injection. Five patients also underwent a second ¹⁸F-FGln study 4-13 wk after initiation of therapy with glutaminase, dual TORC1/2, or programmed death-1 inhibitors. Blood samples were collected to determine plasma and metabolite fractions and to scale the image-derived input function. Regions of interest were manually drawn to calculate SUVs. Pharmacokinetic modeling with both reversible and irreversible 1- and 2-tissue-compartment models was performed to calculate the kinetic rate constants K ₁, k ₂, k ₃, and k ₄ The analysis was repeated with truncated 30-min dynamic datasets. Results: Intratumor ¹⁸F-FGln uptake patterns demonstrated substantial heterogeneity in different lesion types. In most lesions, the reversible 2-tissue-compartment model was chosen as the most appropriate according to the Akaike information criterion. K ₁, a surrogate biomarker for ¹⁸F-FGln intracellular transport, was the kinetic rate constant that was most correlated both with SUV at 30 min (Spearman ρ = 0.71) and with SUV at 190 min (ρ = 0.51). Only K ₁ was reproducible from truncated 30-min datasets (intraclass correlation coefficient, 0.96). k ₃, a surrogate biomarker for glutaminolysis rate, was relatively low in about 50% of lesions. Treatment with glutaminase inhibitor CB-839 substantially reduced the glutaminolysis rates as measured by k ₃ Conclusion: ¹⁸F-FGln dynamic PET is a sensitive tool for studying glutamine transport and metabolism in human malignancies. Analysis of dynamic data facilitates better understanding of ¹⁸F-FGln pharmacokinetics and may be necessary for response assessment to targeted therapies that impact intracellular glutamine pool size and tumor glutaminolysis rates.

Positron Emission Tomography/Computed Tomography-Based Assessments of Androgen Receptor Expression and Glycolytic Activity as a Prognostic Biomarker for Metastatic Castration-Resistant Prostate Cancer

Importance

Androgen receptor-signaling inhibitor (ARSi) drugs prolong life in metastatic castration-resistant prostate cancer (mCRPC), but such tumors eventually become resistant and progress. Comprehensive positron emission tomography/computed tomography (PET/CT) imaging using fluoro-2-D-deoxyglucose F 18 ([18F]-FDG) for glycolysis (Glyc) and fluorodihydrotestosterone F 18 ([18F]-FDHT) for androgen receptor (AR) expression determine heterogeneity of imaging phenotypes, which may be useful in distinguishing patients who will benefit from ARSi drugs from those who need alternative treatments.

Objective

To test the hypothesis that PET/CT-based assessments of AR expression and glycolytic activity would reveal heterogeneity affecting prognosis.

Design, Setting, and Participants

Between April 6, 2007, and October 4, 2012, patients with mCRPC underwent imaging with both [18F]-FDG and [18F]-FDHT at Memorial Sloan Kettering Cancer Center. The patients were naive to ARSi treatment with enzalutamide or abiraterone acetate and were referred during documented disease progression. Image-directed biopsy determined the presence or absence of prostate cancer at positive imaging sites.

Interventions

PET/CT imaging was performed with [18F]-FDHT and [18F]-FDG; select individual lesions were biopsied to correlate imaging phenotype with histologic findings.

Main Outcomes and Measures

All metabolically active lesions were interpreted as [18F]-FDHT-positive (AR1) or [18F]-FDHT-negative (AR0) and as [18F]-FDG-positive (Glyc1) or [18F]-FDG-negative (Glyc0). Correlation was performed with overall survival for both individual lesion imaging phenotype as well as patient-specific imaging phenotype.

Results

The mean (SD) age of the 133 patients was 68 (8.6) years. Imaging phenotypes of 2405 PET/CT-positive lesions (median, 12.0 per patient) included 1713 (71.2%) AR1Glyc1, 386 (16.0%) AR1Glyc0, and 306 (12.7%) AR0Glyc1. On multivariate analysis, each phenotype had an independent negative impact effect on survival, most pronounced for AR0Glyc1 lesions (hazard ratio [HR], 1.11; 95% CI, 1.05-1.16; P < .001), followed by AR1Glyc1 lesions (HR, 1.05; 95% CI, 1.03-1.06; P < .001) and AR1Glyc0 lesions (HR, 1.03; 95% CI, 1.00-1.05; P = .048). When sorted by lesion type, 4 patient-specific groups emerged: (1) concordant, with all AR1Glyc1 (34 patients [25.6%]); (2) AR predominant, with AR1Glyc1 and varying numbers of AR1Glyc0 (33 [24.8%]); (3) Glyc predominant, with AR1Glyc1 and varying numbers of AR0Glyc1 (40 [30.1%]); and (4) mixed, with AR1Glyc1 plus a mixture of varying numbers of AR1Glyc0 and AR0Glyc1 (26 [19.5%]).

Conclusions and Relevance

Heterogeneity of PET/CT imaging phenotype has clinical relevance on a lesion and individual patient level. With regard to mCRPC lesions, most express ARs, consistent with initial benefit of ARSi drugs. On a patient basis, 49% (groups 3 and 4) had at least 1 AR0Glyc1 lesion-the imaging phenotype with the most negative effect on survival, possibly due to ARSi resistance.

