Evidence that adult life risk factors influence the expression of familial propensity to breast cancer.

In the context of a study in Athens comprising 692 cases and 1,261 controls, we have evaluated the effect on breast cancer risk of the joint action of first-degree relative family history and established adult life risk factors. We created a risk score by assigning the value of 1 to women at high risk with respect to any of these risk factors and 0 otherwise, and summing these values, using weights equal to the excess odds ratio. The odds ratio for a tertile increment in the risk score was 1.5 [95% confidence interval (CI) = 1.3-1.8] among women without family history, and 2.3 (95% CI = 1.1-5.1) for women with family history. Our findings imply that women with a family history of breast cancer may benefit disproportionately by reduced exposure to adult life risk factors.