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

## TL;DR

Stereotactic radiotherapy for bone metastases in breast cancer patients shows high control rates and low fracture risks, but more studies are needed to confirm its role.

## Contribution

Demonstrates the effectiveness and safety of SBRT for bone oligometastases in breast cancer through a large European multicenter study.

## Key findings

- SBRT achieved excellent local control with 98.1% and 93.9% freedom from local recurrence at 1 and 3 years.
- Fracture rates were low at 2.7%, indicating a well-tolerated treatment profile.
- Systemic therapy improved overall survival, while higher radiation doses improved progression-free survival.

## Abstract

•Stereotactic radiotherapy of bone oligometastases with excellent local control.•Well-tolerated treatment with especially low fracture rates.•Prospective studies are needed to determine the role of standardized SBRT concepts.

Stereotactic radiotherapy of bone oligometastases with excellent local control.

Well-tolerated treatment with especially low fracture rates.

Prospective studies are needed to determine the role of standardized SBRT concepts.

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.

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.

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 (BED10) ≥ 50 Gy in the gross tumor volume (GTVmean) was 98.1% (95%-CI: 92.7%-99.5%) and 93.9% (95%-CI: 85.5%-97.5%), respectively. For GTVmean BED10 ≥ 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 GTVmean BED10 (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%.

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.