Biodistribution and Dosimetry of Intraventricularly Administered 124I-Omburtamab in Patients with Metastatic Leptomeningeal Tumors

Radiation dose estimations are key for optimizing therapies. We studied the role of ¹²⁴I-omburtamab (8H9) given intraventricularly in assessing the distribution and radiation doses before ¹³¹I-omburtamab therapy in patients with metastatic leptomeningeal disease and compared it with the estimates from cerebrospinal fluid (CSF) sampling. Methods: Patients with histologically proven malignancy and metastatic disease to the central nervous system or leptomeninges who met eligibility criteria for ¹³¹I-omburtamab therapy underwent immuno-PET imaging with ¹²⁴I-8H9 followed by ¹³¹I-8H9 antibody therapy. Patients were imaged with approximately 74 MBq of intraventricular ¹²⁴I-omburtamab via an Ommaya reservoir. Whole-body PET images were acquired at approximately 4, 24, and 48 h after administration and analyzed for dosimetry calculations. Peripheral blood and CSF samples were obtained at multiple time points for dosimetry estimation. Results: Forty-two patients with complete dosimetry and therapy data were analyzed. ¹²⁴I-omburtamab PET-based radiation dosimetry estimations revealed mean (±SD) absorbed dose to the CSF for ¹³¹I-8H9 of 0.62 ± 0.40 cGy/MBq, compared with 2.22 ± 2.19 cGy/MBq based on ¹²⁴I-omburtamab CSF samples and 1.53 ± 1.37 cGy/MBq based on ¹³¹I-omburtamab CSF samples. The mean absorbed dose to the blood was 0.051 ± 0.11 cGy/MBq for ¹²⁴I-omburtamab samples and 0.07 ± 0.04 cGy/MBq for ¹³¹I-omburtamab samples. The effective whole-body radiation dose for ¹²⁴I-omburtamab was 0.49 ± 0.27 mSv/MBq. The mean whole-body clearance half-time was 44.98 ± 16.29 h. Conclusion: PET imaging with ¹²⁴I-omburtamab antibody administered intraventricularly allows for noninvasive estimation of dose to CSF and normal organs. High CSF-to-blood absorbed-dose ratios are noted, allowing for an improved therapeutic index to leptomeningeal disease and reduced systemic doses. PET imaging-based estimates were less variable and more reliable than CSF sample-based dosimetry.

Pharmacokinetics and Biodistribution of a [89Zr]Zr-DFO-MSTP2109A Anti-STEAP1 Antibody in Metastatic Castration-Resistant Prostate Cancer Patients

A six-transmembrane epithelial antigen of prostate-1 (STEAP1) is a newly identified target in prostate cancer. The use of radio-labeled STEAP1-targeting antibodies with positron emission tomography (PET) may allow for detection of sites of metastatic prostate cancer and may refine patient selection for antigen-directed therapies. This was a prospective study in seven patients with metastatic castration-resistant prostate cancer who had at least one archival biopsy that was STEAP1-positive by immunohistochemistry. Patients received intravenous injections of ∼185 MBq and 10 mg of [⁸⁹Zr]Zr-DFO-MSTP2109A, a humanized IgG1 monoclonal antibody directed against STEAP1. PET/CT images, blood samples, and whole-body counts were monitored longitudinally in six patients. Here, we report on safety, biodistribution, pharmacokinetics, dose estimates to normal tissues, and initial tumor targeting for this group of patients. There was no significant acute or subacute toxicity. Favorable biodistribution and enhanced lesion uptake (in both bone and soft tissue) were observed on imaging using a mass of 10 mg of DFO-MSTP2109A. The best lesion discrimination was seen at the latest imaging time, a median of 6 days postadministration. Pharmacokinetics showed a median serum T_1/2 β of 198 h, volume of central compartment of 3.54 L (similar to plasma volume), and clearance of 19.7 mL/h. The median biologic T_1/2 for whole-body retention was 469 h. The highest mean absorbed doses to normal organs (mGy/MBq) were 1.18, 1.11, 0.78, 0.73, and 0.71 for liver, heart wall, lung, kidney, and spleen, respectively. Excellent targeting of metastatic prostate sites in both bone and soft tissue was observed, with an optimal imaging time of 6 days postadministration. The liver and heart were the normal organs that experienced the highest absorbed doses. The pharmacokinetics were similar to other antibodies without major cross-reactivity with normal tissues. A more detailed analysis of lesion targeting in a larger patient population with correlation to immunohistology and standard imaging modalities has been reported.

Assessment of Simplified Methods for Quantification of 18F-FDHT Uptake in Patients with Metastatic Castration-Resistant Prostate Cancer

¹⁸F-fluorodihydrotestosterone (¹⁸F-FDHT) PET/CT potentially provides a noninvasive method for assessment of androgen receptor expression in patients with metastatic castration-resistant prostate cancer (mCRPC). The objective of this study was to assess simplified methods for quantifying ¹⁸F-FDHT uptake in mCRPC patients and to assess effects of tumor perfusion on these ¹⁸F-FDHT uptake metrics. Methods: Seventeen mCRPC patients were included in this prospective observational multicenter study. Test and retest 30-min dynamic ¹⁸F-FDHT PET/CT scans with venous blood sampling were performed in 14 patients. In addition, arterial blood sampling and dynamic ¹⁵O-H₂O scans were obtained in a subset of 6 patients. Several simplified methods were assessed: Patlak plots; SUV normalized to body weight (SUV_BW), lean body mass (SUV_LBM), whole blood (SUV_WB), parent plasma activity concentration (SUV_PP), area under the parent plasma curve (SUV_AUC,PP), and area under the whole-blood input curve (SUV_AUC,WB); and SUV_BW corrected for sex hormone-binding globulin levels (SUV_SHBG). Results were correlated with parameters derived from full pharmacokinetic ¹⁸F-FDHT and ¹⁵O-H₂O. Finally, the repeatability of individual quantitative uptake metrics was assessed. Results: Eighty-seven ¹⁸F-FDHT-avid lesions were evaluated. ¹⁸F-FDHT uptake was best described by an irreversible 2-tissue-compartment model. Replacing the continuous metabolite-corrected arterial plasma input function with an image-derived input function in combination with venous sample data provided similar K _i results (R ² = 0.98). Patlak K _i and SUV_AUC,PP showed an excellent correlation (R ² > 0.9). SUV_BW showed a moderate correlation to K _i (R ² = 0.70, presumably due to fast ¹⁸F-FDHT metabolism. When calculating SUV_SHBG, correlation to K _i improved (R ² = 0.88). The repeatability of full kinetic modeling parameters was inferior to that of simplified methods (repeatability coefficients > 36% vs. < 28%, respectively). ¹⁸F-FDHT uptake showed minimal blood flow dependency. Conclusion: ¹⁸F-FDHT kinetics in mCRPC patients are best described by an irreversible 2-tissue-compartment model with blood volume parameter. SUV_AUC,PP showed a near-perfect correlation with the irreversible 2-tissue-compartment model analysis and can be used for accurate quantification of ¹⁸F-FDHT uptake in whole-body PET/CT scans. In addition, SUV_SHBG could potentially be used as an even simpler method to quantify ¹⁸F-FDHT uptake when less complex scanning protocols and accuracy are required.

