Risk of prostate, ovarian, and endometrial cancer among relatives of women with breast cancer.

OBJECTIVE To investigate the risk of prostate, ovarian, and endometrial cancer among relatives of patients with breast cancer. DESIGN Cohort study of 947 pedigrees in which the proband had breast cancer, linked with the Icelandic cancer registry. SETTING Iceland. SUBJECTS The 947 pedigrees included 29,725 people, of whom 1539 had breast cancer, 467 had prostate cancer, 135 ovarian cancer, and 105 endometrial cancer. MAIN OUTCOME MEASURES Risk of prostate, ovarian, and endometrial cancer among blood relatives of women with breast cancer compared with risk in spouses. RESULTS The risk of prostate cancer was significantly raised for all relatives (1.5), first degree relatives (1.4), and second degree relatives (1.3) of women with breast cancer. Risk of ovarian cancer was raised for all relatives (1.9) and first degree relatives (1.9) and risk of endometrial cancer was raised for all relatives only (1.9). The risk of prostate cancer was raised if the proband with breast cancer had a first degree relative with prostate cancer. CONCLUSIONS Coaggregation exists between breast cancer and cancers of the prostate, ovaries, and endometrium. This risk relation is probably based on genes which act by increasing the risk for cancer at these sites. Environmental factors that are common among relatives may also play a part. Continued research is required into pathophysiological mechanisms that could explain these observations.