Mammographic breast density and race.

OBJECTIVE: Women with increased mammographic breast density are known to be at higher risk of developing breast cancer. Reports of differences in breast density by race have implied that genetic and environmental factors may in part determine breast density. We first compared breast density among white, African American, and Asian women and then correlated breast density and race with age, body mass index (BMI), and breast or cup size.
MATERIALS AND METHODS: A retrospective review of data collected from 15,292 women was conducted. A stepwise multiple regression for an ordered response (breast density) was used to test for a relationship between race or ethnicity and breast density. We then determined whether differences in breast density by race might be caused by differences among races and ethnic groups in the age at imaging and BMI. We informally assessed the strength of the contribution of each term by means of the incremental change in the percent concordance. We also compared models using bra and cup sizes and age with models using BMI and age to try to determine whether the effects of breast size are local or systemic.
RESULTS: We did not find evidence that mammographic breast density differences exist across racial groups (p < 0.0001) other than those associated with BMI and age at screening. Ignoring age and BMI, breast density depends on race for all comparisons (p < 0.0001). To generalize, we found that breast density appears to be greater in Asian women and least in African American women. However, when controlling for BMI and age, breast density differences by race disappeared in all groups except Asians (p < 0.0001). In all racial groups, bra and cup size in addition to age correlated with breast density after controlling for BMI (p < 0.0001). Except in Asian women, in women of any racial group, age and any of the following parameters accounted for all of the breast density differences: BMI, bra size, and cup size.
CONCLUSION: Although breast density is associated with breast cancer risk, our results indicate that innate mammographic breast density differences across racial groups do not explain the risk differences known for the development of breast cancer. Age and BMI or age, bra size, and cup size can account for the reported density differences except among Asians. There may be no innate racial differences in breast density beyond those associated with racial differences in age and body habitus.