Risk of breast cancer to relatives of young breast cancer patients.

Breast cancer risk among sisters and mothers of a population-based series of 1,137 breast cancer patients diagnosed in Metropolitan Detroit before the age of 55 years was compared with risk to the same relatives of 1,001 age-matched, population-based controls. After adjusting for age of the relative, for age of the case or control, and for race, the odds ratio for breast cancer for women with affected sisters was 2.2; for women with affected daughters, 3.2; and for women with affected mothers and sisters, 9.9. Breast cancer in aunts had no independent influence on risk. Among white women, cumulative risk of breast cancer before the age of 50 years was approximately 1% for relatives of controls, 3% for sisters of older cases, but about 17% for women either with sisters diagnosed before the age of 40 years or with affected sisters and mothers. Sisters of the older patients had a 13% risk of breast cancer by 70 years of age, compared to 5% for sisters of controls. White women with affected mothers and sisters were at 50% risk by 65 years of age. This study identifies the criteria for women who could receive particular benefit from screening for breast cancer.