Attributable risks for familial breast cancer by proband status and morphology: a nationwide epidemiologic study from Sweden.

Population attributable factions (PAFs) show the proportion of the disease that could be prevented if the cause could be removed. The PAF for familial breast cancer has not been precisely determined. We used the nationwide Swedish Family-Cancer Database on 10.2 million individuals and 190,000 mothers' and 26,000 daughters' breast cancers to calculate familial standardized incidence ratios (SIRs), proportion of cases with a family history and familial PAFs for all invasive and in situ and morphology-specific breast cancers in daughters who were 0-66 years old. The data were calculated by mother only, sister only or both as probands. More than 5,500 familial breast cancers were recorded. The familial SIRs for all invasive breast cancer were 1.79 by breast cancer in the mother only, 2.03 by breast cancer in a sister only and 2.82 by breast cancer in both a mother and sister. The familial PAFs were 3.61, 3.01 and 0.43%, respectively, giving a total PAF of 7.05%. Age-specific risks were shown for the mother and sister history of breast cancer. The PAF values decreased by age when the daughter had a mother history of breast cancer but not when she had a sister history. PAFs did not depend on the morphologic type of breast cancer. The data show that the familial PAF of breast cancer among a 0-66-year-old population of daughters was 7% and independent of the morphologic type. If contribution from the paternal side was allowed for, the PAF would be 11%.