Family History of Breast Cancer as a Modifier of Other Risk Factors

Summary

A population-based case-control study was conducted to assess the influence of family history of breast cancer, as a surrogate for the actions of one or more susceptibility alleles, upon the effects of other breast cancer risk factors. Seven hundred thirty-four cases of primary invasive breast cancer diagnosed before age 45 were compared with 938 controls identified through random-digit dialing. The associations of selected risk factors for breast cancer were examined separately among women with no history of breast cancer in their mothers, sisters, aunts, or grandmothers (FH−), and among women with a history of breast cancer in their mothers and/or sisters (FH+). Nulliparity and induced abortion were associated with modestly increased relative risk (RR) estimates among FH- women and larger RR estimates among FH+ women. Our results suggest, at most, modest increases in risk associated with long duration and early age at first use of oral contraceptives (OCs) among FH- women. Among FH+ women, there was an increased risk associated with “ever” use of OCs for one or more years (RR = 1.7), but this was not further influenced by duration or age at first use. While these results require confirmation in other studies, they support the hypothesis that some factors preferentially influence the risk of breast cancer in women genetically predisposed to this disease.