Abstract P1-19-03: A phase I trial of sorafenib with whole brain radiotherapy (WBRT) in breast cancer patients with brain metastases and a correlative study of FLT-PET brain imaging in patients receiving WBRT with or without sorafenib

Background: WBRT is a standard therapy for metastatic breast cancer (MBC) patients (pts) with brain metastases (BM), but disease progression in the brain is common. Sorafenib, a tyrosine kinase inhibitor with anti-VEGF activity, has demonstrated anti-tumor efficacy in MBC and radiosensitizing activity preclinically. [18F] 3'deoxy-3'-fluorothymidine (FLT) is a new PET tracer which correlates with cellular proliferation and may improve response assessment in the brain. Methods: A phase I trial of sorafenib with WBRT in MBC pts with BM was conducted using a 3+3 design. Sorafenib was given orally daily at the start of WBRT for a total of 21 days with 3 doses levels: 200mg, 400mg, and 600mg. The primary endpoints were to determine a maximum tolerated dose (MTD) and to evaluate safety and toxicity. The secondary endpoint was central nervous system progression-free survival (CNS-PFS). Macdonald Criteria were used for response assessment with serial MRI brain imaging. Key eligibility criteria include MBC with new or progressive ≥ 1cm BM, ECOG PS 0-2, non-escalating corticosteroid dose, and no other concurrent anti-tumor therapy except trastuzumab. In parallel, we conducted a correlative FLT-PET imaging study (baseline, 7-10 days (FU1), and 10-12 weeks (FU2) after the WBRT) to assess radiographic changes among pts receiving WBRT + sorafenib and in a separate WBRT only cohort. FLT standard uptake value (SUV) and kinetic parameter data were obtained. Results: 13 pts were treated in the dose escalation phase and evaluable for dose-limiting toxicity (DLT). The median age was 56 years (range: 43-77). There were 4 HER2 positive (31%) and 3 triple negative (23%) pts. 2 pts had prior stereotactic radiosurgery. DLTs were: Grade (G) 4 increased lipase at 200mg (1 pt) and G3 rash at 400mg (3 pts) level. MTD was determined to be 200mg. 10 pts were evaluable for response (at least 1 follow up brain imaging). The overall response rate was 70%: 4 complete responses (CR) + 3 partial responses. All 13 pts were evaluated for CNS PFS with a median follow up of 29.7 months (min 19.6, max 57.4mo). Median CNS-PFS was 8.2 months (95%CI: 3.4-31.8). Median OS was 15.4 months (95% CI: 3.4-NR). A total of 10 pts with WBRT and sorafenib and 5 pts with WBRT only were enrolled in the FLT-PET study: all 15 pts had baseline FLT PET, 14 with FU1, and 9 with FU2. 55 baseline lesions, 38 at FU1 and 15 at FU2 were observed and analyzed. All lesions with FLT uptake had MRI correlates. Decline in average SUVmax of ≥25% was seen in 9/10 (90%) of WBRT+sorafenib and 2/4(50%) of WBRT only pts at FU1. A complete disappearance of FLT uptake was noted in 1 pt at FU1 and 2 more pts at FU2. Conclusions: Concurrent WBRT with sorafenib appears safe at 200mg daily dose with a higher rate of CR compared to historical WBRT data. We are currently enrolling patients in the safety-expansion cohort. This combination should be considered for further efficacy evaluation. Additional analysis of FLT-PET as a complementary imaging modality to MRI is currently ongoing. Clinical trial registry: NCT01724606 and NCT01621906. Support: Bayer, Susan G Komen, ASCO Gianni Bonadonna Breast Cancer Award

Citation Format: Morikawa A, Jhaveri K, Grkovski M, Tang K, Humm JL, Holodny A, Beal K, Schoder H, Seidman AD. A phase I trial of sorafenib with whole brain radiotherapy (WBRT) in breast cancer patients with brain metastases and a correlative study of FLT-PET brain imaging in patients receiving WBRT with or without sorafenib [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P1-19-03.

18F-FDG PET/CT for Monitoring of Ipilimumab Therapy in Patients with Metastatic Melanoma

Immune checkpoint inhibitors (ICIs) are now commonly used to treat patients with metastatic malignant melanoma. Although concerns have been raised that the inflammatory response induced by ICIs may limit the ability of ¹⁸F-FDG PET/CT to assess tumor response, systematic analyses on the use of ¹⁸F-FDG PET/CT in this setting are mostly lacking. Thus, we set out to evaluate the association between tumor response on ¹⁸F-FDG PET/CT and prognosis in patients with metastatic malignant melanoma treated with ipilimumab. Methods: We analyzed 60 consecutive patients with metastatic melanoma who underwent ¹⁸F-FDG PET/CT scans both before and after treatment to evaluate treatment response after completion of ipilimumab therapy. Tumor response was assessed by the change in the sum of SULpeak (voxels with the highest average SUL [SUV normalized to lean body mass]) of up to 5 lesions according to PERCIST5. New lesions on PET that appeared suggestive of metastases were considered progressive metabolic disease (PMD). Because immunotherapy may cause new inflammatory lesions that are detectable on ¹⁸F-FDG PET/CT, we also evaluated an immunotherapy-modified response classification (imPERCIST5). In this classification, new lesions do not define PMD per se; rather, PMD requires an increase in the sum of SULpeak by 30%. The correlation between tumor response according to these 3 definitions and overall survival (OS) was evaluated and compared with known prognostic factors. Results: In responders and nonresponders, the 2-y OS was 66% versus 29% for imPERCIST5 (P = 0.003). After multivariate analysis, imPERCIST5 remained prognostic (hazard ratio, 3.853; 95% confidence interval, 1.498-9.911; P = 0.005). New sites of focal ¹⁸F-FDG uptake occurred more often in patients with PMD (n = 24) by imPERCIST5 than in those with stable metabolic disease (n = 7) or partial metabolic response (n = 4). In patients with partial metabolic response, 2 of 4 isolated new lesions regressed spontaneously during follow-up. Conclusion: In patients with metastatic melanoma treated with ipilimumab, tumor response according to PERCIST was associated with OS. Our data suggest that PMD should not be defined by the appearance of new lesions, but rather by an increase in the sum of SULpeak.

