O'Byrne, Jamie T

129 Background: Few studies have explored the use of focal therapies to address select sites of resistant disease in men receiving 177-Lu-PSMA-617 for metastatic castrate resistant prostate cancer (mCRPC). Our institutional practice is to offer SBRT for patients with up to 5 sites of oligoprogression or non-responding lesions while undergoing 177Lu-PSMA-617 (every 6 weeks for 6 cycles total). This is determined by PSMA or choline PET imaging during the course of 177Lu-PSMA-617. Herein, we evaluated the safety of adding target SBRT during treatment with 177Lu-PSMA-617. Methods: This retrospective single institution analysis was conducted via systematic chart review to evaluate for adverse events during or after treatment. Grading of treatment-related adverse events was conducted using Common Terminology Criteria for Adverse Events Version 5.0 (CTCAE v5). Results: Thirty-one patients receiving 177Lu-PSMA-617 with 51 sites of oligoprogressive or non-responding metastatic disease were treated with SBRT. The median number of sites treated was 1 (range 1-4). Most patients received SBRT immediately after cycle 6 (32%) or after cycle 3 or 4 (42%). Bone was the most commonly treated site of disease (90%) with the majority (54%) of bone sites being spine or sacrum, followed by lymph nodes (10%). Ultimately, 84% of patients completed all 6 cycles of 177Lu-PSMA-617 (range 4-6). The median follow-up time was 19.1 months (IQR 15.5-22.4) after 177Lu-PSMA-617 therapy start. A total of 2 patients experienced a pathologic fracture within the SBRT treatment field that could possibly be related to radiotherapy. One patient experienced a grade 2 neuropathy which may have been related to radiation therapy. Other potential adverse events are listed (Table). Conclusions: Our data demonstrates a low rate of significant toxicity with the use SBRT during treatment with 177Lu-PSMA-617. Further study is indicated to determine the oncologic efficacy and potential late side effects of this treatment strategy. Adverse events. Event All Grades Grade > 3 Any Adverse Event 104 2 Anemia 29 (94%) 0 Low Platelets 11 (35.5%) 1 (3.2%) Any Pathologic Fracture 7 (22.5%) 1 (3.2%) Possible SBRT-related Pathologic Fracture 2 (6.5%) 1 (3.2%) Bone Pain Flare 3 (9.6%) 0 Neuropathy 1 (3.2%) 0