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

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