A Phase 2 Trial of Radium and Stereotactic Ablative Radiation Therapy in Hormone-Naïve Men with Oligometastatic Prostate Cancer to Bone: The RadSABR Study.

[PURPOSE] We hypothesized that treatment with Radium (Ra) and Stereotactic ablative radiotherapy (SABR) in patients with bone-only metachronous oligometastastic hormone-sensitive prostate cancer (moHSPC) could safely delay the start of androgen deprivation therapy (ADT) and maintain quality of life (QoL).

[METHODS AND MATERIALS] This prospective trial included 20 men with moHSPC and ≤5 bone-only metastases who previously had definitive treatment to the prostate and pelvic lymph nodes. Eligibility criteria were testosterone ≥ 100 ng/dL and metastases validated by conventional imaging. Exclusion criteria were postinitial treatment (LHRH) therapy or N1 disease at bone metastasis diagnosis. Treatment included 6 cycles of Ra and SABR (30 Gy in 5 fractions). Bone scans and Prostate Specific Antigen (PSA) levels were monitored regularly. The primary endpoint was freedom from ADT use at 15 months in ≥20% of patients. Patients were followed for 2 years, with clinically significant patient-reported outcome changes defined as >1/2 standard deviation from baseline. Statistical analyses used Wilcoxon rank sum, Pearson's χ tests, and univariate Cox regression, with significance set at P < .05 RESULTS: The median number of Ra cycles was 6. Freedom from ADT at 15 and 24 months was 50.0% and 40.0%, respectively (P < .001). Eleven (55%) and 5 (25%) patients had PSA declines exceeding 50% and 90%, respectively, with 2 patients achieving undetectable PSA levels (<0.01) at 2 years. No significant changes were observed in any patient-reported outcome QoL domains. Two patients had grade 3 skeletal-related events, and grade 2+ events attributed to Ra and SABR were observed in 4 and 2 patients, respectively.

[CONCLUSIONS] In this phase 2 trial, the initial use of Ra and SABR for moHSPC significantly delayed ADT use compared with historical controls. The therapy is well tolerated, preserves QoL, and can lead to undetectable PSA levels at 2 years.