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 (2ndBCT) has emerged as an option for patients with low-risk second ipsilateral breast cancer event (2ndiBCE) after prior breast BCT combining lumpectomy and irradiation. However, HER2-positive (HER2+) and triple-negative (TN) subtypes are considered poor candidates for 2ndBCT. We evaluated oncologic outcomes after 2ndBCT versus mastectomy in high-risk 2ndiBCE. METHODS AND MATERIALS: We performed a propensity score-matched cohort study of patients diagnosed with 2ndiBCE 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; 2ndBCT 382), 241 were matched (Mastectomy versus 2ndBCT): HER2+ 70/62 and TN 55/54. Median tumour size (mm) for mastectomy versus 2ndBCT 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 2ndBCT (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 2ndBCT with MIB in patients with HER2+ and TN tumours did not identify clear differences in efficacy. Given the limited sample size, 2ndBCT may warrant consideration in selected patients seeking breast preservation.