The efficacy of metastasis-directed radiation therapy (MDRT) for oligometastatic castration-resistant prostate cancer (CRPC).

240 Background: We have utilized imaging study, especially whole body magnetic resonance imaging (WB-MRI), to detect oligometastatic disease of castration resistant prostate cancer (CRPC) and performed metastasis-directed radiation therapy (MDRT) for oligometastatic disease. We review our institutional experience of MDRT for oligometastatic CRPC. Methods: We retrospectively reviewed the database comprising 26 oligometastatic CRPC patients treated by MDRT for oligometastatic disease combined with baseline androgen deprivation therapy at Osaka International Cancer Institute from January 2013 to August 2019 and investigated treatment efficacy of MDRT. Diagnosis of oligometastatic disease was performed by computed tomography, bone scintigraphy and/or WB-MRI. Treatment efficacy was assessed by change of serum prostate specific antigen (PSA) level and progression free survival (PFS). PFS was analyzed by Kaplan-Meier method and comparison was made by log-rank test. Results: The median age at treatment was 75.5 years (range, 62-86). The oligometastatic diseases were bone in 22 patients, lymph node in 2 patients and both in 2 patients. The number of treated lesions were as follows: 1 lesion for 21 patients, 2 lesions for 4 patients and 3 lesions for 1 patient. WB-MRI was performed in 19 patients (73%). Median dose of MDRT was 35Gy (range, 30-60) for bone and 60Gy (range, 50-60) for lymph node. The median PSA prior MDRT was 6.3 ng/ml (range, 0.5-191). Post MDRT PSA reduction was observed in 24 patients (92%) and over 50% PSA reduction was observed in 14 patients (54%). Median PFS was 8.7 month and patients with over 90 days PSA doubling time (PSADT) showed longer PFS (p=0.0074). Conclusions: Our data suggest that MDRT can achieve PSA reduction in most CRPC patients with oligometastasis and PSADT could be a predictor of disease progression after MDRT.