Evaluating the role of stereotactic body radiation therapy with respect to androgen receptor signaling inhibitors for metastatic prostate cancer.

Abstract

121 Background: Outcomes of stereotactic body radiation therapy (SBRT) with respect to androgen receptor signaling inhibitors (ARSI) have not been characterized for metastatic prostate cancer. Our purpose is to characterize prostate specific antigen (PSA) response and progression free survival (PFS) following SBRT among men who have received ARSI in castration sensitive and resistant settings. Methods: A single institution retrospective analysis was performed for men treated with SBRT and ARSI and categorized into 4 groups: 1) oligometastatic castration-sensitive prostate cancer (omCSPC), 2) ARSI-sensitive (ARSI-s) oligometastatic castration-resistant prostate cancer (omCRPC), 3) ARSI-resistant (ARSI-r) omCRPC, and 4) polymetastatic CRPC (pmCRPC). We calculated the PSA reduction greater than 50% (PSA50) and median PFS (PSA or radiographic progression) as determined by routine care. We also used Cox regression analysis to determine factors influencing PFS for ARSI-r disease. Results: 73 men with 126 lesions were treated with SBRT and followed for a median of 14.4 months. The percentages of men who achieved a PSA50 for omCSPC, ARSI-s omCRPC, ARSI-r omCRPC and pmCRPC were 100%, 90%, 62.9%, and 16.7%, respectively. Respective median PFS values were: not reached, 17.3, 9.0, and 1.6 months. For the 35 men with ARSI-r omCRPC, incomplete ablation (defined as the presence of untreated lesions after SBRT or prior palliative external beam radiation therapy (EBRT)) (HR 3.51 [1.36, 9.06]; p = 0.01) was associated with worse PFS on multivariable analysis. For the subgroup of 22 men with ARSI-r omCRPC without prior palliative EBRT or untreated metastases, the median PFS was 13.1 months. Conclusions: SBRT may augment the efficacy of ARSI, particularly among men with ARSI-r omCRPC, provided that all lesions received ablative radiation doses. Future prospective study of SBRT for men receiving ARSI is warranted.