Body size, mammographic density, and breast cancer risk.

A18

Background. Greater body weight and body mass index (BMI) have been associated with an increased risk of breast cancer in postmenopausal women, but with a reduced risk in premenopausal women. Weight and BMI are also negatively correlated with mammographic density, a strong risk factor for breast cancer. We have examined the associations of body size and mammographic density on breast cancer risk. Method. In 3 nested case-control studies in screening programmes, with 1112 matched case-control pairs, we examined the associations of body size and percent mammographic density in the baseline mammogram, with risk of breast cancer. The effect of each factor on risk of breast cancer was examined before and after adjustment for the other, using linear regression. Results. Before adjustment for mammographic density, BMI and weight were associated positively with breast cancer risk in postmenopausal women, and negatively in premenopausal women. The risk of breast cancer risk in the highest quintlie of BMI, compared to the lowest, was 1.01 (95% CI:0.8, 1.3). After adjustment for density, the risk associated with BMI increased to 1.56 (95% CI:1.1, 2.1), and was positive in both menopausal groups. The breast cancer risk associated with 75% or greater density, compared to 0%, increased from 4.22 (1.6, 11.0) to 5.83 (2.2,15.5) after adjustment for BMI. Conclusion. Body size and mammographic density are independent risk factors for breast cancer, and are thus likely to influence risk through different pathways. They are strongly and negatively correlated with each other, and uncontrolled confounding may lead to underestimation of the effects of either pathway on breast cancer risk.