Technical Note: Rapid multiexponential curve fitting algorithm for voxel-based targeted radionuclide dosimetry

A step toward simplified dosimetry of radiopharmaceutical therapy via SPECT frame duration reduction

Despite being time-consuming, SPECT/CT data is necessary for accurate dosimetry in patient-specific radiopharmaceutical therapy. We investigated how reducing the frame duration (FD) during SPECT acquisition can simplify the dosimetry workflow for [177Lu]Lu-PSMA radioligand therapy (RLT). We aimed to determine the impact of shortened acquisition times on dosimetric precision. Three SPECT scans with FD of 20, 10, and 5 second/frame (sec/fr) were obtained 48 h post-RLT from one metastatic castration-resistant prostate cancer (mCRPC) patient's pelvis. Planar images at 4, 48, and 72 h post-therapy were used to calculate time-integrated activities (TIAs). Using accurate activity calibrations and GATE Monte Carlo (MC) dosimetry, absorbed doses in tumor lesions and kidneys were estimated. Dosimetry precision was assessed by comparing shorter FD results to the 20 sec/fr reference using relative percentage difference (RPD). We observed consistent calibration factors (CFs) across different FDs. Using the same CF, we obtained marginal RPD deviations less than 4% for the right kidney and tumor lesions and less than 7% for the left kidney. By reducing FD, simulation time was slightly decreased. This study shows we can shorten SPECT acquisition time in RLT dosimetry by reducing FD without sacrificing dosimetry accuracy. These findings pave the way for streamlined personalized internal dosimetry workflows.

Clinical Trial Protocol for VIOLET: A Single-Center, Phase I/II Trial Evaluation of Radioligand Treatment in Patients with Metastatic Castration-Resistant Prostate Cancer with [161Tb]Tb-PSMA-I&T

[177Lu]Lu-PSMA is an effective class of therapy for patients with metastatic castration-resistant prostate cancer (mCRPC); however, progression is inevitable. The limited durability of response may be partially explained by the presence of micrometastatic deposits, which are energy-sheltered and receive low absorbed radiation with 177Lu due to the approximately 0.7-mm mean pathlength. 161Tb has abundant emission of Auger and conversion electrons that deposit a higher concentration of radiation over a shorter path, particularly to single tumor cells and micrometastases. 161Tb has shown in vitro and in vivo efficacy superior to that of 177Lu. We aim to demonstrate that [161Tb]Tb-PSMA-I&T will deliver effective radiation to sites of metastatic prostate cancer with an acceptable safety profile. Methods: This single-center, single-arm, phase I/II trial will recruit 30 patients with mCRPC. Key eligibility criteria include a diagnosis of mCRPC with progression after at least one line of taxane chemotherapy (unless medically unsuitable) and androgen receptor pathway inhibitor; prostate-specific membrane antigen-positive disease on [68Ga]Ga-PSMA-11 or [18F]DCFPyL PET/CT (SUVmax ≥ 20); no sites of discordance on [18F]FDG PET/CT; adequate bone marrow, hepatic, and renal function; an Eastern Cooperative Oncology Group performance status of no more than 2, and no prior treatment with another radioisotope. The dose escalation is a 3 + 3 design to establish the safety of 3 prespecified activities of [161Tb]Tb-PSMA-I&T (4.4, 5.5, and 7.4 GBq). The maximum tolerated dose will be defined as the highest activity level at which a dose-limiting toxicity occurs in fewer than 2 of 6 participants. The dose expansion will include 24 participants at the maximum tolerated dose. Up to 6 cycles of [161Tb]Tb-PSMA-I&T will be administered intravenously every 6 wk, with each subsequent activity reduced by 0.4 GBq. The coprimary objectives are to establish the maximum tolerated dose and safety profile (Common Terminology Criteria for Adverse Events version 5.0) of [161Tb]Tb-PSMA-I&T. Secondary objectives include measuring absorbed radiation dose (Gy), evaluating antitumor activity (prostate-specific antigen 50% response rate, radiographic and prostate-specific antigen progression-free survival, overall survival, objective response rate), and evaluating pain (Brief Pain Inventory-Short Form) and health-related quality of life (Functional Assessment of Cancer Therapy-Prostate and Functional Assessment of Cancer Therapy-Radionuclide Therapy). Conclusion: Enrollment was completed in February 2024. Patients are still receiving [161Tb]Tb-PSMA-I&T.

A review of 177Lu dosimetry workflows: how to reduce the imaging workloads?

