Risk of familial breast cancer.

The probabilities of developing breast cancer to and between various ages were estimated for 699 sisters of patients from three pedigree groups: sisters of patients whose (1) mothers, (2) sisters, or (3) second-degree relatives had previous breast cancer. The pedigree source of the patient and the laterality of disease had important effects on her sister's risk, whereas her age at diagnosis had virtually no effect. The highest life-time risks to sisters occurred when the patient had bilateral disease, an affected mother (25 +/- 7.2%), or an affected sister (28 +/- 11%). The risks reduced to 18 +/- 3.3% and 14 +/- 2.6%, respectively, with unilateral disease. Sisters of patients with an affected second-degree relative manifested a risk of 4 +/- 1.8% only with unilateral disease. Although the sisters' probabilities to age 70 were similar in pedigrees with affected mothers and sisters, the interval probabilities for successive 10-year periods suggested an earlier disease occurrence in the mother than sister pedigree groups when the patient had bilateral disease. The current results and those of others underscore the need for caution in applying probabilities for counseling women about their breast cancer risks.