Familial risks in second primary breast cancer based on a family cancer database.

The nationwide Swedish Family Cancer Database was used to analyse the risk of breast cancer in daughters of mothers presenting second, bilateral breast cancer. The database contained information on family relationships and cancers of mothers and daughters when the daughter was born after 1940, totalling 55,411 maternal and 9966 daughters' primary breast cancers. Some 95% of the second breast cancers were diagnosed in the contralateral breast. Familial risk of breast cancer in daughters was 1.70 when the mothers had first breast cancer and 3.28 when the mothers had bilateral breast cancer. Thus, the increased familial effect of the second breast cancer was 1.93. The risk was highest in daughters diagnosed at a young age when the mother was diagnosed before 50 years of age. If the mothers had breast cancer following any other type of cancer, the familial effect was as for the first breast cancer (1.03). The age of onset for breast cancer in daughters was 0.7 years younger for those whose mother had bilateral as compared with unilateral breast cancer, although the difference was not statistically significant. The mothers with bilateral breast cancer whose daughters also had breast cancer were diagnosed with the first breast cancer 3.8 years younger than mothers whose daughters did not have breast cancer. The present results apply to a relatively young population of daughters (< 54 years of age).