Stereotactic body radiotherapy in patients with bone oligometastases from breast cancer – results from a European multicenter cohort study

Background and purpose The interdisciplinary treatment concept for bone metastases (BoM) of oligometastatic breast cancer includes metastasis-directed radiotherapy. We evaluated oncological outcomes of BoM treated with stereotactic body radiotherapy (SBRT) in a large European cohort. Material and methods Data of breast cancer patients treated with SBRT for BoM between 2010 and 2024 at 17 European cancer centers were retrospectively collected. Treatment and dose concepts were analyzed regarding their impact on freedom from local recurrence (FFLR), overall survival (OS), progression-free survival (PFS), and safety profile. Results With a median follow-up of 29.6 months (interquartile range: 15.0–48.0), 109 patients with 147 BoM were analyzed. SBRT was performed with a prescribed dose of median 35 Gy in median 5 fractions. 1-/3-year FFLR for mean biologically effective dose (BED 10 ) ≥ 50 Gy in the gross tumor volume (GTV mean ) was 98.1% (95%-CI: 92.7%-99.5%) and 93.9% (95%-CI: 85.5%-97.5%), respectively. For GTV mean BED 10 ≥ 50 Gy, 1-/3-year PFS was 62.1% (95%-CI: 43.6%-76.0%)/26.7% (95%-CI: 11.0%-45.4%) for non-spine BoM and 66.2% (95%-CI: 50.8%-77.8%)/35.8% (95%-CI: 21.3%-50.4%) for spine BoM. In multivariable analysis, systemic therapy (HR [hazard ratio] 0.22; p < 0.001) was associated with improved OS, while higher GTV mean BED 10 (HR 0.96; p < 0.001) was associated with improved PFS and spine localization with worse PFS (HR 1.81; p = 0.02). Adverse events were rare, with fracture rates of 2.7%. Conclusion In this multicenter retrospective cohort analysis, SBRT of BoM from breast cancer was a well-tolerated, effective treatment. Prospective studies are needed to investigate these findings further and determine the role of standardized SBRT concepts in the multidisciplinary management of oligometastatic breast cancer.