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Study data show risk-reducing mastectomy is effective, reducing the chance of developing breast cancer by as much as 95% in high-risk women. A 1998 study of 639 women at high or moderate risk for breast cancer found prophylactic mastectomy reduced their risk by 90%, compared to their sisters who did not undergo the procedure. (The study did not look specifically at women with mutations in BRCA or other geness associated with cancer risk; the surgeries were done prior to the availability of genetic testing.) Seven participants developed breast cancer after prophylactic mastectomy. A later study which included genetic testing on the same 639 women determined a similar risk reduction from risk-reducing surgery in the 18 mutation carriers identified. These studies were limited—they were retrospective, meaning researchers looked at past records from women who had undergone this procedure.
In a more recent study when researchers followed BRCA carriers for 6 years after bilateral mastectomies, researchers discovered mastectomy surgery alone lowered the risk for breast cancer by about 90%. Risk reducing surgery in women who had both bilateral mastectomies and oophorectomies (removal of the ovaries) reduced risk by 95%.