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.

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.

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.

Absorbed-Dose Specification in Nuclear Medicine: Abstract

A number of reasons have led to a reappraisal of dose specification for nuclear medicine. These include an appreciation of non-uniformities in the distribution of radioactivity in the body, at all levels, for even the most common diagnostic and therapeutic agents; an increasing need to deal with the complexities of varying dose rates; the imperative to provide individual rather than standardised dose estimates as targeted radionuclide therapy becomes more sophisticated; as well as improvements in technology.

This Report deals first with biological considerations that inform the rational use of radionuclide dosimetry. Radiobiological factors in the selection of radionuclides and tumour and normal-tissue dose-responses are discussed. Then, the MIRD (medical internal radiation dose) approach to nuclear medical dosimetry, a robust method that has proven its clinical utility, is described. Following on is an elaboration of non-uniform distributions of radioactivity and of varying dose rates. Lastly, the Report deals with techniques and procedures for measuring time variant activity distributions, image fusion, patient specific dose computations, smallscale dosimetry, and the comparison of calculated and measured doses.

Repeatability of SUV measurements in serial PET

PURPOSE

The standardized uptake value (SUV) is a quantitative measure of FDG tumor uptake frequently used as a tool to monitor therapeutic response. This study aims to (i) assess the reproducibility and uncertainty of SUV max and SUV mean, due to purely statistical, i.e., nonbiological, effects and (ii) to establish the minimum uncertainty below which changes in SUV cannot be expected to be an indicator of physiological changes.

METHODS

Three sets of measurements were made using a GE Discovery STE PET/CT Scanner in 3D mode: (1) A uniform 68Ge 20 cm diameter cylindrical phantom was imaged. Thirty serial frames were acquired for durations of 3, 6, 10, 15, and 30 min. (2) Esser flangeless phantom (Data Spectrum, approximately 6.1 L) with fillable thin-walled cylinders inserts (diameters: 8, 12, 16, and 25 mm; height: approximately 3.8 mm) was scanned for five consecutive 3 min runs. The cylinders were filled with 18FDG with a 37 kBq/cc concentration, and with a target-to-background ratio (T/BKG) of 3/1. (3) Eight cancer patients with healthy livers were scanned approximately 1.5 h post injection. Three sequential 3 min scans were performed for one bed position covering the liver, with the patient and bed remaining at the same position for the entire length of the scan. Volumes of interest were drawn on all images using the corresponding CT and then transferred to the PET images. For each study (1-3), the average percent change in SUV mean and SUV max were determined for each run pair. Moreover, the repeatability coefficient was calculated for both the SUV mean and SUV max for each pair of runs. Finally, the overall ROI repeatability coefficient was determined for each pair of runs.

RESULTS

For the 68Ge phantom the average percent change in SUV max and SUV mean decrease as a function of increasing acquisition time from 4.7 +/- 3.1 to 1.1 +/- 0.6%, and from 0.14 +/- 0.09 to 0.04 +/- 0.03%, respectively. Similarly, the coefficients of repeatability also decrease between the 3 and 30 min acquisition scans, in the range of 10.9 +/- 3.9% - 2.6 +/- 0.9%, and 0.3 +/- 0.1% - 0.10 +/- 0.04%, for the SUV max and SUV mean, respectively. The overall ROI repeatability decreased from 18.9 +/- 0.2 to 6.0 +/- 0.1% between the 3 and 30 min acquisition scans. For the l8FDG phantom, the average percent change in SUV max and SUV mean decreases with target diameter from 3.6 +/- 2.0 to 1.5 +/- 0.8% and 1.5 +/- 1.3 to 0.26 +/- 0.15%, respectively, for targets from 8-25 mm in diameter and for a region in the background (BKG). The coefficients of repeatability for SUV max and SUV mean also decrease as a function of target diameter from 7.1 +/- 2.5 to 2.4 +/- 0.9 and 4.2 +/- 1.5 to 0.6 +/- 0.2, respectively, for targets from 8 mm to BKG in diameter. Finally, overall ROI repeatability decreased from 12.0 +/- 4.1 to 13.4 +/- 0.5 targets from 8 mm to BKG in diameter. Finally, for the measurements in healthy livers the average percent change in SUVmax and SUV mean were in the range of 0.5 +/- 0.2% - 6.2 +/- 3.9% and 0.4 +/- 0.1 and 1.6 +/- 1%, respectively. The coefficients of repeatability for SUV max and SUV men are in the range of 0.6 +/- 0.7% - 9.5 +/- 12% and 0.6 +/- 0.7% - 2.9 +/- 3.6%, respectively. The overall target repeatability varied between 27.9 +/- 0.5% and 41.1 +/- 1.0%.

CONCLUSIONS

The statistical fluctuations of the SUV mean are half as large as those of the SUV max in the absence of biological or physiological effects. In addition, for clinically applicable scan durations (i.e., approximately 3 min) and FDG concentrations, the SUV max and SUV mean have similar amounts of statistical fluctuation for small regions. However, the statistical fluctuations of the SUVmean rapidly decrease with respect tothe SUVmax as the statistical power of the data grows either due to longer scanning times or as the target regions encompass a larger volume.

Renal uptake of bismuth-213 and its contribution to kidney radiation dose following administration of actinium-225-labeled antibody

Clinical therapeutic studies using (225)Ac-labeled antibodies have begun. Of major concern is renal toxicity that may result from the three alpha-emitting progeny generated following the decay of (225)Ac. The purpose of this study was to determine the amount of (225)Ac and non-equilibrium progeny in the mouse kidney after the injection of (225)Ac-huM195 antibody and examine the dosimetric consequences. Groups of mice were sacrificed at 24, 96 and 144 h after injection with (225)Ac-huM195 antibody and kidneys excised. One kidney was used for gamma ray spectroscopic measurements by a high-purity germanium (HPGe) detector. The second kidney was used to generate frozen tissue sections which were examined by digital autoradiography (DAR). Two measurements were performed on each kidney specimen: (1) immediately post-resection and (2) after sufficient time for any non-equilibrium excess (213)Bi to decay completely. Comparison of these measurements enabled estimation of the amount of excess (213)Bi reaching the kidney (γ-ray spectroscopy) and its sub-regional distribution (DAR). The average absorbed dose to whole kidney, determined by spectroscopy, was 0.77 (SD 0.21) Gy kBq(-1), of which 0.46 (SD 0.16) Gy kBq(-1) (i.e. 60%) was due to non-equilibrium excess (213)Bi. The relative contributions to renal cortex and medulla were determined by DAR. The estimated dose to the cortex from non-equilibrium excess (213)Bi (0.31 (SD 0.11) Gy kBq(-1)) represented ∼46% of the total. For the medulla the dose contribution from excess (213)Bi (0.81 (SD 0.28) Gy kBq(-1)) was ∼80% of the total. Based on these estimates, for human patients we project a kidney-absorbed dose of 0.28 Gy MBq(-1) following administration of (225)Ac-huM195 with non-equilibrium excess (213)Bi responsible for approximately 60% of the total. Methods to reduce renal accumulation of radioactive progeny appear to be necessary for the success of (225)Ac radioimmunotherapy.

