Family history, oral contraceptive usage, and breast cancer.

When breast cancer patients were grouped according to their family history of breast cancer striking differences were found in age distribution. Thus, the proportion of cases less than 45 years of age was grandmother greater than aunt greater than mother greater than FH-negative greater than sister. A similar FH-related sequence was observed in regard to current OC usage among breast cancer patients less than 45 years of age. It also appeared that current oral contraceptive (OC) usage increased the risk of invasive breast cancer among grandmother- or aunt-positive women while decreasing the risk among FH-negative women. There are important practical and conceptual reasons for further studies of the influence of interactions between age, OC usage and family history on the stepwise development of breast cancer.