Familial cancer risks in affected sibships: results from the Swedish family-cancer database.

We used the nationwide Swedish Family-Cancer Database to assess familial risks to sibs in sibsibs where at least two sibs had concordant cancer and their parents either concordant or discordant cancer. Familial relative risks (FRRs) were calculated by comparing to concordant sib-pairs whose parents had no cancer. Cancer sites were included if at least ten such concordant sib-pairs were found. In situ cancers were included in order to increase the numbers of cases. Concordant triads, one parent and the sib-pair affected, had an FRR over 100 for thyroid (FRR 399), colon, all bowel, and ovarian cancer. In these cancers, some 40% or more of the concordant sib-pairs belonged to this group. Melanoma and cancer of the nervous system showed FRRs of about 20, and invasive breast cancer of only 2. 9; in these cancers no more than 10% of the concordant sib-pairs belonged to the concordant triads. Discordant family sets, one or two parents presenting with a cancer discordant from that of the sib-pair, showed an FRR of about 3.0 and breast cancer about 2.0, suggesting the involvement of familial effects shared by many forms of cancer.