Using an external gating signal to estimate noise in PET with an emphasis on tracer avid tumors

The purpose of this study is to establish and validate a methodology for estimating the standard deviation of voxels with large activity concentrations within a PET image using replicate imaging that is immediately available for use in the clinic. To do this, ensembles of voxels in the averaged replicate images were compared to the corresponding ensembles in images derived from summed sinograms. In addition, the replicate imaging noise estimate was compared to a noise estimate based on an ensemble of voxels within a region. To make this comparison two phantoms were used. The first phantom was a seven-chamber phantom constructed of 1 liter plastic bottles. Each chamber of this phantom was filled with a different activity concentration relative to the lowest activity concentration with ratios of 1:1, 1:1, 2:1, 2:1, 4:1, 8:1 and 16:1. The second phantom was a GE Well-Counter phantom. These phantoms were imaged and reconstructed on a GE DSTE PET/CT scanner with 2D and 3D reprojection filtered backprojection (FBP), and with 2D- and 3D-ordered subset expectation maximization (OSEM). A series of tests were applied to the resulting images that showed that the region and replicate imaging methods for estimating standard deviation were equivalent for backprojection reconstructions. Furthermore, the noise properties of the FBP algorithms allowed scaling the replicate estimates of the standard deviation by a factor of 1/square root N, where N is the number of replicate images, to obtain the standard deviation of the full data image. This was not the case for OSEM image reconstruction. Due to nonlinearity of the OSEM algorithm, the noise is shown to be both position and activity concentration dependent in such a way that no simple scaling factor can be used to extrapolate noise as a function of counts. The use of the Well-Counter phantom contributed to the development of a heuristic extrapolation of the noise as a function of radius in FBP. In addition, the signal-to-noise ratio for high uptake objects was confirmed to be higher with backprojection image reconstruction methods. These techniques were applied to several patient data sets acquired in either 2D or 3D mode, with (18)F (FLT and FDG). Images of the standard deviation and signal-to-noise ratios were constructed and the standard deviations of the tumors' uptake were determined. Finally, a radial noise extrapolation relationship deduced in this paper was applied to patient data.

An iterative technique to segment PET lesions using a Monte Carlo based mathematical model

PURPOSE

The need for an accurate lesion segmentation tool in 18FDG PET is a prerequisite for the estimation of lesion response to therapy, for radionuclide dosimetry, and for the application of 18FDG PET to radiotherapy planning. In this work, the authors have developed an iterative method based on a mathematical fit deduced from Monte Carlo simulations to estimate tumor segmentation thresholds.

METHODS

The GATE software, a GEANT4 based Monte Carlo tool, was used to model the GE Advance PET scanner geometry. Spheres ranging between 1 and 6 cm in diameters were simulated in a 10 cm high and 11 cm in diameter cylinder. The spheres were filled with water-equivalent density and simulated in both water and lung equivalent background. The simulations were performed with an infinite, 8/1, and 4/1 target-to-background ratio (T/B). A mathematical fit describing the correlation between the lesion volume and the corresponding optimum threshold value was then deduced through analysis of the reconstructed images. An iterative method, based on this mathematical fit, was developed to determine the optimum threshold value. The effects of the lesion volume and T/B on the threshold value were investigated. This method was evaluated experimentally using the NEMA NU2-2001 IEC phantom, the ACNP cardiac phantom, a randomly deformed aluminum can, and a spheroidal shape phantom implemented artificially in the lung, liver, and brain of patient PET images. Clinically, the algorithm was evaluated in six lesions from five patients. Clinical results were compared to CT volumes.

RESULTS

This mathematical fit predicts an existing relationship between the PET lesion size and the percent of maximum activity concentration within the target volume (or threshold). It also showed a dependence of the threshold value on the T/B, which could be eliminated by background subtraction. In the phantom studies, the volumes of the segmented PET targets in the PET images were within 10% of the nominal ones. Clinically, the PET target volumes were also within 10% of those measured from CT images.

CONCLUSIONS

This iterative algorithm enabled accurately segment PET lesions, independently of their contrast value.

Phase and amplitude binning for 4D-CT imaging

We compare the consistency and accuracy of two image binning approaches used in 4D-CT imaging. One approach, phase binning (PB), assigns each breathing cycle 2pi rad, within which the images are grouped. In amplitude binning (AB), the images are assigned bins according to the breathing signal's full amplitude. To quantitate both approaches we used a NEMA NU2-2001 IEC phantom oscillating in the axial direction and at random frequencies and amplitudes, approximately simulating a patient's breathing. 4D-CT images were obtained using a four-slice GE Lightspeed CT scanner operating in cine mode. We define consistency error as a measure of ability to correctly bin over repeated cycles in the same field of view. Average consistency error mue+/-sigmae in PB ranged from 18%+/-20% to 30%+/-35%, while in AB the error ranged from 11%+/-14% to 20%+/-24%. In PB nearly all bins contained sphere slices. AB was more accurate, revealing empty bins where no sphere slices existed. As a proof of principle, we present examples of two non-small cell lung carcinoma patients' 4D-CT lung images binned by both approaches. While AB can lead to gaps in the coronal images, depending on the patient's breathing pattern, PB exhibits no gaps but suffers visible artifacts due to misbinning, yielding images that cover a relatively large amplitude range. AB was more consistent, though often resulted in gaps when no data existed due to patients' breathing pattern. We conclude AB is more accurate than PB. This has important consequences to treatment planning and diagnosis.

Evaluation of an automated deformable image matching method for quantifying lung motion in respiration-correlated CT images

We have evaluated an automated registration procedure for predicting tumor and lung deformation based on CT images of the thorax obtained at different respiration phases. The method uses a viscous fluid model of tissue deformation to map voxels from one CT dataset to another. To validate the deformable matching algorithm we used a respiration-correlated CT protocol to acquire images at different phases of the respiratory cycle for six patients with nonsmall cell lung carcinoma. The position and shape of the deformable gross tumor volumes (GTV) at the end-inhale (EI) phase predicted by the algorithm was compared to those drawn by four observers. To minimize interobserver differences, all observers used the contours drawn by a single observer at end-exhale (EE) phase as a guideline to outline GTV contours at EI. The differences between model-predicted and observer-drawn GTV surfaces at EI, as well as differences between structures delineated by observers at EI (interobserver variations) were evaluated using a contour comparison algorithm written for this purpose, which determined the distance between the two surfaces along different directions. The mean and 90% confidence interval for model-predicted versus observer-drawn GTV surface differences over all patients and all directions were 2.6 and 5.1 mm, respectively, whereas the mean and 90% confidence interval for interobserver differences were 2.1 and 3.7 mm. We have also evaluated the algorithm's ability to predict normal tissue deformations by examining the three-dimensional (3-D) vector displacement of 41 landmarks placed by each observer at bronchial and vascular branch points in the lung between the EE and EI image sets (mean and 90% confidence interval displacements of 11.7 and 25.1 mm, respectively). The mean and 90% confidence interval discrepancy between model-predicted and observer-determined landmark displacements over all patients were 2.9 and 7.3 mm, whereas interobserver discrepancies were 2.8 and 6.0 mm. Paired t tests indicate no significant statistical differences between model predicted and observer drawn structures. We conclude that the accuracy of the algorithm to map lung anatomy in CT images at different respiratory phases is comparable to the variability in manual delineation. This method has therefore the potential for predicting and quantifying respiration-induced tumor motion in the lung.

Quantitation of respiratory motion during 4D-PET/CT acquisition

We report on the variability of the respiratory motion during 4D-PET/CT acquisition. The respiratory motion for five lung cancer patients was monitored by tracking external markers placed on the abdomen. CT data were acquired over an entire respiratory cycle at each couch position. The x-ray tube status was recorded by the tracking system, for retrospective sorting of the CT data as a function of respiration phase. Each respiratory cycle was sampled in ten equal bins. 4D-PET data were acquired in gated mode, where each breathing cycle was divided into ten 500 ms bins. For both CT and PET acquisition, patients received audio prompting to regularize breathing. The 4D-CT and 4D-PET data were then correlated according to their respiratory phases. The respiratory periods, and average amplitude within each phase bin, acquired in both modality sessions were then analyzed. The average respiratory motion period during 4D-CT was within 18% from that in the 4D-PET sessions. This would reflect up to 1.8% fluctuation in the duration of each 4D-CT bin. This small uncertainty enabled good correlation between CT and PET data, on a phase-to-phase basis. Comparison of the average-amplitude within the respiration trace, between 4D-CT and 4D- PET, on a bin-by-bin basis show a maximum deviation of approximately 15%. This study has proved the feasibility of performing 4D-PET/CT acquisition. Respiratory motion was in most cases consistent between PET and CT sessions, thereby improving both the attenuation correction of PET images, and co-registration of PET and CT images. On the other hand, in two patients, there was an increased partial irregularity in their breathing motion, which would prevent accurately correlating the corresponding PET and CT images.

