Does mammographic density reflect ethnic differences in breast cancer incidence rates?

Breast cancer incidence rates in the United States are substantially lower among Asian-American women than among White or African-American women. The authors determined whether mammographic density reflects these ethnic differences by evaluating mammograms from 442 White, African-American, and Asian-American women without breast cancer who served as controls in one of two population-based, breast cancer case-control studies conducted in Los Angeles, California, in 1994-1998. Absolute and percent mammographic densities were determined with a previously developed and validated computer-assisted method. Data were analyzed using multiple regression methods. Mean age-adjusted percent mammographic density was significantly higher in Asian Americans (35.9%) than in African Americans (27.8%, p < 0.05) but was no longer significant after further adjustment for body mass index. After adjustment for age, body mass index, selected menstrual/reproductive factors, and family history, absolute mammographic density was statistically significantly lower in Asian Americans than in African Americans (p < 0.05) but not than in Whites. The ethnic difference in absolute mammographic density was particularly evident among women older than age 50 years. Additional adjustment for breast size reduced these ethnic differences. This study suggests that absolute but not percent mammographic density reflects the lower breast cancer incidence rates of Asian Americans in relation to those of African Americans and Whites.