Outcome of Loco-Regional Radiotherapy in Metastatic Castration-Resistant Prostate Cancer Patients Treated with Abiraterone Acetate

316 Background: To evaluate the potential benefit of curative radiotherapy (RT) to primary tumor in metastatic castration resistant prostate cancer (mCRPC) patients treated with abiraterone. Methods: The clinical parameters of 106 mCRPC patients treated with abiraterone in the either pre- or post-chemotherapy setting were retrospectively evaluated. All patients had PSA progression or radiographic progression suitable for metastasis according to the RECIST guidelines with or without PSA progression. Patients were either oligometastatic (≤5 metastases) at diagnosis or became oligometastatic after systemic treatment were analyzed. Local RT to primary tumor and pelvic lymphatics was delivered in 44 patients (41%), while 62 patients (59%) did not have RT to primary tumor. Results: Median follow-up times for patients overall and for those who survived were 14.2 months (range, 2.3–54.9 months) and median OS for entire cohort was 21.9 months. The median OS was significantly higher in patients treated with local RT to primary tumor compared to patients without local RT (p = 0.04). Local RT to prostate and pelvic lymphatics significantly diminished local recurrence rate ( p = 0.003). In univariate analysis, a decline in PSA levels ≥50% of baseline was significant prognostic factor for both OS and PFS in entire cohort and in patients receiving prostate RT. In multivariate analysis, PSA response ≥50 of baseline obtained ≥3 weeks after abiraterone therapy is the only significant prognostic factor for better OS and PFS. Additionally, local RT to prostate is nearly significant prognosticator for improved OS. Patients treated with primary RT to prostate had significantly less progression under abiraterone and longer abiraterone period compared to patients without local prostate RT. Conclusions: Local prostate RT significantly improves OS and local control but not PFS, in mCRPC treated with abiraterone. Patients treated with primary RT to prostate had significantly less progression under abiraterone and longer abiraterone period compared to patients without local prostate RT.