Four-dimensional (4D) PET/CT imaging of the thorax

We have reported in our previous studies on the methodology, and feasibility of 4D-PET (Gated PET) acquisition, to reduce respiratory motion artifact in PET imaging of the thorax. In this study, we expand our investigation to address the problem of respiration motion in PET/CT imaging. The respiratory motion of four lung cancer patients were monitored by tracking external markers placed on the thorax. A 4D-CT acquisition was performed using a "step-and-shoot" technique, in which computed tomography (CT) projection data were acquired over a complete respiratory cycle at each couch position. The period of each CT acquisition segment was time stamped with an "x-ray ON" signal, which was recorded by the tracking system. 4D-CT data were then sorted into 10 groups, according to their corresponding phase of the breathing cycle. 4D-PET data were acquired in the gated mode, where each breathing cycle was divided into ten 0.5 s bins. For both CT and PET acquisitions, patients received audio prompting to regularize breathing. The 4D-CT and 4D-PET data were then correlated according to respiratory phase. The effect of 4D acquisition on improving the co-registration of PET and CT images, reducing motion smearing, and consequently increase the quantitation of the SUV, were investigated. Also, quantitation of the tumor motions in PET, and CT, were studied and compared. 4D-PET with matching phase 4D-CTAC showed an improved accuracy in PET-CT image co-registration of up to 41%, compared to measurements from 4D-PET with clinical-CTAC. Gating PET data in correlation with respiratory motion reduced motion-induced smearing, thereby decreasing the observed tumor volume, by as much as 43%. 4D-PET lesions volumes showed a maximum deviation of 19% between clinical CT and phase- matched 4D-CT attenuation corrected PET images. In CT, 4D acquisition resulted in increasing the tumor volume in two patients by up to 79%, and decreasing it in the other two by up to 35%. Consequently, these corrections have yielded an increase in the measured SUV by up to 16% over the clinical measured SUV, and 36% over SUV's measured in 4D-PET with clinical-CT Attenuation Correction (CTAC) SUV's. Quantitation of the maximum tumor motion amplitude, using 4D-PET and 4D-CT, showed up to 30% discrepancy between the two modalities. We have shown that 4D PET/CT is clinically a feasible method, to correct for respiratory motion artifacts in PET/CT imaging of the thorax. 4D PET/CT acquisition can reduce smearing, improve the accuracy in PET-CT co-registration, and increase the measured SUV. This should result in an improved tumor assessment for patients with lung malignancies.

A stereotactic method for the three‐dimensional registration of multi‐modality biologic images in animals: NMR, PET, histology, and autoradiography

The objective of this work was to develop and then validate a stereotactic fiduciary marker system for tumor xenografts in rodents which could be used to co-register magnetic resonance imaging (MRI), PET, tissue histology, autoradiography, and measurements from physiologic probes. A Teflon fiduciary template has been designed which allows the precise insertion of small hollow Teflon rods (0.71 mm diameter) into a tumor. These rods can be visualized by MRI and PET as well as by histology and autoradiography on tissue sections. The methodology has been applied and tested on a rigid phantom, on tissue phantom material, and finally on tumor bearing mice. Image registration has been performed between the MRI and PET images for the rigid Teflon phantom and among MRI, digitized microscopy images of tissue histology, and autoradiograms for both tissue phantom and tumor-bearing mice. A registration accuracy, expressed as the average Euclidean distance between the centers of three fiduciary markers among the registered image sets, of 0.2 +/- 0.06 mm was achieved between MRI and microPET image sets of a rigid Teflon phantom. The fiduciary template allows digitized tissue sections to be co-registered with three-dimensional MRI images with an average accuracy of 0.21 and 0.25 mm for the tissue phantoms and tumor xenografts, respectively. Between histology and autoradiograms, it was 0.19 and 0.21 mm for tissue phantoms and tumor xenografts, respectively. The fiduciary marker system provides a coordinate system with which to correlate information from multiple image types, on a voxel-by-voxel basis, with sub-millimeter accuracy--even among imaging modalities with widely disparate spatial resolution and in the absence of identifiable anatomic landmarks.

Effect of respiratory gating on reducing lung motion artifacts in PET imaging of lung cancer

Positron emission tomography (PET) has shown an increase in both sensitivity and specificity over computed tomography (CT) in lung cancer. However, motion artifacts in the 18F fluorodioxydoglucose (FDG) PET images caused by respiration persists to be an important factor in degrading PET image quality and quantification. Motion artifacts lead to two major effects: First, it affects the accuracy of quantitation, producing a reduction of the measured standard uptake value (SUV). Second, the apparent lesion volume is overestimated. Both impact upon the usage of PET images for radiation treatment planning. The first affects the visibility, or contrast, of the lesion. The second results in an increase in the planning target volume, and consequently a greater radiation dose to the normal tissues. One way to compensate for this effect is by applying a multiple-frame capture technique. The PET data are then acquired in synchronization with the respiratory motion. Reduction in smearing due to gating was investigated in both phantoms and patient studies. Phantom studies showed a dependence of the reduction in smearing on the lesion size, the motion amplitude, and the number of bins used for data acquisition. These studies also showed an improvement in the target-to-background ratio, and a more accurate measurement of the SUV. When applied to one patient, respiratory gating showed a 28% reduction in the total lesion volume, and a 56.5% increase in the SUV. This study was conducted as a proof of principle that a gating technique can effectively reduce motion artifacts in PET image acquisition.

PET imaging of (86)Y-labeled anti-Lewis Y monoclonal antibodies in a nude mouse model: comparison between (86)Y and (111)In radiolabels

Absorbed doses in (90)Y radioimmunotherapy are usually estimated by extrapolating from (111)In imaging data. PET using (86)Y (beta(+) 33%; half-life, 14.7 h) as a surrogate radiolabel could be a more accurate alternative. The aim of this study was to evaluate an (86)Y-labeled monoclonal antibody (mAb) as a PET imaging agent and to compare the biodistribution of (86)Y- and (111)In-labeled mAb.

METHODS

The humanized anti-Lewis Y mAb hu3S193 was labeled with (111)In or (86)Y through CHX-A"-diethylenetriaminepentaacetic acid chelation. In vitro cell binding and cellular retention of radiolabeled hu3S193 were evaluated using HCT-15 colon carcinoma cells, a cell line expressing Lewis Y. Nude mice bearing HCT-15 xenografts were injected with (86)Y-hu3S193 or (111)In-hu3S193. The biodistribution was studied by measurements of dissected tissues as well as by PET and planar imaging.

RESULTS

The overall radiochemical yield in hu3S193 labeling and purification was 42% +/- 2% (n = 2) and 76% +/- 3% (n = 6) for (86)Y and (111)In, respectively. Both radioimmunoconjugates specifically bound to HCT-15 cells. When cellular retention of hu3S193 was studied using (111)In-hu3S193, 80% of initially cell-bound (111)In activity was released into the medium as high-molecular-weight compounds within 8 h. When coadministered, in vivo tumor uptake of (86)Y-hu3S193 and (111)In-hu3S193 reached maximum values of 30 +/- 6 and 29 +/- 6 percentage injected dose per gram and tumor sites were easily identifiable by PET and planar imaging, respectively.

CONCLUSION

At 2 d after injection of (111)In-hu3S193 and (86)Y-hu3S193 radioimmunoconjugates, the uptake of (111)In and (86)Y activity was generally similar in most tissues. After 4 d, however, the concentration of (86)Y activity was significantly higher in several tissues, including tumor and bone tissue. Accordingly, the quantitative information offered by PET, combined with the presumably identical biodistribution of (86)Y and (90)Y radiolabels, should enable more accurate absorbed dose estimates in (90)Y radioimmunotherapy.

Relative therapeutic efficacy of (125)I- and (131)I-labeled monoclonal antibody A33 in a human colon cancer xenograft

A33, a monoclonal antibody that targets colon carcinomas, was labeled with (125)I or (131)I and the relative therapeutic efficacy of the 2 radiolabeled species was compared in a human colon cancer xenograft system.