177 Lu radiopharmaceutical therapy is a standardized systemic treatment, with a typical dose of 7.4 GBq per injection, but its response varies from patient to patient. Dosimetry provides the opportunity to personalize treatment, but it requires multiple post-injection images to monitor the radiopharmaceutical's biodistribution over time. This imposes an additional imaging burden on centers with limited resources. This review explores methods to lessen this burden by optimizing acquisition types and minimizing the number and duration of imaging sessions. After summarizing the different steps of dosimetry and providing examples of dosimetric workflows for177 Lu -DOTATATE and177 Lu -PSMA, we examine dosimetric workflows based on a reduced number of acquisitions, or even just one. We provide a non-exhaustive description of simplified methods and their assumptions, as well as their limitations. Next, we detail the specificities of each normal tissue and tumors, before reviewing dose-response relationships in the literature. In conclusion, we will discuss the current limitations of dosimetric workflows and propose avenues for improvement.

Tumor-Targeted Interleukin 2 Boosts the Anticancer Activity of FAP-Directed Radioligand Therapeutics

We studied the antitumor efficacy of a combination of 177Lu-labeled radioligand therapeutics targeting the fibroblast activation protein (FAP) (OncoFAP and BiOncoFAP) with the antibody-cytokine fusion protein L19-interleukin 2 (L19-IL2) providing targeted delivery of interleukin 2 to tumors. Methods: The biodistribution of 177Lu-OncoFAP and 177Lu-BiOncoFAP at different molar amounts (3 vs. 250 nmol/kg) of injected ligand was studied via SPECT/CT in mice bearing subcutaneous HT-1080.hFAP tumors, and self-absorbed tumor and organ doses were calculated. The in vivo anticancer effect of 5 MBq of the radiolabeled preparations was evaluated as monotherapy or in combination with L19-IL2 in subcutaneously implanted HT-1080.hFAP and SK-RC-52.hFAP tumors. Tumor samples from animals treated with 177Lu-BiOncoFAP, L19-IL2, or both were analyzed by mass spectrometry-based proteomics to identify therapeutic signatures on cellular and stromal markers of cancer and on immunomodulatory targets. Results: 177Lu-BiOncoFAP led to a significantly higher self-absorbed dose in FAP-positive tumors (0.293 ± 0.123 Gy/MBq) than did 177Lu-OncoFAP (0.157 ± 0.047 Gy/MBq, P = 0.01) and demonstrated favorable tumor-to-organ ratios at high molar amounts of injected ligand. Administration of L19-IL2 or 177Lu-BiOncoFAP as single agents led to cancer cures in only a limited number of treated animals. In 177Lu-BiOncoFAP-plus-L19-IL2 combination therapy, complete remissions were observed in all injected mice (7/7 complete remissions for the HT-1080.hFAP model, and 4/4 complete remissions for the SK-RC-52.hFAP model), suggesting therapeutic synergy. Proteomic studies revealed a mechanism of action based on the activation of natural killer cells, with a significant enhancement of the expression of granzymes and perforin 1 in the tumor microenvironment after combination treatment. Conclusion: The combination of OncoFAP-based radioligand therapeutics with concurrent targeting of interleukin 2 shows synergistic anticancer effects in the treatment of FAP-positive tumors. This experimental finding should be corroborated by future clinical studies.

Future trends for patient-specific dosimetry methodology in molecular radiotherapy

Molecular radiotherapy is rapidly expanding, and new radiotherapeutics are emerging. The majority of treatments is still performed using empirical fixed activities and not tailored for individual patients. Molecular radiotherapy dosimetry is often seen as a promising candidate that would allow personalisation of treatments as outcome should ultimately depend on the absorbed doses delivered and not the activities administered. The field of molecular radiotherapy dosimetry has made considerable progress towards the feasibility of routine clinical dosimetry with reasonably accurate absorbed-dose estimates for a range of molecular radiotherapy dosimetry applications. A range of challenges remain with respect to the accurate quantification, assessment of time-integrated activity and absorbed dose estimation. In this review, we summarise a range of technological and methodological advancements, mainly focussed on beta-emitting molecular radiotherapeutics, that aim to improve molecular radiotherapy dosimetry to achieve accurate, reproducible, and streamlined dosimetry. We describe how these new technologies can potentially improve the often time-consuming considered process of dosimetry and provide suggestions as to what further developments might be required.

Simultaneous Imaging of Ga-DOTA-TATE and Lu-DOTA-TATE in Murine Models of Neuroblastoma

