Breast carcinoma chemoprevention in the community setting. Estimating risks and benefits

The United States Preventive Services Task Force recommends that women who are at both high risk for breast carcinoma and low risk for adverse events should receive counseling regarding tamoxifen for chemoprevention. Estimates of the risks and benefits of tamoxifen based on results from clinical trials may not reflect the real-world experience. The authors determined the prevalence of women in a community-based cohort who would meet the definition of high risk for breast carcinoma and calculated the number of women needed to screen to determine one for whom the benefits of tamoxifen would outweigh the risks. Baseline incidence also was examined for adverse health events in this community-based cohort compared with participants in the Breast Cancer Prevention Trial.

The study participants were women ages 40-70 years (n = 6048 women) who were members of the CLUE II cohort, which started in 1989, and who responded to questionnaire surveys in 1996 and 2000.

Eighteen percent of all women had a 5-year risk of invasive breast carcinoma > or = 1.66%. The number of women needed to screen to find 1 woman for whom the benefits outweighed the risks of tamoxifen ranged from 26 women ages 40-49 years to 142 women ages 60-70 years. For women who had undergone a hysterectomy, the numbers needed to screen were lower. Baseline incidence rates of fracture and thromboembolic disease were higher in the community-based cohort compared with the rates observed among prevention trial participants; thus, fewer women had to be treated with tamoxifen to prevent one fracture. However, fewer women in the community also had to be treated to observe harm with a thromboembolic event.

Clinicians who counsel women about tamoxifen should take into consideration community-level risks and benefits.