ESTRO 2025 - Abstract Book

S514

Clinical - Breast

ESTRO 2025

Purpose/Objective: After breast-conservative surgery, standard care for non-metastatic breast cancer includes radiotherapy. An additional radiation dose (boost) to the tumor bed reduces local recurrence risk (1) but has not shown an overall survival benefit and is associated with increased adverse events (2). Therefore, the benefit-risk ratio of the boost is debated and guidelines are heterogeneous, highlighting the need to identify patient subgroups in whom the boost can be safely omitted. This study aims to use real-world data to assess boost. Material/Methods: This study emulates a target trial in CANTO-RT, a sub-study of the multicenter French prospective CANTO cohort that includes detailed radiotherapy data (3). Eligible patients were post-menopausal women with non-metastatic, stage I to IIIa, breast cancer treated with breast-conservative surgery. The primary endpoint was invasive breast cancer-free survival (IBCFS), with event being invasive-breast tumor, loco-regional or distant recurrence; invasive contralateral breast cancer; or death from any cause. We used propensity score weighting to account for confounding and multiple imputations for missing data. The propensity score was conditioned on age, cT, pTNM, SBR, ER, PR, histology, in-situ contingent, nodal irradiation, number of removed nodes, nodal involvement, adjuvant chemotherapy, trastuzumab, smoking, BMI, and Charlson index. Results: Of the 3932 patients included in the CANTO-RT study, 1646 were eligible for the emulated target trial, of whom 1255 received a boost and 391 did not. Compared to patients who did not received the boost, those who did were younger (median age 61.9 versus 68.2 years), had higher tumor grade (27% versus 9% of grade 3 tumors), were less frequently hormone receptor (HR) positive (86% versus 96%), and more frequently HER2 positive (11% versus 3%), with all differences being statistically significant (p < 0.01) (Table 1). The median follow-up was 7.6 years. At 10 years, the local relapse rate was 2.2% and IBCSF events occurred in 162 patients: 121 (10%) with and 41 (10%) without boost. A total of 47, 62 and 53 IBCSF events occurred in 547 (8.6%) patients younger than 60 years, 793 (7.8%) aged of 60-69 and 306 (17.3%) aged of 70 years and older respectively. After propensity score weighting, the HR for IBCFS was 0.91, 95%CI [0.55-1.49] in the overall population (Figure 1).