68Ga-DOTA-TATE and 177Lu-DOTA-TATE are radiolabeled somatostatin analogs used to detect or treat neuroendocrine tumors. They are administered separately for either diagnostic or therapeutic purposes but little experimental data for their biokinetics are measured simultaneously in the same biological model. By co-administering 68Ga-DOTA-TATE and 177Lu-DOTA-TATE in three laboratory mice bearing two IMR32 tumor xenografts expressing different levels of somatostatin receptors (SSTRs) on their shoulders and imaging both 68Ga and 177Lu simultaneously, we investigated the relationship between the uptake of 68Ga-DOTA-TATE and 177Lu-DOTA-TATE in organs and tumors. In addition, using the percent of injected activity (%IA) values of 68Ga-DOTA-TATE at 0 hr and 4 hr, we investigated the correlation between 68Ga-DOTA-TATE %IA and the time-integrated activity coefficients (TIACs) of 177Lu-DOTA-TATE to estimate the organ-based and tumor-based doses of 177Lu-DOTA-TATE. The results showed that the extrapolated clearance time of 68Ga-DOTA-TATE linearly correlated with the TIACs of 177Lu-DOTA-TATE in the IMR32-SSTR2 tumor, kidneys, brain, heart, liver, stomach and remainder body. The extrapolated %IA value at 0 hr of 68Ga-DOTA-TATE linearly correlated with the TIACs of 177Lu-DOTA-TATE in the IMR32 tumor and lungs. In our murine study, both kidneys and lungs were organs that showed high absorbed doses of 177Lu-DOTA-TATE.

Patient-specific dosimetry adapted to variable number of SPECT/CT time-points per cycle for [Formula: see text]Lu-DOTATATE therapy

BACKGROUND

The number of SPECT/CT time-points is important for accurate patient dose estimation in peptide receptor radionuclide therapy. However, it may be limited by the patient's health and logistical reasons. Here,  an image-based dosimetric workflow adapted to the number of SPECT/CT acquisitions available throughout the treatment cycles was proposed, taking into account patient-specific pharmacokinetics and usable in clinic for all organs at risk.

METHODS

Thirteen patients with neuroendocrine tumors were treated with four injections of 7.4 GBq of [Formula: see text]Lu-DOTATATE. Three SPECT/CT images were acquired during the first cycle (1H, 24H and 96H or 144H post-injection) and a single acquisition (24H) for following cycles. Absorbed doses were estimated for kidneys (LK and RK), liver (L), spleen (S), and three surrogates of bone marrow (L2 to L4, L1 to L5 and T9 to L5) that were compared. 3D dose rate distributions were computed with Monte Carlo simulations. Voxel dose rates were averaged at the organ level. The obtained Time Dose-Rate Curves (TDRC) were fitted with a tri-exponential model and time-integrated. This method modeled patient-specific uptake and clearance phases observed at cycle 1. Obtained fitting parameters were reused for the following cycles, scaled to the measure organ dose rate at 24H. An alternative methodology was proposed when some acquisitions were missing based on population average TDRC (named STP-Inter). Seven other patients with three SPECT/CT acquisitions at cycles 1 and 4 were included to estimate the uncertainty of the proposed methods.

RESULTS

Absorbed doses (in Gy) per cycle available were: 3.1 ± 1.1 (LK), 3.4 ± 1.5 (RK), 4.5 ± 2.8 (L), 4.6 ± 1.8 (S), 0.3 ± 0.2 (bone marrow). There was a significant difference between bone marrow surrogates (L2 to L4 and L1 to L5, Wilcoxon's test: p value < 0.05), and while depicting very doses, all three surrogates were significantly different than dose in background (p value < 0.01). At cycle 1, if the acquisition at 24H is missing and approximated, medians of percentages of dose difference (PDD) compared to the initial tri-exponential function were inferior to 3.3% for all organs. For cycles with one acquisition, the median errors were smaller with a late time-point. For STP-Inter, medians of PDD were inferior to 7.7% for all volumes, but it was shown to depend on the homogeneity of TDRC.

CONCLUSION

The proposed workflow allows the estimation of organ doses, including bone marrow, from a variable number of time-points acquisitions for patients treated with [Formula: see text]Lu-DOTATATE.

Quantitative SPECT/CT for dosimetry of peptide receptor radionuclide therapy

Neuroendocrine tumors (NETs) are uncommon malignancies of increasing incidence and prevalence. As these slow growing tumors usually overexpress somatostatin receptors (SSTRs), the use of ⁶⁸Ga-DOTA-peptides (gallium-68 chelated with dodecane tetra-acetic acid to somatostatin), which bind to the SSTRs, allows for PET based imaging and selection of patients for peptide receptor radionuclide therapy (PRRT). PRRT with radiolabeled somatostatin analogues such as ¹⁷⁷Lu-DOTATATE (lutetium-177-[DOTA,Tyr³]-octreotate), is mainly used for the treatment of metastatic or inoperable NETs. However, PRRT is generally administered at a fixed injected activity in order not to exceed dose limits in critical organs, which is suboptimal given the variability in radiopharmaceutical uptake among patients. Advances in SPECT (single photon emission computed tomography) imaging enable the absolute quantitative measure of the true radiopharmaceutical distribution providing for PRRT dosimetry in each patient. Personalized PRRT based on patient-specific dosimetry could improve therapeutic efficacy by optimizing effective tumor absorbed dose while limiting treatment related radiotoxicity.