A Phase 2 Trial of Radium223 and Stereotactic Ablative Radiotherapy in Hormone-Naïve Men with Oligometastatic Prostate Cancer to Bone: The RadSABR study.

Abstract

Purpose: We hypothesized that treatment with Radium²²³ (Ra²²³) 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: This prospective trial included twenty 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 post-initial treatment LHRH therapy or N1 disease at bone metastasis diagnosis. Treatment included six cycles of Ra²²³ and SBRT (30 Gy in five fractions). Bone scans and PSA levels were monitored regularly. The primary endpoint was freedom from ADT (FFAdt) use at 15 months in ≥20% of patients. Patients were followed for 2 years, with clinically significant PRO changes defined as >1/2 standard deviation from baseline. Statistical analyses used Wilcoxon rank sum, Pearson's Chi2 tests, and univariate Cox regression, with significance set at P<0.05 RESULTS: The median number of Ra²²³ cycles was six. FFAdt at 15 and 24 months was 50.0% and 40.0%, respectively (p<0.001). Eleven (55%) and five (25%) patients had PSA declines exceeding 50% and 90%, respectively, with two patients achieving undetectable PSA levels (<0.01) at two years. No significant changes were observed in any PRO QoL domains. Two patients had grade 3 SREs, and grade 2+ events attributed to Ra²²³ and EBRT were observed in four and two patients, respectively.

Conclusions: In this phase II trial, the initial use of Ra²²³ and SABR for moHSPC significantly delayed ADT use compared to historical controls. The therapy is well tolerated, preserves QoL, and can lead to undetectable PSA levels at two years.