Incidence rates and risk factors of benign breast neoplasms.

A case-control study of benign breast disease was conducted in Greater Boston in 1968-1969. Cases were nearly all women living in the reference population who were initially diagnosed as having fibrocystic disease, fibroadenoma or a "mixed" lesion during the study period. Controls were a random sample of the entire reference population. A mail questionnaire was completed for 678 cases and for 1807 controls. For fibrocystic disease the age-standardized incidence rate was 89.4 per 100,000 woman-years; for fibroadenoma it was 32.8. Fibrocystic disease rises in incidence to age 45 and then declines sharply. The incidence rate of fibroadenoma peaks during the 20's, while that of mixed tumors has a mode at 30 to 34. Among young women, the highest rates occur in married nulliparae, but this is not so at higher ages. Neither for fibroadenoma nor fibrocystic disease was there a consistent relationship of risk with parity or with age at first birth. Fibrocystic disease risk was strongly and directly related to age at natural menopause, directly but not strongly related to an index of socioeconomic status, and was increased among women who gave a history of arthritis. Both fibrocystic disease and fibroadenoma were much less frequent in more obese women. Neither fibrocystic disease nor fibroadenoma has an epidemiologic pattern which corresponds closely to that of breast cancer. Therefore, it seems reasonable to suggest that the apparent increased risk of breast cancer among women with benign breast disease is concentrated within a subset of these women.