Breastfeeding After Hormone Receptor–Positive Breast Cancer: Results From the POSITIVE Trial

We investigated breastfeeding patterns, behaviors, and association with breast cancer (BC) outcomes in women with early hormone receptor-positive (HR+) BC who had a live birth in the POSITIVE trial. POSITIVE is a prospective trial that demonstrated no increased short-term risk of BC events in women with early HR+ BC who interrupted endocrine therapy (ET) to attempt pregnancy. We describe the frequency, duration, and laterality of breastfeeding and estimate the cumulative incidence of BC events by breastfeeding status. At a median follow-up of 41 months, 317 patients had at least one live birth and 313 were eligible for this analysis. A total of 196 of 313 (62.6%) patients breastfed. A total of 130 of the 167 women (77.8%) who had breast-conserving surgery breastfed, and 90 of 130 (69.2%) breastfed from the unaffected breast only. Sixty-six of the 146 women (45.2%) who underwent unilateral mastectomy breastfed. The frequency of breastfeeding was higher in women older than 35 years (67.6% v 55.7%) and in those without previous children (66.4% v 48.5%). Over half (103 of 196, 52.6%) of women breastfed their first live birth for >4 months (median 4.4 months; 95% CI, 4.0 to 5.3). The cumulative incidence of a BC event at 24 months from first on-study live birth was 3.6% and 3.1% in the breastfeeding and nonbreastfeeding groups, respectively (0.5% difference; 95% CI, -4.3% to 5.2%). In POSITIVE, two thirds of women who gave birth after BC diagnosis breastfed, mostly for 4 months or more. In early follow-up, we did not observe differences in BC-related events in women who breastfed compared with those who did not. These results are key for women who wish to pursue pregnancy and breastfeeding after BC.