Local treatment for HER2-positive or triple negative second ipsilateral breast cancer event: A propensity score-matched analysis of the GEC-ESTRO database

Purpose Second breast conserving treatment (2 nd BCT) has emerged as an option for patients with low-risk second ipsilateral breast cancer event (2 nd iBCE) after prior breast BCT combining lumpectomy and irradiation. However, HER2-positive (HER2+) and triple-negative (TN) subtypes are considered poor candidates for 2 nd BCT. We evaluated oncologic outcomes after 2 nd BCT versus mastectomy in high-risk 2 nd iBCE. Methods and Materials We performed a propensity score-matched cohort study of patients diagnosed with 2 nd iBCE between 1995 and 2017. Data were collected from 15 centers across seven European countries. Patients underwent mastectomy or lumpectomy plus Multicatheter-interstitial brachytherapy (MIB). One-to-one matching used ten clinicopathologic variables. The primary endpoint was 5-year overall survival (OS). Secondary endpoints included 5-year cumulative incidence of third breast events, regional relapse, distant metastasis, disease-free survival (DFS) and cause-specific survival (CSS). Results Among 1327 patients (Mastectomy 945; 2 nd BCT 382), 241 were matched (Mastectomy versus 2 nd BCT): HER2+ 70/62 and TN 55/54. Median tumour size (mm) for mastectomy versus 2 nd BCT was 17/13 in HER2+ and 13/12 in TN. Median follow-up was 79.9 and 80.5 months for HER2+ and TN, respectively, without group differences. No statistical differences in 5-year OS were observed between mastectomy and 2 nd BCT (HER2+: 85% vs 79%, p=0.85; TN: 89% vs 81%, p=0.4). Similarly, the secondary endpoints were also comparable between groups. Conclusion This first matched comparison of mastectomy and 2 nd BCT with MIB in patients with HER2+ and TN tumours did not identify clear differences in efficacy. Given the limited sample size, 2 nd BCT may warrant consideration in selected patients seeking breast preservation.