METHODS

Nude mice bearing human SW1222 colon carcinoma xenografts were administered escalating activities of (125)I-A33 (9.25-148 MBq) or (131)I-A33 (0.925-18.5 MBq), (125)I- and (131)I-labeled control antibodies, unlabeled antibody, or no antibody. The effects of treatment were assessed using the endpoints of tumor growth delay and cure.

RESULTS

Tumor growth delay increased with administered activity for all radiolabeled antibodies. Approximately 4.5 times more activity was required for (125)I-A33 to produce therapeutic effects that were equivalent to those of (131)I-A33. This ratio was approximately 7 for a nonspecific, noninternalizing isotype-matched, radiolabeled control antibody. Unlabeled A33 antibody had no effect on tumor growth. Approximately 10 times more activity of (125)I-A33 produced toxicity similar to that of (131)I-A33, and this ratio fell to approximately 6 for radiolabeled control antibody.

CONCLUSION

Treatment with (125)I-A33 resulted in a relative therapeutic gain of approximately 2 compared with (131)I-A33 in this experimental system.

Estimate of absorbed dose based on two-dimensional autoradiographic information in internal radionuclide therapy

In radiation therapies using radionuclides emitting short-range particles, such as radioimmunotherapy or boron neutron capture therapy, the biological effects are strongly affected by the heterogeneity of the absorbed dose distribution delivered to tumor cells. The three-dimensional (3D) information of the source distribution at the cellular level is required to accurately determine the absorbed dose distribution to the individual tumor cells. Two-dimensional distribution of cell and nuclide with a resolution of 1 microm can be obtained from individual tissue sections by microautoradiography. To obtain such information in 3D, an ideal approach would be to align the serial tissue sections from a block and analyze all of them. This is straightforward in theory, but extremely difficult in practice. Furthermore, every section in the block has to be processed and analyzed, and the usage of the data from this laborious work is very inefficient. An approach presented here is to estimate the absorbed dose based on individual sections without 3D reconstruction. It is realistically workable since it avoids the most difficult task of alignment for the serial tissue sections. In addition, the absorbed dose can be estimated based on a limited number of noncontiguous sections. The validity of this approach has been tested by a Monte Carlo simulation for two representative radionuclide configurations: (a) a uniform distribution of sources and (b) a cell membrane bound source distribution. With only a limited number of sampling sections, the uncertainties in the dose estimation were estimated to approximately 15% for short-range particles.

Hypoxia-Induced increase in FDG uptake in MCF7 cells

Recent clinical data indicate that tumor hypoxia negatively affects the treatment outcome of both radiotherapy and surgery in various cancers, emphasizing the need for noninvasive detection of tumor hypoxia. Several studies have shown an increased uptake of FDG in hypoxic regions of xenografts, suggesting the use of PET with FDG as a potential technique. In this study, we examine the mechanism underlying the hypoxia-induced increase of FDG uptake in the human breast carcinoma cell line MCF7.

METHODS

The uptake of 3H-FDG into MCF7 cells was determined after incubation under hypoxic (0% oxygen) or normoxic conditions, with or without redox agents, for varying time periods. In addition, the effects of the redox agents on the glucose transporter activity and the hexokinase activity were determined independently, and the effects of hypoxia on glucose transporter protein and hexokinase levels were assessed.

RESULTS

A more than twofold increase (2.53 +/- 0.79; P < 0.005) in 3H-FDG uptake was observed under hypoxic conditions, but no changes in the cellular levels of glucose transporter proteins or hexokinase were observed. A reducing agent, dithiothreitol (DTT), also caused an increase in 3H-FDG uptake but failed to affect uptake under hypoxic conditions. This indicates that the mechanisms by which hypoxia and DTT affect 3H-FDG uptake might be the same. The oxidizing agent p-chloromercuribenzenesulfonic acid (pCMBS) had no effect on 3H-FDG uptake under normoxic conditions but counteracted the effect of hypoxia. DTT caused an increase in glucose transporter activity, whereas it had no effect on hexokinase activity. pCMBS had no effect on either glucose transporter activity or hexokinase activity.

CONCLUSION

The hypoxia-induced increase in 3H-FDG uptake in MCF7 cells is the result, in part, of an increase in glucose transporter activity resulting from the modification (reduction) of thiol group(s) in the glucose transport protein(s). Modulation of hexokinase activity is probably not involved in the hypoxia-induced increase in 3H-FDG uptake in these cells.

Targeted radioimmunotherapy for leptomeningeal cancer using (131)I-3F8

BACKGROUND

Intrathecal antibody-based targeted therapies may have clinical potential for patients with leptomeningeal (LM) cancer.

PROCEDURE

Five patients with GD2-positive LM tumors were injected with 1-2 mCi intra-Ommaya (131)I-3F8, a murine IgG3 antibody specific for GD2. Serial cerebrospinal fluid (CSF) and serum samples and SPECT imagings (4, 24, and 48 hr) were performed to predict radiation doses to the tumor and normal brain and blood prior to the administration of larger therapeutic doses.

RESULTS

Side effects included self-limited fever, headache, and vomiting. Focal (131)I-3F8 uptake consistent with tumors was seen along the craniospinal axis in four patients. Calculated radiation dose to the CSF was 14.9-56 cGy/mCi and to blood and other organs outside the CNS less than 2 cGy/mCi.

CONCLUSIONS

Intraventricular (131)I-3F8 successfully detected LM disease and resulted in a large favorable CSF/blood ratio. Intraventricular (131)I-3F8 may have clinical utility in the diagnosis and radioimmunotherapy of GD2-positive LM cancers. Med. Pediatr. Oncol. 35:716-718. 2000.

Optimizing the sequence of combination therapy with radiolabeled antibodies and fractionated external beam

The purpose of this study was to determine the optimum sequence for combined modality therapy with radiolabeled antibodies and fractionated external beam radiation.

METHODS

The uptake and distribution of a nontherapeutic activity of 125I-labeled tumor-associated A33 monoclonal antibody was determined in SW1222 human colon carcinoma xenografts in nude mice for 4 study groups: group 1, radiolabeled antibody alone; group 2, radiolabeled antibody administered (day 0) immediately before the first of 5 daily fractions of 2-Gy, 320-kilovolt peak x-rays; group 3, radiolabeled antibody administered after the fifth radiation fraction (day 5); and group 4, radiolabeled antibody administered 5 d after irradiation (day 10). Tumors were excised 5 d after antibody administration. Tumors were frozen and sectioned for histology and phosphor plate autoradiography. The percentage of A33 antigen-expressing cells was estimated by immunohistochemical staining.

RESULTS

The average tumor uptake values relative to control group 1 were 1.47 (group 2), 0.78 (group 3), and 0.21 (group 4), which illustrates that tumor uptake is increased by almost 50% when the antibody is present in the blood at the start of irradiation. Five days into a fractionated irradiation protocol, antibody uptake was reduced, falling more significantly on day 10. Phosphor plate autoradiographs showed decreased uptake uniformity for groups 3 and 4. Immunohistochemical data showed a reduction in A33 antigen-positive cells from 85%, 64%, 50%, to 41% for groups 1-4, respectively.

CONCLUSION

Maximum radiolabeled antibody tumor uptake was achieved when the antibody was administered just before radiation therapy. This might be explained by a transient increase in capillary leakage to macromolecules, followed by a reduction at later times, possibly the result of capillary damage and occlusion.

Use of PET to monitor the response of lung cancer to radiation treatment

Approximately 170,000 people are diagnosed with lung cancer in the United States each year. Many of these patients receive external beam radiation for treatment. Fluorine-18 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG PET) is increasingly being used in evaluating non-small cell lung cancer and may be of clinical utility in assessing response to treatment. In this report, we present FDG PET images and data from two patients who were followed with a total of eight and seven serial FDG PET scans, respectively, through the entire course of their radiation therapy. Changes in several potential response parameters are shown versus time, including lesion volume (V(FDG)) by PET, SUVav, SUVmax, and total lesion glycolysis (TLG) during the course of radiotherapy. The response parameters for patient 1 demonstrated a progressive decrease; however, the response parameters for patient 2 showed an initial decrease followed by an increase. The data presented here may suggest that the outcome of radiation therapy can be predicted by PET imaging, but this observation requires a study of additional patients.