11C-Choline Pharmacokinetics in Recurrent Prostate Cancer

The aim of this study was to investigate the value of pharmacokinetic modeling for quantifying 11C-choline uptake in patients with recurrent prostate cancer. Methods: In total, 194 patients with clinically suspected recurrence of prostate cancer underwent 11C-choline dynamic PET over the pelvic region (0-8 min), followed by a 6-min static acquisition at about 25 min after injection. Regions of interest were drawn over sites of disease identified by a radiologist with experience in nuclear medicine. 11C-choline uptake and pharmacokinetics were evaluated by SUV, graphical analysis (Patlak plot; KiP), and 1- and 2-compartment pharmacokinetic models (K1, K1/k2, k3, k4, and the macro parameter KiC). Twenty-four local recurrences, 65 metastatic lymph nodes, 19 osseous metastases, and 60 inflammatory lymph nodes were included in the analysis, which was subsequently repeated for regions of interest placed over the gluteus maximus muscle and adipose tissue as a control. Results: SUVmean and KiP were 3.60 ± 2.16 and 0.28 ± 0.22 min-1 in lesions, compared with 2.11 ± 1.33 and 0.15 ± 0.10 min-1 in muscle and 0.26 ± 0.07 and 0.02 ± 0.01 min-1 in adipose tissue. According to the Akaike information criterion, the 2-compartment irreversible model was most appropriate in 85% of lesions and resulted in a K1 of 0.79 ± 0.98 min-1 (range, 0.11-7.17 min-1), a K1/k2 of 2.92 ± 3.52 (range, 0.31-20.00), a k3 of 0.36 ± 0.30 min-1 (range, 0.00-1.00 min-1) and a KiC of 0.28 ± 0.22 min-1 (range, 0.00-1.33 min-1). The Spearman ρ between SUV and KiP, between SUV and KiC, and between KiP and KiC was 0.94, 0.91, and 0.97, respectively, and that between SUV and K1, between SUV and K1/k2, and between SUV and k3 was 0.70, 0.44, and 0.33, respectively. Malignant lymph nodes exhibited a higher SUV, KiP, and KiC than benign lymph nodes. Conclusion: Although 11C-choline pharmacokinetic modeling has potential to uncouple the contributions of different processes leading to intracellular entrapment of 11C-choline, the high correlation between SUV and both KiP and KiC supports the use of simpler SUV methods to evaluate changes in 11C-choline uptake and metabolism for treatment monitoring.

Reproducibility and Repeatability of Semiquantitative 18F-Fluorodihydrotestosterone Uptake Metrics in Castration-Resistant Prostate Cancer Metastases: A Prospective Multicenter Study

18F-fluorodihydrotestosterone (18F-FDHT) is a radiolabeled analog of the androgen receptor's primary ligand that is currently being credentialed as a biomarker for prognosis, response, and pharmacodynamic effects of new therapeutics. As part of the biomarker qualification process, we prospectively assessed its reproducibility and repeatability in men with metastatic castration-resistant prostate cancer. Methods: We conducted a prospective multiinstitutional study of metastatic castration-resistant prostate cancer patients undergoing 2 (test/retest) 18F-FDHT PET/CT scans on 2 consecutive days. Two independent readers evaluated all examinations and recorded SUVs, androgen receptor-positive tumor volumes, and total lesion uptake for the most avid lesion detected in each of 32 predefined anatomic regions. The relative absolute difference and reproducibility coefficient (RC) of each metric were calculated between the test and retest scans. Linear regression analyses, intraclass correlation coefficients (ICCs), and Bland-Altman plots were used to evaluate repeatability of 18F-FDHT metrics. The coefficient of variation and ICC were used to assess interobserver reproducibility. Results: Twenty-seven patients with 140 18F-FDHT-avid regions were included. The best repeatability among 18F-FDHT uptake metrics was found for SUV metrics (SUVmax, SUVmean, and SUVpeak), with no significant differences in repeatability among them. Correlations between the test and retest scans were strong for all SUV metrics (R2 ≥ 0.92; ICC ≥ 0.97). The RCs of the SUV metrics ranged from 21.3% (SUVpeak) to 24.6% (SUVmax). The test and retest androgen receptor-positive tumor volumes and TLU, respectively, were highly correlated (R2 and ICC ≥ 0.97), although variability was significantly higher than that for SUV (RCs > 46.4%). The prostate-specific antigen levels, Gleason score, weight, and age did not affect repeatability, nor did total injected activity, uptake measurement time, or differences in uptake time between the 2 scans. Including the most avid lesion per patient, the 5 most avid lesions per patient, only lesions 4.2 mL or more, only lesions with an SUV of 4 g/mL or more, or normalizing of SUV to area under the parent plasma activity concentration-time curve did not significantly affect repeatability. All metrics showed high interobserver reproducibility (ICC > 0.98; coefficient of variation < 0.2%-10.8%). Conclusion: Uptake metrics derived from 18F-FDHT PET/CT show high repeatability and interobserver reproducibility.

In Vitro and In Vivo Comparison of Gemcitabine and the Gemcitabine Analog 1-(2'-deoxy-2'-fluoroarabinofuranosyl) Cytosine (FAC) in Human Orthotopic and Genetically Modified Mouse Pancreatic Cancer Models

PURPOSE

Although gemcitabine is a mainstay of pancreatic cancer therapy, it is only moderately effective, and it would be desirable to measure drug uptake in patients. 1-(2'-deoxy-2'-fluoroarabinofuranosyl) cytosine (FAC), is an analog of gemcitabine, and when labeled with F-18, it may be a potential surrogate PET tracer for the drug.

