Answer:
Lynn Hartmann reviewed the experience at the Mayo Clinic (NEJM 1999; 340:77-84). There were 639 women with a family of history of breast cancer who had undergone prophylactic bilateral mastectomy. Of these, 214 were considered high risk by the following criteria:
Two or more first-degree relatives with breast cancer
One first-degree relative and two or more second-degree or third-degree relatives with breast cancer
One first-degree relative with breast cancer before the age of 45 years and one other relative with breast cancer
One first-degree relative with breast cancer and one or more relatives with ovarian cancer
Two second-degree or third-degree relatives with breast cancer and one or more with ovarian cancer
One second-degree or third-degree relative with breast cancer and two or more with ovarian cancer
Three or more second-degree or third degree relatives with breast cancer
One first-degree relative with bilateral breast cancer.
The numbers of breast cancers in these 214 women who had prophylatic mastectomies were compared with the numbers in 403 of their sisters. Breast cancer was diagnosed in 3 of 214 or 1.4 percent of women with mastectomies compared with 156 of 403 or 38.7 percent of their sisters. This represents a 90 percent reduction in risk.