Stereotactic Ablative Radiotherapy (SABR/SBRT) for Up to Five Oligometastases: Progression-Free Survival, Local Control, and Prognostic Factors in the Population-Based Phase 2 SABR-5 Trial

Abstract

Purpose: Although stereotactic ablative radiotherapy (SABR), also termed stereotactic body radiotherapy (SBRT), is increasingly used for oligometastatic cancer, prospective outcome data remain limited. This analysis of the population-based phase 2 SABR-5 trial evaluated progression-free survival (PFS), local control (LC), and baseline prognostic factors after SABR for oligometastases. Methods and Materials: SABR-5 was a single-arm phase 2 trial with toxicity as the primary endpoint, conducted at the 6 regional cancer centers in British Columbia, Canada, during a period when SABR for oligometastases was available only within the trial. Eligible patients had up to 5 oligometastases in total, or up to 5 lesions not controlled by prior treatment, including induced oligometastatic disease, and received SABR to all known lesions. Patients were aged 18 years or older, had an Eastern Cooperative Oncology Group performance status of 0 to 2, and had life expectancy ≥6 months. Secondary endpoints of PFS and LC are reported here. Results: From November 2016 to July 2020, 381 patients underwent protocol SABR. Median follow-up was 27 months (interquartile range, 18-36). Median PFS was 15 months (95% confidence interval [CI], 12-18). LC at 1 and 3 years was 93% (95% CI, 91-95) and 87% (95% CI, 84-90), respectively. On multivariable analysis, larger tumor diameter (hazard ratio [HR], 1.09; P < .001), worse performance status (HR, 2.13; P < .001), disease-free interval <18 months (HR, 1.52; P = .003), 4 or more metastases at SABR (HR, 1.48; P = .048), initiation or change in systemic treatment (HR, 0.50; P < .001), and oligoprogression (HR, 1.56; P = .008) were independent predictors of PFS. Larger tumor diameter (sub-hazard ratio [SHR], 1.28; P < .001), colorectal histology (SHR, 4.33; P = .002), and "other" histology (SHR, 3.90; P < .001) were associated with worse LC. Conclusions: In this population-based cohort of patients with genuine oligometastatic, oligoprogressive, and induced oligometastatic disease treated with SABR to all lesions, median PFS was 15 months and 3-year LC was 87%. These findings support continued efforts to randomize patients in phase 3 trials, including outside the original 1 to 5 metachronous oligometastatic paradigm.