PROCEDURES

[¹⁸F]FAC was synthesized to a radiochemical purity of >96 %. The human tumor lines AsPC1, BxPC3, Capan-1, Panc1, and MiaPaca2 were grown orthotopically in nude mice. KPC mice that conditionally express oncogenic K-ras and p53 mutations in pancreatic tissue were also used. The intra-tumoral distributions of [¹⁴C]gemcitabine and [¹⁸F]FAC were mapped with autoradiography. The inter-tumor correlation between [¹⁴C]gemcitabine and [¹⁸F]FAC was established in the orthotopic tumors. Expression of the equilibrative and concentrative nucleoside transporters (ENT, CNT) in vitro was detected by western blotting. Drug uptake was characterized in vitro using [³H]gemcitabine and the effect of transporter inhibition on gemcitabine and FAC uptake was investigated. The relative affinity of cells for gemcitabine and FAC was tested in competition assays. The cell lines differed in sensitivity to transport inhibitors and in competition studies. There was a good in vivo correlation between the total uptake of [¹⁸F]FAC and [¹⁴C]gemcitabine, measured across all orthotopic tumors. Using the KPC and BxPC3 models, we found that [¹⁴C]gemcitabine and [¹⁸F]FAC were largely co-localized.

CONCLUSIONS

In the lines examined here, [¹⁸F]FAC uptake correlates well with gemcitabine in vivo, supporting the notion that [¹⁸F]FAC can serve as a PET radiotracer surrogate to determine the uptake and distribution of gemcitabine within pancreatic tumors.

18F-fluoromisonidazole predicts evofosfamide uptake in pancreatic tumor model

Background

Quantitative imaging can facilitate patient stratification in clinical trials. The hypoxia-activated prodrug evofosfamide recently failed a phase III trial in pancreatic cancer. However, the study did not attempt to select for patients with hypoxic tumors. We tested the ability of ¹⁸F-fluoromisonidazole to predict evofosfamide uptake in an orthotopic xenograft model (BxPC3).

Methods

Two forms of evofosfamide were used: (1) labeled on the active moiety (³H) and (2) on the hypoxia targeting nitroimidazole group (¹⁴C). Tumor uptake of evofosfamide and ¹⁸F-fluoromisonidazole was counted ex vivo. Autoradiography of ¹⁴C and ¹⁸F coupled with pimonidazole immunohistochemistry revealed the spatial distributions of prodrug, radiotracer, and hypoxia.

Results

There was significant individual variation in ¹⁸F-fluoromisonidazole uptake, and a significant correlation between normalized ¹⁸F-fluoromisonidazole and both ³H-labeled and ¹⁴C-labeled evofosfamide. ¹⁸F-fluoromisonidazole and ¹⁴C-evofosfamide both localized in hypoxic regions as identified by pimonidazole.

Conclusion

¹⁸F-fluoromisonidazole predicts evofosfamide uptake in a preclinical pancreatic tumor model.

TMSOTf assisted synthesis of 2'-deoxy-2'-[18F]fluoro-β-D-arabinofuranosylcytosine ([18F]FAC)

[¹⁸F]FAC (2’-deoxy-2’-[¹⁸F]fluoro-β-D-arabinofuranosylcytosine, 1) is a versatile probe for imaging deoxycytidine kinase (dCK) expression levels in vivo. dCK is responsible for phosphorylation of deoxycytidine (dC, 2) and other nucleoside analogs, plays a key role in immune activation and has demonstrated to be one of the key enzymes in activating nucleoside based drugs including gemcitabine. Reported synthesis of [¹⁸F]FAC is high yielding but is quite challenging requiring bromination using HBr and careful drying of excess HBr which is critical for successful synthesis. Here in we report a simplified trimethylsilyl trifluoromethanesulfonate (TMSOTf) assisted synthesis of [¹⁸F]FAC eliminating the need of bromination and drying. [¹⁸F]FAC (β-anomer) was synthesized with average isolated decay corrected yield of 10.59 + 4.2% (n = 6) with radiochemical purity of >98% and total synthesis time of 158 + 19 min.

Predicting hypoxia status using a combination of contrast-enhanced computed tomography and [18F]-Fluorodeoxyglucose positron emission tomography radiomics features

BACKGROUND AND PURPOSE

Hypoxia is a known prognostic factor in head and neck cancer. Hypoxia imaging PET radiotracers such as 18F-FMISO are promising but not widely available. The aim of this study was therefore to design a surrogate for 18F-FMISO TBRmax based on 18F-FDG PET and contrast-enhanced CT radiomics features, and to study its performance in the context of hypoxia-based patient stratification.

METHODS

121 lesions from 75 head and neck cancer patients were used in the analysis. Patients received pre-treatment 18F-FDG and 18F-FMISO PET/CT scans. 79 lesions were used to train a cross-validated LASSO regression model based on radiomics features, while the remaining 42 were held out as an internal test subset.

RESULTS

In the training subset, the highest AUC (0.873±0.008) was obtained from a signature combining CT and 18F-FDG PET features. The best performance on the unseen test subset was also obtained from the combined signature, with an AUC of 0.833, while the model based on the 90th percentile of 18F-FDG uptake had a test AUC of 0.756.

CONCLUSION

A radiomics signature built from 18F-FDG PET and contrast-enhanced CT features correlates with 18F-FMISO TBRmax in head and neck cancer patients, providing significantly better performance with respect to models based on 18F-FDG PET only. Such a biomarker could potentially be useful to personalize head and neck cancer treatment at centers for which dedicated hypoxia imaging PET radiotracers are unavailable.

Technical Note: Scintillation well counters and particle counting digital autoradiography devices can be used to detect activities associated with genomic profiling adequacy of biopsy specimens obtained after a low activity 18 F-FDG injection

PURPOSE

Genomic profiling of biopsied tissue is the basis for precision cancer therapy. However, biopsied materials may not contain sufficient amounts of tumor deoxyribonucleonic acid needed for the analysis. We propose a method to determine the adequacy of specimens for performing genomic profiling by quantifying their metabolic activity.

