Patterns of failure after stereotactic body radiotherapy to non-spine bone metastases

Background and purposeStereotactic body radiotherapy (SBRT) has proven useful for non-spine bone metastases (NSBM). We analyzed local relapse rates and patterns of failure after NSBM-SBRT, contrasting our results with existing contouring guidelines.Materials and methodsWe conducted a retrospective analysis of NSBM-SBRT treatments performed between 2013 and 2024 in a single institution. Clinical, pathologic, and treatment-related data were collected. Failure patterns were assessed based on imaging tests and categorized as in-field, marginal/out-of-field.ResultsAmong 119 NSBM-SBRT treatments in 85 patients, the most common primary tumors were prostate (36.1%) and breast cancer (24.4%). The coxal bone was the predominant metastatic site (52.9%). The median follow-up was 32.8 months. OS rates at 1, 2, and 3 years were 90.1%, 83.5%, and 75.8%, respectively. Twenty seven relapses were observed in the treated bone with a median recurrent tumor volume of 9.9 cm3 and a median time to recurrence of 15.1 months. Relapse-free survival in the treated bone was 89.4%, 78.5%, and 74.2% at 1, 2, and 3 years, respectively. Median recurrent tumor volume within the CTV was 50.6% and the median distance from the relapse center to the initial tumor was 11.4 mm.ConclusionNSBM-SBRT provides effective local control with relapses often occurring near the initial tumor lesion. While adherence to existing contouring guidelines captures most scenarios, consideration of larger CTV expansions may be warranted in cases with poorer prognosis. Further studies are needed to identify risk factors for relapses outside the margins and optimize volume delineation in these scenarios.