Differential Patterns of Risk Factors for Early-Onset Breast Cancer by ER Status in African American Women.

Abstract

Given the disproportionately high incidence of early-onset breast cancer and aggressive subtypes, such as estrogen receptor (ER)-negative tumors, in African American (AA) women, elucidation of risk factors for early onset of specific subtypes of breast cancer is needed. We evaluated associations of reproductive, anthropometric, and other factors with incidence of invasive breast cancer by age at onset (<45, ≥45) in 57,708 AA women in the prospective Black Women's Health Study. From 1995 to 2013, we identified 529 invasive breast cancers among women <45 years of age (151 ER-, 219 ER+) and 1,534 among women ≥45 years (385 ER-, 804 ER+). We used multivariable Cox proportional hazards regression to estimate hazard ratios (HRs) for associations by age and ER status. Higher parity, older age at first birth, never having breastfed, and abdominal adiposity were associated with increased risk of early-onset ER- breast cancer: HRs were 1.71 for ≥3 births versus one birth; 2.29 for first birth after age 25 versus <20 years; 0.61 for ever having breastfed versus never; and 1.64 for highest versus lowest tertile of waist-to-hip ratio. These factors were not associated with ER- cancer in older women or with ER+ cancer regardless of age. Differences in risk factors by ER subtype were observed for breast cancer diagnosed before the age of 45 years. Etiological heterogeneity by tumor subtype in early-onset breast cancer, in combination with a higher prevalence of the risk factors in AA women, may explain, in part, racial disparities in breast cancer incidence. Cancer Epidemiol Biomarkers Prev; 26(2); 270-7. ©2016 AACR.