METHODS

We estimated the average density of tumor cells in biopsy specimens needed to successfully perform genomic analysis following the Memorial Sloan Kettering Integrated Mutation Profiling of Actionable Cancer Targets (MSK-IMPACT) protocol from the minimum amount of deoxyribonucleonic acid needed and the volume of tissue typically used for analysis. The average ¹⁸ F-FDG uptake per cell was assessed by incubating HT-29 adenocarcinoma tumor cells in ¹⁸ F-FDG containing solution and then measuring their activity with a scintillation well counter. Consequently, we evaluated the response of two devices around the minimum expected activities which would indicate genomic profiling adequacy of biopsy specimens obtained under ¹⁸ F-FDG PET/CT guidance. Surrogate samples obtained using 18G core needle biopsies of gels containing either ¹⁸ F-FDG-loaded cells in the expected concentrations or the corresponding activity were measured using autoradiography and a scintillation well counter. Autoradiography was performed using a CCD-based device with real-time image display as well as with digital autoradiography imaging plates following a 30-min off-line protocol for specimen activity determination against previously established calibration.

RESULTS

Cell incubation experiments and estimates obtained from quantitative autoradiography of biopsy specimens (QABS) indicate that specimens acquired under ¹⁸ F-FDG PET/CT guidance that contained the minimum amount of cells needed for genomic profiling would have an average activity concentration in the range of about 3 to about 9 kBq/mL. When exposed to specimens with similar activity concentration, both a CCD-based autoradiography device and a scintillation well counter produced signals with sufficient signal-to-background ratio for specimen genomic adequacy identification in less than 10 min, which is short enough to allow procedure guidance.

CONCLUSION

Scintillation well counter measurements and CCD-based autoradiography have adequate sensitivity to detect the tumor burden needed for genomic profiling during ¹⁸ F-FDG PET/CT-guided 18G core needle biopsies of liver adenocarcinoma metastases.

PARP-1-Targeted Radiotherapy in Mouse Models of Glioblastoma

The DNA repair enzyme poly(ADP-ribose) polymerase 1 (PARP-1) is overexpressed in glioblastoma, with overall low expression in healthy brain tissue. Paired with the availability of specific small molecule inhibitors, PARP-1 is a near-ideal target to develop novel radiotherapeutics to induce DNA damage and apoptosis in cancer cells, while sparing healthy brain tissue. Methods: We synthesized an ¹³¹I-labeled PARP-1 therapeutic and investigated its pharmacology in vitro and in vivo. A subcutaneous tumor model was used to quantify retention times and therapeutic efficacy. A potential clinical scenario, intratumoral convection-enhanced delivery, was mimicked using an orthotopic glioblastoma model combined with an implanted osmotic pump system to study local administration of ¹³¹I-PARPi (PARPi is PARP inhibitor). Results:¹³¹I-PARPi is a 1(2H)-phthalazinone, similar in structure to the Food and Drug Administration-approved PARP inhibitor AZD-2281. In vitro studies have shown that ¹³¹I-PARPi and AZD-2281 share similar pharmacologic profiles. ¹³¹I-PARPi delivered 134.1 cGy/MBq intratumoral injected activity. Doses to nontarget tissues, including liver and kidney, were significantly lower. Radiation damage and cell death in treated tumors were shown by p53 activation in U87-MG cells transfected with a p53-bioluminescent reporter. Treated mice showed significantly longer survival than mice receiving vehicle (29 vs. 22 d, P < 0.005) in a subcutaneous model. Convection-enhanced delivery demonstrated efficient retention of ¹³¹I-PARPi in orthotopic brain tumors, while quickly clearing from healthy brain tissue. Conclusion: Our results demonstrate ¹³¹I-PARPi's high potential as a therapeutic and highlight PARP's relevance as a target for radionuclide therapy. Radiation plays an integral role in brain tumor therapy, and radiolabeled PARP therapeutics could ultimately lead to improvements in the standard of care.

Correlation Between Tumor Metabolism and Semiquantitative Perfusion Magnetic Resonance Imaging Metrics in Non-Small Cell Lung Cancer

PURPOSE

To correlate semiquantitative parameters derived from dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG-PET) for non-small cell lung cancer (NSCLC).

METHODS AND MATERIALS

Twenty-four NSCLC patients who underwent pretreatment ¹⁸F-FDG-PET and DCE-MRI were analyzed. The maximum standardized uptake value (SUVmax) was measured from 18F-FDG-PET. Dynamic contrast-enhanced MRI was obtained on a 3T MRI scanner using 4-dimensional T1-weighted high-resolution imaging with a volume excitation sequence. The DCE-MRI parameters, consisting of mean, median, standard deviation (SD), and median absolute deviation (MAD) of peak enhancement, time to peak (TTP), time to half peak (TTHP), wash-in slope (WIS), wash-out slope (WOS), initial gradient, wash-out gradient, signal enhancement ratio, and initial area under the relative signal enhancement curve taken up to 30, 60, 90, 120, 150, and 180 seconds, TTP, and TTHP (IAUCtthp), were calculated for each lesion. Univariate analysis (UVA) was performed using Spearman correlation. A linear regression model to predict SUVmax from DCE-MRI parameters was developed by multivariate analysis (MVA) using least absolute shrinkage selection operator in combination with leave-one-out cross-validation (LOOCV).

RESULTS

In UVA, mean(WOS) (ρ = -0.456, P = .025), mean(IAUCtthp) (ρ = -0.439, P = .032), median(IAUCtthp) (ρ = -0.543, P = .006), and MAD(IAUCtthp) (ρ = -0.557, P = .005) were statistically significant; all these parameters were negatively correlated with SUVmax. In MVA, a linear combination of SD(WIS), SD(TTP), MAD(TTHP), and MAD(IAUCtthp) was statistically significant for predicting SUVmax (LOOCV-based adjusted R² = 0.298, P = .0006). A decrease in SD(WIS), MAD(TTHP), and MAD(IAUCtthp) and an increase in SD(TTP) were associated with a significant increase in SUVmax.

