Robert Olson, Mitchell Liu, Alanah Bergman, Benjamin Mou, Hadassah Abraham, Curtis Leclerc, Hannah Carolan, Devin Schellenberg, Will Jiang, Abraham Alexander, F. Hsu, Stacy Miller, Siavash Atrchian, Elisa Chan, Clement Ho, Islam Mohamed, Angela Lin, Tanya Berrang, Andrew Bang, Deepa Gautam, Nick Chng, Quinn Matthews, Vicky Huang, Ante Mestrovic, Derek Hyde, Chad Lund, Howard Pai, Boris Valev, Shilo Lefresene, Scott Tyldesley, Sarah Baker
BACKGROUND AND PURPOSE: SABR-5 was a provincially coordinated, single-arm phase II trial of SABR for patients with up to five extracranial metastases across all six BC Cancer centres. The primary objective was to prospectively quantify treatment-related toxicity in a population-based setting. This analysis reports extended follow-up and long-term toxicity outcomes. MATERIALS AND METHODS: Adults with oligometastatic or oligoprogressive disease (≤5 extracranial metastases) were enrolled between 2016 and 2020, when SABR for these indications in BC was available only through this trial. All grade ≥ 3 events underwent central review by a provincial toxicity committee. Toxicity was evaluated using crude per-patient rates, cumulative incidence methods, annual landmark analyses to year 5, and assessment of toxicity duration. Median follow-up for this update was 54.2 months. RESULTS: A total of 380 patients were treated (median age 69 years; 32% female). Most patients (91%) had one or two lesions treated. Crude per-patient rates of treatment-related toxicity were 18.9% for grade 2, 5.8% for grade 3, 0.0% for grade 4, and 0.3% for grade 5. There were no additional treatment-related deaths beyond the single previously reported case. The most frequent grade ≥ 3 toxicities were fracture (2.9%) and pain (1.8%). The cumulative incidence of grade 2 and grade ≥ 3 toxicity at 5 years was 24% and 7%, respectively. Annual landmark analyses demonstrated that most evaluable patients remained grade 0-1 at each timepoint. Analysis of toxicity duration demonstrated that most grade 3 events resolved within a year (median durations typically < 12 months), and persistent long-term toxicity was uncommon and predominantly low-grade. CONCLUSION: SABR delivered within a population-based, quality-assured program is associated with a low incidence of late high-grade toxicity and no new treatment-related deaths. These findings provide durable reassurance regarding the long-term safety of SABR for patients with limited extracranial metastatic disease. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT02933242.