Breastfeeding after breast cancer in young BRCA carriers

Background: We investigated safety of breastfeeding after breast cancer in patients carrying germline BRCA pathogenic or likely pathogenic variants. Methods: This was an international, multicentre, 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 centres 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 37% in the no breastfeeding group (adjusted subdistribution hazard ratio[HR]=1.08, 95%CI 0.57-2.06). No difference in DFS (aHR = 0.83, 95%CI 0.49-1.41) nor in OS (aHR = 1.32, 95%CI 0.31-5.66) 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.