Stereotactic Body Radiation Therapy (SBRT/SABR) After Androgen Receptor Signaling Inhibitor Progression in Oligometastatic Castration-Resistant Prostate Cancer

Abstract

Purpose: Clinical outcomes of stereotactic body radiation therapy (SBRT), also termed stereotactic ablative radiotherapy (SABR), in relation to androgen receptor signaling inhibitors (ARSI) have not been characterized for oligometastatic prostate cancer. We sought to characterize prostate-specific antigen (PSA) response and progression-free survival (PFS) after SBRT among men who had progressed on ARSI therapy in the oligometastatic castration-resistant prostate cancer (omCRPC) setting. Methods and Materials: A single-institution retrospective analysis was performed in men with ARSI-resistant omCRPC. The intervention was SBRT to metastatic lesions. Outcome measures were PSA reduction greater than 50% and median PFS, defined by PSA progression or radiographic progression as determined during routine care. Cox regression analysis was used to identify factors associated with PFS. Results: Thirty-five men with ARSI-resistant omCRPC and 65 lesions treated with SBRT were followed for a median of 17.2 months. PSA reduction greater than 50% was achieved in 63% of men. Median PFS was 9.0 months. Incomplete ablation, defined as the presence of untreated lesions after SBRT or receipt of prior palliative radiation therapy doses, was associated with worse PFS (hazard ratio, 4.21 [1.74-10.19]; P < .01). In a subgroup of 22 men with complete ablation of metastatic sites using SBRT, median PFS was 13.1 months. One-year overall survival was 93.1% (95% confidence interval, 84.4-100). Conclusions: SBRT may augment the efficacy of ARSI in omCRPC, provided that all lesions receive ablative radiation doses. Prospective study of SBRT for men receiving ARSI is warranted.