Radioiodine uptake in thyroid remnants during therapy after tracer dosimetry

Our objective was to evaluate the effect of a diagnostic tracer dose of 131I on the uptake of the therapeutic dose of 1311 in the ablation of a thyroid remnant or residual tumor in patients with differentiated thyroid cancer.

METHODS

Twelve consecutive patients referred for a dosimetric study and subsequent radioiodine treatment of focal neck uptake of 131I were studied. The 24-h (in 1 case, 48-h) neck activity was calculated by the region-of-interest method, after both dosimetric and therapeutic administrations. The focal activity in the neck was corrected for decay and compared with the total activity administered to obtain the percentage uptake at 24 h. This procedure was performed for both the scanning dose (range, 19.8-196.1 MBq; mean, 85.1 MBq; median, 40 MBq) and the therapeutic dose (range, 1.073-5.713 GBq; mean, 2.991 GBq). The uptake of the therapeutic dose was then expressed as a percentage of the uptake of the diagnostic dose (%T/D). Counting rate linearity was established up to 350 MBq in the field of view of the gamma camera used in the study.

RESULTS

Thirteen of a total of 16 lesions exhibited reduced uptake from the therapeutic dose, 2 remained the same, and in 1 the uptake actually increased from 0.26% to 1.01%. The %T/D ranged from 7.0% to 388.5%, with a mean of 71%. If the lesion with increased uptake is excluded, the range becomes 7.0%-102.1%, with a mean of 50%. Linear regression between the percentage uptake of the diagnostic dose to that of the therapeutic dose results in a slope of 0.42, with a correlation coefficient of only 0.75. We were unable to accurately calculate the radiation dose to the lesion from the diagnostic activity of 131I, because of uncertainty about the tumor mass.

CONCLUSION

The percentage uptake of the therapeutic dose is on average only one half of that predicted from the dosimetric uptake in thyroid remnants after surgery, even though the median dosimetric dose was only 40 MBq. This reduced uptake should be accounted for in the therapeutic prescription for thyroid ablation or treatment of residual thyroid cancer. We postulate that this effect is caused by radiation damage from the tracer dose during dosimetry.

Evaluation of 11C-colchicine for PET imaging of multiple drug resistance

Overexpression of P-glycoprotein (P-gp) can confer multiple drug resistance (MDR) phenotype on cancer cells and tumors by reducing intracellular accumulation of various cytotoxic agents. Early diagnosis of MDR in the clinic will serve to improve the efficacy of chemotherapeutic intervention and the quality of life of patients. In this article we describe use of a positron-emitting MDR tracer, 11C-colchicine (CHC), to evaluate MDR by PET imaging. Unlike existing MDR tracers such as 99mTc-sestamibi, this compound is electroneutral, with biodistribution not affected by perturbations of membrane potential.

METHODS

In vitro studies showed that resistance to CHC is correlated to resistance to Taxol (paclitaxel). The results of biodistribution experiments were found to be consistent with previously reported experiments with CHC labeled with other isotopes. On the basis of in vitro experiments with a series of drug-resistant variants of the human neuroblastoma BE (2)-C cell line, a mathematic model of 11C-CHC distribution in tumors was formulated. Dynamic PET 11C-CHC imaging experiments were performed with nude rats xenografted with the BE (2)-C-sensitive and -resistant strains. Each scan was accompanied by a transmissions scan and a static FDG scan. These scans allowed improved image localization.

RESULTS

We observed an approximately 2-fold difference between 11C-CHC accumulation in sensitive and resistant tumors. Imaging data were analyzed using the mathematic model, and various parameters characterizing resistance could be identified and estimated. In particular, the parameter r, proportional to the level of resistance of the tumors, was obtained. We showed that the ratio of these r parameters determined from the sensitive and resistant tumors was identical to the ratio of CHC accumulation in the corresponding sensitive and resistant cell lines used for xenografting.

CONCLUSION

These in vivo experiments provided additional evidence for the indirect effect of P-gp action on CHC-to-tubulin binding, which in turn determines CHC uptake in tumors. The significance of these findings and future plans is discussed.

Single-dose versus fractionated radioimmunotherapy: model comparisons for uniform tumor dosimetry

Targeting molecules with reduced immunogenicity will enable repetitive administrations of radioimmunotherapy. In this work a mathematical model was used to compare 2 different treatment strategies: large single administrations (LSAs) and rapid fractionation (RF) of small individual administrations separated by short time intervals.

METHODS

An integrated compartmental model of treatment pharmacokinetics and tumor response was used to compare alternative treatments that delivered identical absorbed doses to red marrow.

RESULTS

Based on the key assumption of uniform dose distributions, the LSA approach consistently produced smaller nadir values of tumor cell survival and tumor size. The predicted duration of remission was similar for both treatment structures. These findings held for both macroscopic and microscopic tumors and were independent of tumor cell radiosensitivity, proliferation rate, rate of tumor shrinkage, and uptake characteristics of radiolabeled material in tumor.

CONCLUSION

Clinical situations for which each treatment is most appropriate may be tentatively identified. An LSA using a short-range-emitting radionuclide would be most appropriate for therapy of microscopic disease, if uptake is relatively homogeneous. RF using a longer range emitter would be most appropriate for macroscopic disease, if uptake is heterogeneous and varies from one administration to another. There is a rationale for combining LSA and RF treatments in clinical situations in which slowly growing macroscopic disease and rapidly growing microscopic disease exist simultaneously.

Cortical injury impairs contralateral forelimb immobility during swimming: a simple test for loss of inhibitory motor control

Most animal models of focal injury to the sensorimotor cortex have been aimed at detecting non-use or impairment of the limbs in specific tasks or during spontaneous exploratory behaviors. However, the inability to hold a limb still can be an equally disabling movement disorder. The present study investigated the loss of control of limb immobility that occurs following damage to the forelimb region of the rat sensorimotor cortex (FL-SMC). When swimming forward in a tank of water, adult rats typically hold both forepaws mostly motionless underneath the chin, using primarily the hindlimbs for stroking movements. Following a unilateral FL-SMC lesion, rats hold only the non-impaired forelimb immobile under the chin, and make 'immature' stroking movements with the impaired forelimb. We have devised a simple means of assessing and quantifying this deficit. While the criterion for most tests of motor recovery involves appropriate movement of an impaired limb, this test depends on adequate inhibition of movement as the norm, and may be a useful way to assess the loss of inhibitory motor control and the efficacy of potential restorative interventions.

Pharmacokinetics and dosimetry of an alpha-particle emitter labeled antibody: 213Bi-HuM195 (anti-CD33) in patients with leukemia

Data from nine patients with leukemia participating in a phase I activity-escalation study of HuM195, labeled with the alpha-particle emitter 213Bi (half-life = 45.6 min), were used to estimate pharmacokinetics and dosimetry. This is the first trial using an alpha-particle emitter in humans. The linear energy transfer of alpha particles is several hundredfold greater than that of beta emissions. The range in tissue is approximately 60-90 microm.

METHODS

The activity administered to patients ranged from 0.6 to 1.6 GBq. Patient imaging was initiated at the start of each injection. Thirty 1-min images followed by ten 3-min images were collected in dynamic mode; a 20% photopeak window centered at 440 keV was used. Blood samples were collected until 3 h postinjection and counted in a gamma counter. Contours around the liver and spleen were drawn on the anterior and posterior views and around a portion of the spine on the posterior views. No other organs were visualized.

