Racial differences in the incidence of breast cancer subtypes defined by combined histologic grade and hormone receptor status

Breast cancer encompasses several distinct clinical entities of very different characteristics and behaviors, a fact which likely contributes to the higher breast cancer mortality in African-Americans (AA) despite the higher incidence in European-Americans (EA). We are interested in how incidence variability in cancer subtypes defined by combined estrogen receptor (ER) and grade contributes to racial mortality disparities. As an initial step, we compared age-specific and age-adjusted incidence rates for each ER/ Grade subtype in South Carolina (SC-a southern state) with Ohio (a northern mid-western state), using state registry data for 1996-2004. Each ER/Grade subtype had a distinct incidence pattern and rate, with three striking racial/geographic differences. First, the racial incidence disparity in ER negative (ER-) cancers was mostly within the ER-/G3 subtype, of which AAs had similar to 65% higher incidence than did EAs; ER-/G2 was much less common, but of significantly higher incidence in AAs. Second, the racial disparity in ER positive (ER+) cancers was in the ER+/lower-grade cancers, with a marked EA excess in both states. Third, AA incidence of the ER+/lower-grade subtypes was similar to 26% higher in Ohio than in SC. The other subtypes (ER-/G1 and ER+/G3) varied minimally by race and state, and the latter showed a strong association with age. Age adjustment halved the racial difference in mean age at diagnosis to about 2 years younger in AAs, compared to 4 years younger in case comparisons. Use of age-adjusted and age-specific rates of breast cancer subtypes may improve understanding of racial incidence and mortality disparities over time and geography. This approach also may aid in estimating the race-specific incidence rates of triple-negative breast cancer.