Objective.-The purpose of this study is to assess the impact of family history on the risk of developing breast cancer. Design.-A case-control study design was used. Setting.-To provide a comprehensive assessment of family history risk, we used the Utah Population Database, a linked database compiled of genealogy data of the descendants of Mormon pioneer families, cancer data from the Utah Cancer Registry, and mortality data from the Utah Department of Vital Statistics. Patients.-All women diagnosed with breast cancer who were in the genealogy database and the Utah Cancer Registry were included. Controls were women selected from the genealogy, who like cases had no record of previous cancer. They were matched to the cases by age and place of birth. Outcome.-Several definitions of family history were used. The total familial risk variable, developed to work effectively in the Utah Genealogy Database, accounts for all family members, their degree of relatedness to the case, and the amount of time they were observed for possible cancer diagnosis. Results.-A threefold increase in risk, estimated by the odds ratio (OR), of breast cancer among those with the highest family history score (6% of cases) was observed when compared with those with the lowest family history score. The OR for women with a first-degree relative with breast cancer was 2.45 (95% confidence interval [CI], 1.84 to 3.06). If the nearest relative was a second-degree relative, the OR was 1.82 (95% CI, 1.39 to 2.24); if the nearest relative was a third-degree relative, the OR was 1.35 (95% CI, 1.07 to 1.64). A slightly greater risk was observed if the first-degree relative was a woman's mother (OR, 2.44; 95% CI, 1.77 to 3.42) rather than a sister (OR, 2.01; 95% CI, 1.66 to 2.43). Among subjects diagnosed before the age of 50 years, the disease experience of relatives prior to age 50 was most important, while for older subjects the experience of relatives of all ages was of roughly equal importance. Women who developed contralateral breast cancer within 3 years of initial diagnosis were nearly 10 times as likely as women without breast cancer to have a first-degree relative with breast cancer. Based on the risk estimates in this study, we have estimated that approximately 17% to 19% of breast cancer in the population could be attributed to family history. Women who had a first-degree relative with colon cancer had a 30% increased risk of breast cancer. Conclusions.-In this study population, women with a family history of breast cancer, even if the nearest relative with breast cancer is a third-degree relative, are at increased risk of the disease.
