PT  - JOURNAL ARTICLE
AU  - Catherine Meyer
AU  - David Mirando
AU  - Tim Adams
AU  - Johannes Czernin
AU  - Jeremie Calais
AU  - Magnus Dahlbom
TI  - Patient-specific tumor dosimetry in mCRPC patients treated with <sup>177</sup>Lu-PSMA-617
DP  - 2021 May 01
TA  - Journal of Nuclear Medicine
PG  - 1572--1572
VI  - 62
IP  - supplement 1
4099  - http://jnm.snmjournals.org/content/62/supplement_1/1572.short
4100  - http://jnm.snmjournals.org/content/62/supplement_1/1572.full
SO  - J Nucl Med2021 May 01; 62
AB  - 1572Introduction: We present early findings of a larger retrospective dosimetry study in metastatic castrate-resistant prostate cancer (mCRPC) patients treated with 177Lu-PSMA-617. In this work, tumor doses are estimated based on a hybrid imaging approach using both quantitative SPECT/CT and planar imaging following treatment. Methods: This is a retrospective dosimetry analysis in patients treated with 177Lu PSMA-617 who were included in a prospective phase II trial (NCT03042312). Each patient underwent whole body 177Lu gamma imaging at 4h, 24h, 48h, and 72h after administration of 177Lu-PSMA (5.9-8.1 GBq) with an optional late imaging time point after 9 days. Quantitative SPECT/CT was also acquired 24h after administration. Image analysis and dose calculations were performed with MIM dosimetry software utilizing a 177Lu dose kernel derived from Monte Carlo simulations. Tumor ROIs were segmented semi-automatically on SPECT images and propagated to the serial planar image series. Time-activity curves were generated from the planar images and scaled by the respective SPECT counts, followed by exponential curve-fitting and integration to yield mean tumor doses. Results: This preliminary report includes a total of 40 segmented metastatic lesions in 6 mCRPC patients. Dosimetry calculations estimated mean tumor absorbed doses of 21.9 ± 22.1 Gy, or 3.48 ± 3.46 Gy/GBq. There was significant variability in tumor doses within individual patients and across the patient cohort with tumor doses ranging from 0.2-11.9 Gy/GBq. This early observation suggests significant heterogeneity in tumor absorbed doses, possibly due to differences in inter-lesion PSMA expression. Conclusions: Dosimetry calculations were performed using a hybrid SPECT and serial planar imaging approach to estimate tumor uptake in a cohort of mCRPC patients treated with 177Lu-PSMA-617. The mean absorbed dose in metastatic lesions is approximately 3.48 ± 3.46 Gy/GBq and these early findings suggest significant inter- and intra-patient tumor dose heterogeneity.