Breastfeeding and triple-negative breast cancer: potential implications for racial/ethnic disparities.

Breast cancer, like most cancers, is increasingly understood to be a biologically heterogeneous disease. That biological heterogeneity, in turn, has considerable implications for breast cancer epidemiology , such that separate evaluation of biologically-distinct tumor subtypes may offer greater insight into disease etiology and prognosis. In this issue of the Journal, Palmer et al. present a resource for characterizing the epidemiology of breast cancer subtypes: the African American Breast Cancer Epidemiology and Risk (AMBER) Consortium (1). Through this consortium of studies in African American women, Palmer et al. provide an important opportunity not only to better characterize risk factors for breast cancer in an understudied demographic, but also to identify risk factors for the triple-negative subtype of breast cancer that disproportionately affects this demographic. Triple-negative breast cancer, defined by a lack of estrogen-receptor, progesterone-receptor, and HER2/neu expression, is an aggressive, poor-prognosis, poorly understood disease subtype (2). Incidence rates for triple-negative breast cancer are higher among African American women than among non-Hispanic white women in the United States within every age category (3). The disproportionate burden of this aggressive disease in African American women may also contribute to noted disparities in breast cancer survival by race/ethnicity (4). In order to address, reduce, and ultimately eliminate these well-documented inequalities, it is important to identify those modifiable factors contributing to the unequal burden of triple-negative disease. To date, however, few risk factors for triple-negative breast cancer have been consistently noted. To the contrary, it has become clear that many of the so-called " traditional " risk factors for breast cancer overall do not impact risk of triple-negative disease (2). Breastfeeding has been one possible exception in the search for risk factors for this poor-prognosis disease subtype. Several studies have now documented a 25% to 50% lower risk of triple-negative breast cancer in parous women who have breastfed for at least four to six months relative to parous women who have never breastfed (5–9). Results from the AMBER Consortium add further support to these previous findings (1). Although information on breastfeeding duration was not provided by Palmer et al., the authors reported an odds ratio of 0.81 comparing risk of estrogen receptor-negative and, more specifically, triple-negative breast cancer in parous women who ever vs never breastfed. Given that the biological mechanisms underlying this repeatedly observed association have not yet been elucidated, it remains plausible that breastfeeding is simply a correlate for some other causal factor; …