CONCLUSION

An association was found between SUVmax, the SD, and MAD of DCE-MRI metrics derived during contrast uptake in NSCLC, reflecting that intratumoral heterogeneity in wash-in contrast kinetics is associated with tumor metabolism. Although MAD(IAUCtthp) was a significant feature in both UVA and MVA, the LASSO-based multivariate regression model yielded better predictability of SUVmax than a univariate regression model using MAD(IAUCtthp). This study will facilitate understanding of the complex relationship between tumor vascularization and metabolism and eventually help in guiding targeted therapy.

Inter-operator variability in compartmental kinetic analysis of 18F-fluoromisonidazole dynamic PET

PURPOSE

To assess the inter-operator variability in compartment analysis (CA) of dynamic-FMISO (dyn-FMISO) PET.

METHODS

Study-I: Five investigators conducted CA for 23 NSCLC dyn-FMISO tumor time-activity-curves. Study-II: Four operators performed CA for four NSCLC dyn-FMISO datasets. Repeatability of Kinetic-Rate-Constants (KRCs) was assessed.

RESULTS

Study-I: Strong correlation (ICC > 0.9) and interchangeable results among operators existed for all KRCs. Study-II: Up to 103% variability in tumor segmentation, and weaker ICC in KRCs (ICC-V_B = 0.53; ICC-K₁ = 0.91; ICC-K₁/k₂ = 0.25; ICC-k3 = 0.32; ICC-K_i = 0.54) existed. All KRCs were repeatable among the different operators.

CONCLUSIONS

Inter-operator variability in CA of dyn-FMISO was shown to be within statistical errors.

A phase II study of radioimmunotherapy with intraventricular 131 I-3F8 for medulloblastoma

BACKGROUND

High-risk and recurrent medulloblastoma (MB) is associated with significant mortality. The murine monoclonal antibody 3F8 targets the cell-surface disialoganglioside GD2 on MB. We tested the efficacy, toxicity, and dosimetry of compartmental radioimmunotherapy (cRIT) with intraventricular ¹³¹ I-labeled 3F8 in patients with MB on a phase II clinical trial.

METHODS

Patients with histopathologically confirmed high-risk or recurrent MB were eligible for cRIT. After determining adequate cerebrospinal fluid (CSF) flow, patients received 2 mCi (where Ci is Curie) ¹²⁴ I-3F8 or ¹³¹ I-3F8 with nuclear imaging for dosimetry, followed by up to four therapeutic (10 mCi/dose) ¹³¹ I-3F8 injections. Dosimetry estimates were based on serial CSF and blood samplings over 48 hr plus region-of-interest analyses on serial imaging scans. Disease evaluation included pre- and posttherapy brain/spine magnetic resonance imaging approximately every 3 months for the first year after treatment, and every 6-12 months thereafter.

RESULTS

Forty-three patients received a total of 167 injections; 42 patients were evaluable for outcome. No treatment-related deaths occurred. Toxicities related to drug administration included acute bradycardia with somnolence, headache, fatigue, and CSF pleocytosis consistent with chemical meningitis and dystonic reaction. Total CSF absorbed dose was 1,453 cGy (where Gy is Gray; 350.0-2,784). Median overall survival from first dose of cRIT was 24.9 months (95% confidence interval [CI]:16.3-55.8). Patients treated in radiographic and cytologic remission were at a lower risk of death compared to patients with radiographically measurable disease (hazard ratio: 0.40, 95% CI: 0.18-0.88, P = 0.024).

CONCLUSIONS

cRIT with ¹³¹ I-3F8 is safe, has favorable dosimetry to CSF, and when added to salvage therapy using conventional modalities, may have clinical utility in maintaining remission in high-risk or recurrent MB.

A Combination of Radiation and the Hypoxia-Activated Prodrug Evofosfamide (TH-302) is Efficacious against a Human Orthotopic Pancreatic Tumor Model

This study was designed to investigate the effect of single-dose radiation therapy (RT) in combination with evofosfamide (TH-302), a hypoxia-activated prodrug, in a pre-clinical model of pancreatic cancer. AsPC1 tumors were implanted orthotopically in the pancreas of nude mice. Tumors were treated with 15 Gy of RT, using a 1 cm diameter field, and delivered as a continuous arc. Image-guidance to center the field on the tumor was based on CT imaging with intraperitoneal contrast. Evofosfamide (100 mg/kg, i.p.) was administered 3 hours before RT. Tumor volumes were measured using ultrasound, and regrowth curves were plotted. Tumor hypoxia and cell proliferation were measured using pimonidazole and the thymidine analog EdU, respectively. In vitro clonogenic assays were performed. Tumors were shown to contain substantial areas of hypoxia, as calculated by percent pimonidazole staining. Evofosfamide was active in these tumors, as demonstrated by a significant reduction in uptake of the thymidine analog EdU. This effect was visible in oxygenated tissue, consistent with the previously reported bystander effects of evofosfamide. RT produced significant regrowth delay, as did evofosfamide. The combination of both agents produced a growth delay that was at least equal to the sum of the two treatments given separately. The improvement in tumor response when evofosfamide is combined with RT supports the hypothesis that hypoxia is a cause of radioresistance in high dose RT for pancreatic cancer. Assessing the efficacy and safety of stereotactic radiation treatment and evofosfamide is warranted in patients with locally advanced pancreatic cancer.

Evaluation of the tumor registration error in biopsy procedures performed under real-time PET/CT guidance

PURPOSE

The purpose of this study is to quantify tumor displacement during real-time PET/CT guided biopsy and to investigate correlations between tumor displacement and false-negative results.

METHODS

19 patients who underwent real-time ¹⁸ F-FDG PET-guided biopsy and were found positive for malignancy were included in this study under IRB approval. PET/CT images were acquired for all patients within minutes prior to biopsy to visualize the FDG-avid region and plan the needle insertion. The biopsy needle was inserted and a post-insertion CT scan was acquired. The two CT scans acquired before and after needle insertion were registered using a deformable image registration (DIR) algorithm. The DIR deformation vector field (DVF) was used to calculate the mean displacement between the pre-insertion and post-insertion CT scans for a region around the tip of the biopsy needle. For 12 patients one biopsy core from each was tracked during histopathological testing to investigate correlations of the mean displacement between the two CT scans and false-negative or true-positive biopsy results. For 11 patients, two PET scans were acquired; one at the beginning of the procedure, pre-needle insertion, and an additional one with the needle in place. The pre-insertion PET scan was corrected for intraprocedural motion by applying the DVF. The corrected PET was compared with the post-needle insertion PET to validate the correction method.