RESULTS

The percentage injected dose in the liver and spleen volumes increased rapidly over the first 10-15 min to a constant value for the remaining hour of imaging, yielding a very rapid uptake followed by a plateau in the antibody uptake curves. The kinetic curves were integrated to yield cumulated activity. The mean energy emitted per nuclear transition for 213Bi and its daughters, adjusted by a relative biologic effectiveness of 5 for alpha emissions, was multiplied by the cumulated activity to yield the absorbed dose equivalent. Photon dose to the total body was determined by calculating a photon-absorbed fraction. The absorbed dose equivalent to liver and spleen volumes ranged from 2.4 to 11.2 and 2.9 to 21.9 Sv, respectively. Marrow (or leukemia) mean dose ranged from 6.6 to 12.2 Sv. The total-body dose (photons only) ranged from 2.2 x 10(-4) to 5.8 x 10(-4) Gy.

CONCLUSION

This study shows that patient imaging of 213Bi, an alpha-particle emitter, labeled to HuM195 is possible and may be used to derive pharmacokinetics and dosimetry. The absorbed dose ratio between marrow, liver and spleen volumes and the whole body for 213Bi-HuM195 is 1000-fold greater than that commonly observed with beta-emitting radionuclides used for radioimmunotherapy.

131I radioimmunotherapy and fractionated external beam radiotherapy: comparative effectiveness in a human tumor xenograft

This article compares the effectiveness of radiation delivered by a radiolabeled monoclonal antibody, 131I-labeled A33, that targets colorectal carcinoma, with that of 10 fractions of conventional 320 kVp x-rays.

METHODS

Human colorectal cancer xenografts (SW1222) ranging between 0.14 and 0.84 g were grown in nude mice. These were treated either with escalating activities (3.7-18.5 MBq) of 131I-labeled A33 or 10 fractions of 320 kVp x-rays (fraction sizes from 1.5 to 5 Gy). Tumor dosimetry was determined from a similar group of tumor-bearing animals by serial kill, tumor resection and counting of radioactivity in a gamma counter. The relative effectiveness of the two radiation therapy treatment approaches was compared in terms of tumor regrowth delay and probability of tumor cure.

RESULTS

The absorbed dose to tumor per MBq administered was estimated as 3.7 Gy (+/-1 Gy; 95% confidence interval). We observed a close to linear increase in tumor regrowth delay with escalating administered activity. Equitumor response of 1311 monoclonal antibody A33 was observed at average radiation doses to the tumor three times greater than when delivered by fractionated external beam radiotherapy. The relationship between the likelihood of tumor cure and administered activity was less predictable than that for regrowth delay.

CONCLUSION

The relative effectiveness per unit dose of radiation therapy delivered by 131I-labeled A33 monoclonal antibodies was approximately one third of that produced by fractionated external beam radiotherapy, when measured by tumor regrowth delay.

Metabolic activation of the subependymal zone after cortical injury

The subependymal zone (SEZ) of the adult mammalian forebrain contains a population of progenitor cells that proliferate in response to brain injury. This study examined the effect of cortical injury on metabolic activity in the SEZ using quantitative histochemistry of cytochrome oxidase. The SEZ showed significantly enhanced cytochrome oxidase activity in rats with electrolytic cortical injuries relative to sham-operated controls, while other brain regions showed no such changes. The results indicate that the SEZ had increased oxidative energy demands, and thus provide metabolic evidence that SEZ cells are activated in response to brain injury.

Use-dependent exaggeration of brain injury: is glutamate involved?

Extreme overreliance on the impaired forelimb following unilateral lesions of the forelimb representation area of the rat sensorimotor cortex (FL-SMC) leads to exaggeration of the initial cortical injury. Glutamate has repeatedly been implicated in the secondary processes leading to neuronal death following traumatic insult, chiefly because of the neuroprotective properties of excitatory amino acid antagonists in a variety of animal models of brain injury. The present study investigated the possibility that NMDA receptor-mediated processes are involved in use-dependent exaggeration of neuronal injury. Rats were fitted with one-sleeved casts that immobilized the intact forelimb for the first 7 days following FL-SMC lesion, a procedure previously shown to result in use-dependent exaggeration of injury and more severe and persistent limb-use deficits. In the present investigation, administration of MK-801 (1 mg/kg ip once daily on alternate days) during the casting period spared neural tissue surrounding the lesion and enhanced functional recovery of the impaired forelimb. These results suggest a role for NMDA receptor-mediated processes in use-dependent exaggeration of injury.

Thyroid cancer dosimetry using clearance fitting

Since 1962, Memorial Sloan Kettering Cancer Center has used an individually optimized dosimetry method for patients with thyroid carcinoma undergoing radioiodine therapy. This traditional dosimetry method involves a determination of the maximum tolerated activity or the activity that will deliver 2 Gy to the blood (A(max)), and the corresponding ablative lesion dose (D(lesion)). However, the traditional calculations of A(max) and D(lesion) were based on empirical assumptions. The objective of this work was to develop a dosimetry method that eliminates these assumptions by incorporating patient kinetics and that is not restricted to 131I as a tracer and therapeutic agent.

METHODS

Patient kinetics were incorporated into the dosimetry algorithm by fitting parameters to patient clearance measurements. The radioiodines 123I, 124I, 125I and 131I were accommodated as tracers and therapeutic agents by incorporating their physical half lives and by precalculating photon-absorbed fractions for these radionuclides for several thousand patient geometries using Monte Carlo simulations.

RESULTS

A(max) and D(lesion) have been calculated using the traditional and new method for a group of patients, and errors associated with each of the above assumptions were examined. Assuming that the initial blood activity is distributed instantaneously in 5 L was found to introduce an error in A(max) of up to 30%, whereas assuming physical decay beyond the last data point introduced an error of up to 50%.

CONCLUSION

Individualized fitting of clearance data is a practical method to accurately account for inter-patient kinetics variations. The substitution of standard kinetics beyond measured data might lead to substantial errors in estimating A(max) and D(lesion). In addition, gamma camera images, rather than neck probe readings, should be used to determine lesion uptakes for thyroid cancer patients.

Radioimmunotherapy with alpha-emitting nuclides

This review discusses the application of alpha particle-emitting radionuclides in targeted radioimmunotherapy. It will outline the production and chemistry of astatine-211, bismuth-212, lead-212, actinium-225, bismuth-213, fermium-255, radium-223 and terbium-149, which at present are the most promising alpha-emitting isotopes available for human clinical use. The selective cytotoxicity offered by alpha particle-emitting radioimmunoconstructs is due to the high linear energy transfer and short particle path length of these radionuclides. Based upon the pharmacokinetics of alpha particle-emitting radioimmunoconstructs, both stochastic and conventional dosimetric methodology is discussed, as is the preclinical and initial clinical use of these radionuclides conjugated to monoclonal antibodies for the treatment of human neoplasia.

Pharmacokinetic model of iodine-131-G250 antibody in renal cell carcinoma patients

A model that describes the pharmacokinetic distribution of 131I-labeled G250 antibody is developed.

METHODS

Previously collected pharmacokinetic data from a Phase I-II study of 131I-G250 murine antibody against renal cell carcinoma were used to develop a mathematical model describing antibody clearance from serum and the whole body. Survey meter measurements, obtained while the patient was under radiation precautions, and imaging data, obtained at later times, were combined to evaluate whole-body clearance kinetics over an extended period.

RESULTS

A linear two-compartment model was found to provide good fits to the data. The antibody was injected into Compartment 1, the initial distribution volume (Vd) of the antibody, which included serum. The antibody exchanged with the rest of the body, Compartment 2, and was eventually excreted. Data from 13 of the 16 patients fit the model with unique parameters; the maximum, median and minimum values for model-derived Vd were 6.3, 3.7 and 2.11, respectively. The maximum, median and minimum values for the excretion rate were 8 x 10(-2), 2.4 x 10(-2) and 1.3 x 10(-2) hr(-1), respectively. Parameter sensitivity analysis showed that a change in the transfer rate constant from serum to the rest of the body had the greatest effect on serum cumulative activity and that the rate constant for excretion had the greatest effect on whole-body cumulative activity.

CONCLUSION

A linear two-compartment model was adequate in describing the serum and whole-body kinetics of G250 antibody distribution. The median initial distribution volume predicted by the model was consistent with the nominal value of 3.81. A wide variability in fitted parameters was observed among patients, reflecting the differences in individual patient clearance and exchange kinetics of G250 antibody. By selecting median parameter values, such a model may be used to evaluate and design prolonged multiple administration radioimmunotherapy protocols.

