Background We investigated safety of breastfeeding after breast cancer in patients carrying germline BRCA pathogenic or likely pathogenic variants. Methods This was an international, multicenter, hospital-based, retrospective cohort study including BRCA carriers diagnosed with stage I-III invasive breast cancer at age 40 years or younger between January 2000 and December 2020 (NCT03673306). Locoregional recurrences and/or contralateral breast cancers, disease-free survival (DFS), and overall survival (OS) were compared between patients who breastfed after delivery and those who did not. Results Among 4732 patients included from 78 centers worldwide, 659 had a pregnancy after breast cancer diagnosis, of whom 474 delivered a child. After excluding patients with uptake of bilateral risk-reducing mastectomy prior to delivery (n = 225) or unknown breastfeeding status (n = 71), 110 (61.8%) breastfed (median duration 5 months) and 68 (38.2%) did not breastfeed. Compared to patients in the no breastfeeding group, those who breastfed were more frequently nulliparous at breast cancer diagnosis (61.8% vs 45.6%) and did not report prior smoking habit (71.8% vs 57.4%). After a median follow-up of 7.0 years following delivery, 7-year cumulative incidence of locoregional recurrences and/or contralateral breast cancers was 29% in the breastfeeding group and 36% in the no breastfeeding group (adjusted subdistribution hazard ratio [HR] = 1.08, 95% CI = 0.57 to 2.06). No difference in DFS (adjusted hazard ratio [aHR] = 0.83, 95% CI = 0.49 to 1.41) nor in OS (aHR = 1.32, 95% CI = 0.31 to 5.66) was observed. Conclusions Breastfeeding did not appear to be associated with a higher risk of developing locoregional recurrences or contralateral breast cancers, emphasizing the possibility of achieving a balance between maternal and infant needs without compromising oncological safety.
