Simultaneous integrated boost intensity-modulated radiotherapy for treatment of bone metastases: analysis of a breast cancer cohort.

Background: Bone metastases occur in up to 75% of metastatic breast cancer (MBC) cases. Advances in imaging now allow earlier detection, even during the oligometastatic phase. Radiotherapy (RT) is increasingly used in asymptomatic patients with ≤5 bone lesions, however standardised guidelines for dose and target volumes remain lacking. This study evaluates the outcomes of a simultaneous integrated boost (SIB) using intensity-modulated radiotherapy (IMRT) to deliver ablative doses to macroscopic bone lesions.

Methods: This retrospective study analysed MBC patients treated with SIB-IMRT for bone metastases between January 2014 and January 2022. The primary endpoint was freedom from local progression (FFLP); secondary endpoints included disease progression after radiotherapy (DP-AR) and overall survival (OS). Subgroup analyses were performed according to age, immunophenotype, and line of therapy.

Results: Among 954 patients treated with RT, 85 received SIB-IMRT (6-8 Gy per fraction, 5 fractions). Median follow-up was 41 months. Nineteen patients (22.4%) had a single bone metastasis, 23.5% were oligometastatic, and 54.1% were plurimetastatic. Median FFLP was 17 months; only 7% experienced local relapse at the SIB site. While DP-AR was 13.2 months, median OS reached 82.7 months. No significant correlation was found between local relapse and age, immunophenotype, or systemic therapy. Immunophenotype significantly influenced DP-AR (p = 0.002), while DP-AR and OS were not significantly associated with local progression.

Interpretation: SIB-IMRT for bone metastases in MBC is feasible and effective, with encouraging local control and minimal toxicity. Prospective studies are warranted to optimise dose escalation and explore synergistic effects with systemic therapies.