Stereotactic Ablative Radiation Therapy for Abiraterone-Resistant, Oligoprogressive Metastatic Prostate Cancer

Inclusion Criteria: * ECOG performance status 0-1. * Histologic confirmation of prostate adenocarcinoma. * Documentation of metastatic, castration-resistant prostate cancer. * Patient being treated with abiraterone. * Evidence of oligoprogression (according to RECIST \[V1.1\] and/or Prostate Cancer Working Group criteria \[PCWG3\], as applicable), applying any of the following: (i) ≤ 5 metastatic lesions progressing on conventional imaging (≤ 3 progressing metastases in any one organ system) while all other metastases are controlled or responding; (ii) PSA progression only, but in the setting of oligometastases (≤ 5 metastatic lesions seen on imaging, with ≤ 3 metastases in any one organ system); in this setting, all metastases will be irradiated. * All metastases of interest amenable to SABR. Exclusion Criteria: * Patients presenting with unequivocal clinical progression, defined as one of the following: (i) cancer pain requiring the initiation of opioid therapy; (ii) immediate need for cytotoxic chemotherapy as per treating physician's discretion; or (iii) deterioration of performance status to grade ≥ 3 according to ECOG. * Evidence of spinal cord compression. * Prior malignancy within the past 5 years, excluding non-melanoma skin cancer, and in-situ cancer.