Family history and risk of breast cancer: nurses' health study.

Family history of cancer remains underused in general clinical practice. We assess age at diagnosis and the role of family history in risk of breast cancer. Prospective follow-up of nurses' health study participants from 1980 to 2006. Updated assessment of family history in mother and sister including age at diagnosis. We used youngest age at diagnosis for family member when classifying risk. We confirmed 4327 incident invasive breast cancers confirmed. Breast cancer incidence models fitted to women with and without family history to assess variation in the risk for established risk factors. Compared to women with no family history those whose mother was diagnosed before age 50 had an adjusted relative risk of 1.69 (95% CI 1.39-2.05) and those with mother diagnosed at 50 or older had a relative risk of 1.37 (1.22-1.53). Sister history was associated with increased relative risk; 1.66 (1.38-1.99) for those with sister history before age 50 and 1.52 (1.29-1.77) for those with sister diagnosed at age 50 or older. Women with either mother or sister diagnosed before age 50 had a relative risk of 1.70 (1.48-1.95) significantly higher than those with diagnosis at age 50 or older (RR = 1.30; (1.27-1.54), P = 0.016). The magnitude of risk associated with established reproductive and lifestyle risk factors did not differ significantly between women with and those without family history with the exception of risk after bilateral oophorectomy in which setting women with family history had greater reduction in risk of subsequent breast cancer. Women with a family member diagnosed with breast cancer before age 50 had increased risk of breast cancer compared to women with family members diagnosed at older ages. Consistent findings for risk factors regardless of family history adds to robust evidence for risk identification and risk stratification in clinical settings where prevention strategies will apply equally to women with and without family history.