Background: Breast cancer incidence is considerably lower among Japanese and Chinese women than among Caucasian and Native Hawaiian even in second and third generation migrants. Mammographic densities, which refer to the radiological appearance of the healthy female breast, are related to breast cancer risk. The purpose of this project was to explore the hypothesis that women from ethnic groups at high breast cancer risk are more likely to have high levels of densities than women from low breast cancer risk groups.
Methods: In a cross-sectional design, 514 pre- and post-menopausal women recruited at mammography screening clinics completed a self-administered questionnaire. We used a computer-assisted method to measure the dense and the total areas of the breast and to compute per cent breast density. Student's t-tests and multiple linear regression were applied to examine ethnic differences and to explore determinants of mammographic densities, respectively.
Results: The unadjusted mean dense area was 15% smaller in Chinese and Japanese women than in the Caucasian/Hawaiian group. However, because of their smaller breast size, the per cent of the breast occupied by dense tissue in Chinese and Japanese women was 20% higher than in Caucasian women. Body mass index, age, menopausal status, parity, and oestrogen therapy were associated with mammographic densities, but they did not account for all ethnic differences.
Conclusions: Whereas this study detected some ethnic differences in mammographic densities, the importance of dense areas and per cent densities as indicators of breast cancer risk in ethnically diverse populations remains to be clarified.