Breastfeeding Associated With Reduced Risk of Breast Cancer Recurrence and Mortality in Luminal A Subtype
In a study reported in the Journal of the National Cancer Institute, Kwan et al found that women with basal-like tumors were less likely to have breastfed than those with luminal A tumors and that breastfeeding was associated with a significantly reduced risk of recurrence and breast cancer–related death among all women and among those with luminal A tumors but not other tumor subtypes.
Study Details
The study involved data from 1,636 women from two prospective breast cancer cohorts. Luminal A, luminal B, HER2-enriched, and basal-like subtypes were identified by PAM50 gene expression assay. Overall, 754 women were never-breastfeeders, and 882 were ever-breastfeeders; 17.7% were nulliparous, 26.1% were parous with no breastfeeding, 21.7% breastfed for < 6 months, and 34.5% breastfed for ≥ 6 months.
Breastfeeding by Subtype
The proportion of women who ever breastfed was similar across luminal A (57.7%), luminal B (55.8%), and HER2-enriched (57.7%) subtypes and lower among the basal-like subtype (47.6%); on analysis adjusted for age at diagnosis, race/ethnicity, stage, and parity, women with the basal-like subtype were less likely to have breastfed than those with the luminal A subtype (odds ratio [OR] = 0.56, 95% CI = 0.39–0.80), with no significant differences for luminal A vs other subtypes being found.
Breastfeeding and Risk for Recurrence and Mortality
In analysis adjusted for age at diagnosis, race/ethnicity, stage, chemotherapy, radiotherapy, hormonal therapy, surgery type, PAM50 subtype, and parity, compared with never breastfeeding, ever breastfeeding was associated with a reduced risk of breast cancer recurrence (hazard ratio [HR] = 0.70, 95% CI = 0.53–0.93), with hazard ratios of 0.81 (95% CI = 0.58–1.14) among those breastfeeding for < 6 months and 0.63 (95% CI = 0.46–0.87; P = .01 for trend) among those breastfeeding for ≥ 6 months. Ever breastfeeding was also associated with a reduced risk for breast cancer mortality (HR = 0.72, 95% CI = 0.53–0.98), with hazard ratios of 0.90 (95% CI = 0.61–1.32) among those breastfeeding for < 6 months and 0.61 (95% CI = 0.43–0.88; P = .01 for trend) among those breastfeeding for ≥ 6 months.
In analysis by subtypes, ever vs never breastfeeding was associated with a reduced risk of recurrence (HR = 0.52, 95% CI = 0.31–0.89) and breast cancer death (HR = 0.52, 95% CI = 0.29–0.93) among women with luminal A subtype. Hazard ratios for recurrence and mortality favored ever breastfeeding in luminal B and basal-like subtypes and never breastfeeding for the HER2-enriched subtype, but none of the associations was statistically significant.
The reduction in risk for recurrence or breast cancer death associated with breastfeeding among all patients was greater in those with expression of proliferation genes below the median value.
The investigators concluded: “History of breastfeeding might affect prognosis and survival by establishing a luminal tumor environment with lower proliferative activity.”
Marilyn L. Kwan, PhD, of Kaiser Permanente Northern California, is the corresponding author of the Journal of the National Cancer Institute article.
The study was supported by the National Institutes of Health. For full disclosures of the study authors, visit jnci.oxfordjournals.org.
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