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Breast Cancer Prevention (PDQ®): Prevention - Health Professional Information [NCI] - Description of Evidence

Incidence of Outcomes Per 1,000 Women continued...

An RCT has reported the effect of aromatase inhibitors in preventing invasive breast cancer among women who have no history of breast cancer. In this study, 4,560 women aged 35 years and older who had at least one risk factor (e.g., women aged 60 years and older or those having a Gail 5-year risk >1.66% or a history of DCIS with mastectomy) were randomly assigned to receive exemestane 25 mg daily or a placebo. After 35 months median follow-up, 32 women of the 2,275 in the placebo group had been diagnosed with invasive breast cancer, compared with 11 women in the exemestane group (HR, 0.35; 95% CI, 0.18–0.70; NNT, about 100 for 35 months). There was a small increase in adverse effects in the exemestane group compared with the placebo group, primarily in hot flashes (increase, 8%) and fatigue (increase, 2%). There was no difference in the occurrence of fractures or cardiovascular events.[107] A second trial (IBIS-2) is under way.

Prophylactic mastectomy

A retrospective cohort study was conducted to evaluate the impact of bilateral prophylactic mastectomy on the subsequent occurrence of breast cancer among women at high and moderate risk of breast cancer on the basis of family history.[108] Most women in this retrospective series (90%) had undergone subcutaneous rather than total mastectomy, which is the procedure of choice for maximum breast tissue removal. Median follow-up after surgery was 14 years. All women included in the report had some family history of cancer and were classified as high risk or moderate risk for breast cancer based on the pattern of breast cancer in the family. Expected cases of breast cancer were estimated for moderate- and high-risk women using the Gail model and the observed rates of breast cancer among sisters of the probands. The reduction in risk for moderate-risk women was 89%; for high-risk women, the reduction ranged from 90% to 94% depending on the method used to calculate expected rates of breast cancer. The reduction in risk of death from breast cancer ranged from 100% among moderate-risk women to 81% among high-risk women. Information on BRCA1 or BRCA2 mutation status was not known. Although this study provides the best evidence available to date that prophylactic surgery offers benefits despite the fact that some breast tissue remains postsurgery, some factors may bias the estimate of benefit.[109] Criteria used to classify women at high risk would include women from families misclassified as having an autosomal-dominant inherited pattern and women from inherited-syndrome families who are not at high risk because they did not inherit the susceptibility genotype. These factors may tend to overestimate the benefits of prophylactic surgery. Most of the women, however, who underwent prophylactic surgery would never have gone on to develop breast cancer. Thus, many were treated for the few who truly benefited by having their breast cancer prevented. Among the 425 moderate-risk women who had prophylactic mastectomy, the estimated number of breast cancer cases expected to occur was 37.4; among the 214 high-risk women, the estimates ranged from 30.0 to 52.9, depending on the model used to estimate breast cancer occurrence. Thus, bilateral prophylactic mastectomy as an option for women should be considered in association with cancer risk assessment and counseling regarding all the available preventive options, which now include tamoxifen as a preventive agent.[84]

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