RESULTS

The mean displacement of tissue around the needle between the pre-biopsy CT and the postneedle insertion CT was 5.1 mm (min = 1.1 mm, max = 10.9 mm and SD = 3.0 mm). For mean displacements larger than 7.2 mm, the biopsy cores gave false-negative results. Correcting pre-biopsy PET using the DVF improved the PET/CT registration in 8 of 11 cases.

CONCLUSIONS

The DVF obtained from DIR of the CT scans can be used for evaluation and correction of the error in needle placement with respect to the FDG-avid area. Misregistration between the pre-biopsy PET and the CT acquired with the needle in place was shown to correlate with false negative biopsy results.

Abstract 4122: Tumor-specific PI3K inhibition via nanoparticle targeted delivery in head and neck squamous cell carcinoma

Purpose: PI3K-pathway activation is the second most common genetic abnormality occurring in head and neck squamous cell carcinoma (HNSCC). Mutation or amplification of PIK3CA, the gene coding for the p110α subunit of PI3K, occur in 34%-56% of HNSCC and there is evidence that targeting PI3Kα in these tumors can be radiosensitizing. Small molecule kinase inhibitors of PI3Kα are showing promising activity. However, the use of this molecule is limited by serious side effects such as hyperglycemia, diarrhea and rush. In this study we aimed to apply novel nanotechnology for targeted drug delivery via the cell adhesion molecule P-selectin, which was previously shown to be overexpressed in tumor microvasculature. Furthermore, P-selectin is up regulated following radiation therapy (RT), which could enhance drug delivery using targeted nanoparticles.

Experimental design: We explored the efficacy and specificity of targeted delivery of BYL719, a potent alpha-specific PI3K inhibitor, using nanoparticles that selectively target P-selectin present in the tumor microvasculature in PIK3CA-mutated HNSCC. We compared the antitumor effects of nanoparticle delivery versus standard oral gavage with free drug in PIK3CA mutated HNSCC cell line- and patient-derived xenografts (PDXs). In addition, we compared blood glucose and insulin levels between standard BYL719 administration and nanoparticle drug delivery at different time points after treatment. Furthermore, we sought to evaluate the radiosensitizing properties of BYL719 when combined with fractionated RT of 5X4Gy.

Results: P-selectin targeted delivery of 50 mg/kg/week BYL719 resulted in a significant tumor growth delay, which was comparable to the standard drug administration of 350 mg/kg/week (50 mg/kg/daily). While blood glucose and insulin levels were spiking after standard oral gavage of BYL719, these parameters were virtually unchanged upon nanoparticle administration of the drug. The radiosensitizing abilities of low-dose nanoparticle-linked BYL719 were comparable to the drug concentrations used in standard daily BYL719 administration. When given concomitantly with fractionated RT, both methods showed robust enhancement of radiotherapy response in all the models tested resulting in durable control of tumor growth.

Conclusions: P-selectin targeted delivery of a PI3Kα inhibitor resulted in inhibition of the PI3K/Akt/mTOR pathway without affecting glucose or insulin levels. The magnitude of pathway inhibition was sufficient to radiosensitize several HNSCC animal models. This novel targeting strategy could be translated to the clinic to treat patients with PIK3CA activated and radioresistant HNSCC tumors sparing most of the systemic adverse effects of PI3K inhibition.

Citation Format: Aviram Mizrachi, Yosi Shamay, Janki Shah, Samuel I. Brook, Joanne Soong, Vinagolu K. Rajasekhar, John L. Humm, Simon N. Powell, José Baselga, Daniel A. Heller, Adriana Haimovitz-Friedman, Maurizio Scaltriti. Tumor-specific PI3K inhibition via nanoparticle targeted delivery in head and neck squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4122. doi:10.1158/1538-7445.AM2017-4122

Quantitative Assessment of Early [18F]Sodium Fluoride Positron Emission Tomography/Computed Tomography Response to Treatment in Men With Metastatic Prostate Cancer to Bone

Purpose [¹⁸F]Sodium fluoride (NaF) positron emission tomography (PET)/computed tomography (CT) is a promising radiotracer for quantitative assessment of bone metastases. This study assesses changes in early NaF PET/CT response measures in metastatic prostate cancer for correlation to clinical outcomes. Patients and Methods Fifty-six patients with metastatic castration-resistant prostate cancer (mCRPC) with osseous metastases had NaF PET/CT scans performed at baseline and after three cycles of chemotherapy (n = 16) or androgen receptor pathway inhibitors (n = 40). A novel technology, Quantitative Total Bone Imaging, was used for analysis. Global imaging metrics, including maximum standardized uptake value (SUV_max) and total functional burden (SUV_total), were extracted from composite lesion-level statistics for each patient and tracked throughout treatment. Progression-free survival (PFS) was calculated as a composite end point of progressive events using conventional imaging and/or physician discretion of clinical benefit; NaF imaging was not used for clinical evaluation. Cox proportional hazards regression analyses were conducted between imaging metrics and PFS. Results Functional burden (SUV_total) assessed midtreatment was the strongest univariable PFS predictor (hazard ratio, 1.97; 95% CI, 1.44 to 2.71; P < .001). Classification of patients based on changes in functional burden showed stronger correlation to PFS than did the change in number of lesions. Various global imaging metrics outperformed baseline clinical markers in predicting outcome, including SUV_total and SUV_mean. No differences in imaging response or PFS correlates were found for different treatment cohorts. Conclusion Quantitative total bone imaging enables comprehensive disease quantification on NaF PET/CT imaging, showing strong correlation to clinical outcomes. Total functional burden assessed after three cycles of hormonal therapy or chemotherapy was predictive of PFS for men with mCRPC. This supports ongoing development of NaF PET/CT-based imaging biomarkers in mCRPC to bone.