Prophylactic Radiation Therapy vs. Standard-of-Care for Patients with High-Risk, Asymptomatic Bone Metastases: A Multicenter, Randomized Phase II Trial

Purpose/Objective(s) External beam radiotherapy (RT) is standard-of-care (SOC) for pain relief of symptomatic bone metastases. We aimed to test whether radiation for asymptomatic bone metastases prevents skeletal-related events (SRE). Materials/Methods We conducted a multicenter, randomized phase 2 trial (NCT03523351) of prophylactic RT to high-risk, asymptomatic bone metastases vs SOC. Patients were eligible if they were at least 18 years old with a metastatic solid tumor malignancy and more than 5 metastatic lesions, including at least one asymptomatic high-risk bone lesion, defined as: bulky disease (≥2cm in longest diameter), junctional spine or posterior spinal element disease, disease involving hip or sacroiliac joint, or disease in long bone involving 1/3-2/3 cortical thickness. Patients were stratified by histology and planned SOC (systemic therapy or observation) and randomly assigned in a 1:1 ratio to RT to all enrolled high-risk bone metastases or SOC alone. The primary outcome of SRE (defined as fracture, cord compression, or intervention with surgery or radiation) was analyzed from randomization to death or 12 months. A sample size of 66 was deemed necessary to achieve 80% power to detect a difference of 30% (experimental arm) versus 60% (SOC arm) using a two-sample, one-sided proportion test with alpha<0.05. We used the log-rank test for time-to-event analyses and Wilcoxon rank sum test to compare hospitalizations, brief pain inventory and quality of life (using EQ-5D-5L) between arms. Results Between May 8, 2018 and August 9, 2021, 78 patients with 122 bone metastases were enrolled and randomized to prophylactic RT (n=39) or SOC (n=39). The most common primary cancer types were lung (27%), breast (24%), and prostate (22%). Seventy-one patients (91%) were evaluable for the primary endpoint. At one year, SRE occurred in 1 of 62 lesions (1.6%) in the RT arm and 14 of 49 lesions (29%) in the SOC arm (p<0.001). There was a significant difference in time-to-SRE by type of high-risk feature (p=0.016), with most events occurring in junctional spine and bulky disease. There were significantly fewer patients hospitalized for SRE in the RT arm compared to the SOC arm (4 vs 0, p=0.045). At a median follow-up of 2.4 years, overall survival was significantly longer in the RT arm (HR 0.50, 95% CI 0.28, 0.91; p=0.02). Median OS among 11 patients with an SRE was 1.1 years compared to 1.5 years among 67 patients without an SRE. Pain was reduced at 3 months in the RT arm (p<0.05) compared to the SOC arm. EQ-5D-5L was similar between the groups at all time points. Conclusion Radiation delivered prophylactically to asymptomatic, high-risk bone metastases reduced skeletal-related events, meeting the primary endpoint of the study. Importantly, we observed an improvement in overall survival, with potential mechanisms of palliation or debulking warranting further investigation in a phase 3 trial.