Use-dependent exacerbation of brain damage occurs during an early post-lesion vulnerable period

For a period of time after unilateral brain injury, surviving neural tissue surrounding the lesion may be vulnerable to extremely high behavioral demand. Previously, we found that lesions of the forelimb representation area of the sensorimotor cortex (FL-SMC) in rats increase in size substantially when the intact forelimb is immobilized with a plaster of paris cast during the first 15 days after surgery, which forces overuse of the impaired forelimb. The present study was designed to determine whether the adult brain is more vulnerable to forced overuse of the impaired forelimb during the first 7 days post-lesion than during the second 7 days post-lesion. Using behavioral tests of forelimb use and stereological analysis of remaining tissue volume 40 days after FL-SMC lesions, we found that forced overuse of the impaired forelimb during the first 7 days after the initial damage caused expansion of neural injury and greatly interfered with restoration of function. In contrast, forced overuse of the impaired forelimb during the second 7 days had no significant effect on lesion size but nevertheless interfered with restoration of function. Thus, surviving neural tissue in the damaged hemisphere and recovery of function appear to be vulnerable to prolonged forced overuse of the impaired forelimb throughout the first 15 days, but tissue loss was detectable only when the animal was forced to use the impaired forelimb during the first 7 days after injury.

Electron wedges for radiation therapy

PURPOSE

Brain tumors can be advantageously treated with electron over photon radiation, by exploiting the rapid fall-off in dose with depth. This advantage could be further enhanced by utilizing multiple electron beams. However, in some beam configurations, wedged dose profiles would be necessary for the dose uniformity. Unlike photons, shaped pieces of material placed in electron beam severely degrade the energy, give additional scattering and, therefore, are suboptimal. The purpose of this study was to create wedged electron fields, using intensity modulation. The combination of electron wedges enables a more uniform coverage of brain tumors with a reduced dose to normal tissue.

METHODS AND MATERIALS

Intensity modulation was performed for 10 to 50 MeV electrons using a narrow scanning elementary beam of a racetrack Microtron accelerator, delivering radiation pulses with coordinates and intensities prescribed by a custom scan matrix. Dispensing more pulses (or longer pulses) within the field to increase the local dose, one can sharpen the penumbra at depth and generate wedged dose distributions of arbitrary angle as well as many other desired profiles. We modulated the electron beams, measured dose distributions using film in an anthropomorphic phantom, and compared the results with conventional techniques.

RESULTS

Intensity modulation of electron beams decreases the 50-90% penumbra at depth by 40% and increases the flatness by 80%. Wedged profiles at depth can be created for any angle up to about 70 degrees, depending on the beam energy. Multiple modulated electron beams give smaller 20-70% but larger 70-100% isodose regions than photon beams.

CONCLUSIONS

Electron beams can improve dose distributions in brain compared to the same number of photon beams, reducing the 20-70% isodoses region in normal tissue by 30%. Intensity modulation significantly improves the dose distribution from combined electron beams providing a sharper penumbra, better conformity, and reduced margin.

Segmentation of lung lesion volume by adaptive positron emission tomography image thresholding

BACKGROUND

It is common protocol in radionuclide therapies to administer a tracer dose of a radiopharmaceutical, determine its lesion uptake and biodistribution by gamma imaging, and then use this information to determine the most effective therapeutic dose. This treatment planning approach can be used to quantitate accurately the activity and volume of lesions and organs with positron emission tomography (PET). In this article, the authors focus on the specification of appropriate volumes of interest (VoI) using PET in association with computed tomography (CT).

METHODS

The authors have developed an automatic image segmentation schema to determine the VoI of metastases to the lung from PET images, under conditions of variable background activity. An elliptical Jaszczak phantom containing a set of spheres with volumes ranging from 0.4 to 5.5 mL was filled with F-18 activity (2-3 microCi/mL) corresponding to activities clinically observed in lung lesions. Images were acquired with a cold background and then with variable source-to-background (S/B) ratios of: 7.4, 5.5, 3.1, and 2.8. Lesion VoI analysis was performed on 10 patients with 17 primary or metastatic lung lesions, applying the optimum threshold values derived from the phantom experiments. Initial volume estimates for lung lesions were determined from CT images. Approximate S/B ratios were obtained for the corresponding lesions on F-18-fluoro-2-deoxy-D-glucose (18FDG)-PET images. From the CT estimate of the lesion size and the PET estimate of the S/B ratio, the appropriate optimum threshold could be chosen. The threshold was applied to the PET images to obtain lesion activity and a final estimate of the lesion volume.

RESULTS

Phantom data analysis showed that image segmentation converged to a fixed threshold value (from 36% to 44%) for sphere volumes larger than 4 mL, with the exact value depending on the S/B ratios. For patients, the use of optimum threshold schema demonstrated a good correlation (r = 0.999) between the initial volume from CT and the final volume derived from the 18FDG-PET scan (P < 0.02). The mean difference for those volumes was 8.4%.

CONCLUSIONS

The adaptive thresholding method applied to PET scans enables the definition of tumor VoI, which hopefully leads to accurate tumor dosimetry. This method can also be applied to small lesions (<4 mL). It should enable physicians to track objectively changes in disease status that could otherwise be obscured by the uncertainties in the region-of-interest drawing, even when the scans are delineated by the same physician.

Preoptic area infusions of morphine disrupt--and naloxone restores--parental-like behavior in juvenile rats

As in the adult lactating female, opioids disrupt (and naloxone restores), parental behavior in juvenile rats (approximately 25 days of age). Because the preoptic area regulates the display of parental behavior in lactating females, we examined its parental behavior role in the juvenile rat. At 21 days of age, juvenile rats were implanted with bilateral cannulae aimed at the preoptic area using a modified Kopf stereotaxic and extrapolating from a developing-rat brain atlas [58], and divided into two groups: Initiation and maintenance. On day 25, the initiation group received bilateral infusions of either morphine (0.50 microgram), saline (0.25 microliter), or morphine plus naloxone (0.25 microgram). Thirty minutes later, they were exposed to three 1-6-day-old pups; the maintenance group was exposed to pups until they displayed 2 consecutive days of parental behavior, then infused. Morphine disrupted parental behavior in both the initiation and Maintenance groups, and naloxone restored the behavior to control/ saline levels. Parental behavior in the juvenile animal of both sexes, therefore, is under opioid regulation that parallels the adult female.

Quantitative bone metastases analysis based on image segmentation

Preliminary evidence indicates that the fraction of bone containing metastatic lesions is a strong prognostic indicator of survival longevity for prostate and breast cancer. Our current approach to quantify metastatic bone lesions, called the Bone Scan Index, is based on an inspection of the bone scan, estimating visually the fraction of each bone involved and then summing across all bones to determine the percentage of total skeletal involvement. This approach, however, is time consuming, subjective and dependent on individual interpretation.

METHODS

To overcome these problems, a semiautomated image segmentation program was developed for the quantitation of metastases from planar whole-body bone scans. The user is required to insert a seed point into each metastatic region on the image. The algorithm then connects pixels to the seed pixel in all directions until a contrast-dependent threshold is reached. The optimal threshold for cessation of the region growing is determined from phantom studies. On the images, lesion delineation and size measurements were performed by the algorithm. Each delineated lesion is associated with a bone site using pull-down menus. The program then computes the fraction of lesion involvement in each bone based on look-up-tables containing the relationship of bone mass with race, sex, height and age. These look-up-tables were obtained by multiple regression of the skeletal mass measurements in humans. The total fraction of skeletal involvement is then obtained from the individual fractional masses. For individual fractional mass, values given in International Commission on Radiation Protection Publication No. 23 were used.

RESULTS

The bone metastases analysis system has been used on 11 scans from 6 patients. The correlation was high (r = 0.83) between conventional (manually drawn region-of-interest) and this analysis system. Bone metastases analysis results in consistently lower estimates of fractional involvement in bone compared with the conventional region-of-interest drawing or visual estimation method. This is due to the apparent broadening of objects at and below the limits of resolution of the gamma camera.

