Background: Women who have had a first-degree relative diagnosed with breast cancer (ie, a positive family history) have a rate of breast cancer that is approximately twice that of all women their age, but it is unclear how they should perceive this risk at different ages or if they should be considered at higher risk for the remainder of their lifetime.
Methods: We used Swedish population-based data to assess the risk of breast cancer in 23,654 sisters of women diagnosed with breast cancer and in 1,732,775 sisters of unaffected women from 1958 through 2001. Poisson models were used to express the rate of breast cancer as a function of current age, whether a woman had an affected sister, time since the first diagnosis in the family, and family size (number of sisters). The effect of the age of the index case (the first sister diagnosed in the family) at diagnosis and whether her "at-risk" sisters had achieved this age were examined in stratified analyses. Incidence rate ratios of breast cancer in exposed compared with unexposed sisters were calculated with 95% confidence intervals. All estimates were adjusted for calendar time.
Results: Sisters of breast cancer patients had higher breast cancer incidence than unexposed sisters at all ages. The association of exposure (ie, a diagnosis of breast cancer in a sister) with the risk of breast cancer was most pronounced in young women (age 20-39; incidence rate ratio = 6.64, 95% confidence interval = 4.66 to 9.48), and the relative risk decreased to approximately 2 in women older than 50 years. The risk associated with having a sister diagnosed with breast cancer was not modified substantially by the age of the index case at diagnosis (< or = 45 years vs > 45 years). The risk was similar for women who were approaching the age at which the first sister was diagnosed in their family and those who had already attained it. The incidence rate ratio of breast cancer in exposed sisters compared with unexposed sisters was constant over time for all age categories of at-risk women.
Conclusions: Women who have a sister diagnosed with breast cancer have an increased risk of breast cancer throughout much of their lifetimes.