Simultaneous Integrated Boost (Sib) Intensity-Modulated Radiotherapy for Treatment of Bone Metastases: Analysis of a Breast Cancer Cohort

Aims: . Bone metastases have an incidence of up to 75% in metastatic breast cancer, thanks to the evolution of imaging we can identify them in the oligometastatic phase. With the introduction of target therapies and increasingly effective conjugated antibodies, it is increasingly common to treat patients with radiotherapy although asymptomatic (if single or ≤5 bone lesions). Volumes and doses in this setting do not yet have a clear guidelines. In this study, the aim is to administer to the macroscopically evident disease an ablative dose that favors the local control of disease. Aim of this retrospective analysis is to evaluate the outcome of pts receiving SIB for treatment of bone metastases. Material and Methods. A retrospective analysis including mBC patients (pts) undergone RT on bone metastases was conducted for pts treated between January 2014 and January 2022. Primary endpoint was freedom from local progression (FFLP). Rate of disease progression after radiotherapy (DP-AR) and overall survival (OS) were secondary endpoints. Subgroup analysis (age, immunophenotype, line of therapy) were performed. Results. Among 954 mBC who underwent radiotherapy for metastases, 85 pts underwent SIB-IMRT with a boost on macroscopic lesion of 8-6 Gy in 5 fr. Mean follow up was 41 months (6-61.5 m). Nineteen pts had single bone metastase (22.4%), 20 pts were oligometastatic (23,5%), 46 pts were plurimetastatic (54,1%). FFLP was 17 months (95% CI 3.2-61.5 m). Only 6 pts (7%) had local relapse on SIB-IMRT site during follow up. DP-AR was 13,2 m (95% CI 3.1-56.9 m). OS was 82,7 m (95% CI 10.6-343 m). Local-relapse was not associated with age, immunophenotype or sistemic line ongoing. Among secondary outcomes, DP-AR resulted associated to immunophenotype (p 0.002). DP-AR and OS were not significantly associated with local relapse (respectively p 0.148 and p 0.4). Conclusions. Pts with breast tumor can be treated with SIB-IMRT on bone metastases with a 93% of local control at 17 months from radiotherapy. Further data are needed to individuate pts who can benefit of dose-escalation and also sinergic effect with systemic therapies to possibly increase DP-AR.