CONCLUSION

Image segmentation reduces the delineation and quantitation time of lesions by at least two compared with manual region-of-interest drawing. The objectivity of this technique allows the detection of small variations in follow-up patient scans for which the manual region-of-interest method may fail, due to performance variability of the user. This method preserves the diagnostic skills of the nuclear medicine physician to select which bony structures contain lesions, yet combines it with an objective delineation of the lesion.

Distribution of radiolabeled monoclonal antibody MX35 F(ab')2 in tissue samples by storage phosphor screen image analysis: evaluation of antibody localization to micrometastatic disease in epithelial ovarian cancer

Our objective was to quantify the targeting of the monoclonal antibody (mAb) MX35 F(ab')2 to micrometastatic epithelial ovarian cancer. This mAb detects a Mr 95,000 glycoprotein with homogeneous distribution on 80% of ovarian tumor specimens. Six patients with minimal residual disease from an imaging trial were injected with 2 or 10 mg of 131I- and 125I-labeled mAb MX35 F(ab')2. Biopsied samples were removed at second-look laparotomy 1-5 days post-i.v. or -i.p. infusion of antibody. Serial cryostat sections were stained by indirect immunoperoxidase method for antigen distribution and exposed to storage phosphor screens for quantitative autoradiography. Coregistration of tumor histology, antigen expression, and radionuclide distribution demonstrated specific localization in micrometastatic tumor foci (50 micrometer to 1 mm) found within tissue stroma. The radiolabeled antibody uptake determined by well scintillation counts ranged between 5.2 and 223.5 x 10(-4) percentage of injected dose/g of tumor tissue for 131I. Specific localization of mAb in tumor was determined by tumor:normal tissue (fat) ratios ranging from 0.9:1 to 35.9:1 for 131I. The high resolution and linear response of the storage phosphor screen imager was used to estimate the radionuclide activity localized in each micrometastatic site. Quantitation of phosphor screen response revealed microCi/g values of 0.026-0.341 for normal tissue and 0.184-6.092 for tumor biopsies, evaluated 4 or 5 days post-antibody injection. The tumor:normal tissue (adjacent to tumor) ratios were between 1 and 4 times greater using the phosphor screen method than well counter measurements, but even larger variations of ratios up to 20:1 were observed between tumor cell foci and stromal cells within the same tissue section. This study has demonstrated that mAb MX35 F(ab')2 localizes to the micrometastatic ovarian carcinoma deposits within the peritoneal cavity. The dosimetry results suggest a therapeutic potential for this antibody in patients with minimal residual disease (<5 mm).

Pharmacokinetics and dosimetry of iodine-125-IUdR in the treatment of colorectal cancer metastatic to liver

The radiotoxicity of 125I is highly sensitive to the site of decay relative to nuclear DNA. This paper describes a new approach, based upon pharmacokinetic clearance of radioactivity from the tumor, with which to quantify the fraction of [125I]IUdR incorporated within the DNA of tumor cells.

METHODS

Patients were injected with [125I]IUdR through the hepatic artery. Iodine-131-IUdR was used as a tracer for imaging and quantitation. Both conventional and DNA-level dosimetry were performed.

RESULTS

We calculated that if 15% of the tumor cells were in S phase at the time of injection, there would be 250 decays of 125I in the DNA per tumor cell after an infusion of 5 mCi [125I]IUdR. According to in vitro data based on 5 x 10(8) cells per g tumor, 99% of these cells in S phase would be killed.

CONCLUSION

The estimate of cell inactivation is strongly dependent on the number of cells per gram and the fraction of cells in S phase at the time of injection, which indicates that repeat injections would be necessary to achieve a therapeutic effect.

Dosimetric properties of a scanned beam microtron at low monitor unit settings: importance for conformal therapy

The dosimetric stability, linearity, dose rate dependence, and flatness of both photon and electron beams have been evaluated for a racetrack microtron at low monitor unit settings. For photons, the variation in dosimetric output about the mean is < 0.4% at 100 monitor units (MU), < 1% at 10 MU, and < 4% at 2 MU. The output dependence on the dose rate varied by < 0.6% between 85-300 MU/min. Flatness and symmetry for the 25- and 50-MV beams showed deviations of < 3% at both dmax and 10-cm depths, and only slightly > 3% at 20 cm, even at only 3 MU, in contrast with other scanned beam accelerators. Broad electron beams on the microtron are created by the superposition of the elementary beam pulses either directly from the scan magnets, or after their broadening through a scattering foil. The dosimetric instability both with and without the foil was less than 0.6% for both the 25- and 50-MeV electrons. Dose nonlinearity was < 1% above 10 MU. Field flatness was determined for scan matrices designed to produce a flat field both with and without a scattering foil. Symmetry and flatness deviations were < 3% for both electron energies when a scattering foil was used, even for a single scan. The variation of the electron dose per monitor unit between dose rates of 85-300 MU/min was < 1% (25 MeV) and < 4% (50 MeV) when a scattering foil was used, but as high as 22% (25 MeV) and 36% (50 MeV) for broad beams generated by elementary beam pulses directly from the scanning magnets. The microtron exhibits dosimetric properties which fulfill the recommendations of Task Groups 21 and 25. Based on the stability of the scanned beam at low monitor unit settings, the microtron can be used for 3-D conformal therapy with both photons and electrons.

Electron dose profile shaping by modulation of a scanning elementary beam

The use of multiple high energy electron beams has been limited in the treatment of deep seated tumors. This is principally because of the rapid increase in the physical electron beam penumbra as a result of the rise in large angle scattering with depth in the patient. This decreases the transverse dose gradient between the target volume and sensitive dose limiting structures and diminishes the ability to conform electron isodose lines to the target volume. If the beam is flat in air, then its profile will become progressively more rounded with depth, due to the increase of scatter out of the beam edges. With a scanning elementary electron beam, such as produced by the Microtron MM50, the characteristics of a broad beam profile are determined by the scan pattern. Using an appropriate scan pattern one can create, at any depth within the range of electrons, various dose profiles with the sharpness not exceeding that of the elementary beam. The objective of this work was to study methods that produce the desired electron beam profile at the depth of the target volume, and to derive the surface fluence profile required. Two approaches were explored to modulate the elementary beam distribution: "amplitude modulation" (AM) and "frequency modulation" (FM). We calculated coordinate and intensity distributions of the 25, 40, and 50 MeV elementary beam pulses at the surface that would yield a flat field at various prescribed depths. The results are in good qualitative agreement with iterative deconvolution calculations by Brahme et al. [Acta Radiol. Oncol. 19, 305-319 (1980)]. The scattering penumbra between the 50%-90% isodose lines can be reduced by up to 40% by beam modulation. The modulation should also enable the combination of multiple electron beams so as to achieve the desired conformal isodose profile as is customarily seen with photon beam planning, but with greater normal tissue sparing due to higher electron longitudinal depth--dose gradients. The results can be also used for electron accelerators that do not use a scanning elementary beam.

Collision detection and avoidance during treatment planning

PURPOSE

To develop computer software that assists the planner avoid potential gantry collisions with the patient or patient support assembly during the treatment planning process.

METHODS AND MATERIALS

The approach uses a simulation of the therapy room with a scale model of the treatment machine. Because the dimensions of the machine and patient are known, one can calculate a priori whether any desired therapy field is possible or will result in a collision. To assist the planner, we have developed a graphical interface enabling the accurate visualization of each treatment field configuration within a "room's eye view" treatment planning window. This enables the planner to be aware of, and alleviate any potential collision hazards. To circumvent blind spots in the graphic representation, an analytical software module precomputes whether each update of the gantry or turntable position is safe.

RESULTS

If a collision is detected, the module alerts the planner and suggests collision evasive actions such as either an extended distance treatment or the gantry angle of closest approach.

CONCLUSIONS

The model enables the planner to experiment with unconventional noncoplanar treatment fields, and immediately test their feasibility.