Family history and the risk of breast cancer: a systematic review and meta-analysis.

An increased risk of breast cancer in women with a family history of breast cancer has been demonstrated by many studies using a variety of study designs. However, the extent of this risk varies according to the nature of the family history (type of relative affected, age at which relative developed breast cancer and number of relatives affected) and may also vary according to age of the individual. The aim of our study was to identify all the published studies which have quantified the risk of breast cancer associated with a family history of the disease, and to summarise the evidence from these studies, with particular emphasis on age-specific risks according to subject and relative age. Seventy-four published studies were identified. The pooled estimate of relative risk (RR) associated with various family histories was as follows: any relative, RR = 1.9 (95% CI, 1.7-2.0); a first-degree relative, RR = 2.1 (CI = 2.0, 2.2); mother, RR = 2.0 (CI = 1.8, 2.1); sister, RR = 2.3 (CI = 2.1, 2.4); daughter, RR = 1.8 (CI = 1.6, 2.0); mother and sister, RR = 3.6 (CI = 2.5, 5.0); and a second-degree relative, RR = 1.5 (CI = 1.4, 1.6). Risks were increased in subjects under age 50 and when the relative had been diagnosed before age 50.