Chemoprevention of Breast Cancer: A Summary of the Evidence for the U.S. Preventive Services Task Force

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Abstract
Chemoprevention offers promise as a strategy for reducing morbidity and mortality from breast cancer in women. This review examined the evidence for the effectiveness of chemoprevention in women without a history of breast cancer. MEDLINE (1966 to December 2001). English-language, randomized, controlled trials (RCTs) of chemoprevention of breast cancer in women without a previous diagnosis of breast cancer were examined, and 4 relevant trials, 3 involving tamoxifen and 1 involving raloxifene, were selected. Trials that provided data on the harms of tamoxifen or raloxifene, studies of the costs of chemoprevention, and studies of risk assessment were also reviewed. Four reviewers independently abstracted data on key variables, including study population, sample size, randomization, treatment, and outcomes. The largest of the RCTs of tamoxifen reported a 49% reduction in relative risk (0.51 [95% CI, 0.39 to 0.66]) for invasive cancer among women with an estimated 5-year breast cancer risk of at least 1.66%. The other tamoxifen trials did not observe a statistically significant benefit, but only a few women in each trial took tamoxifen during the entire study period. The raloxifene study of postmenopausal women with osteoporosis found a 76% reduction in relative risk (0.24 [CI, 0.13 to 0.44]) for invasive breast cancer. Tamoxifen and raloxifene were effective only against estrogen receptor-positive tumors. Both drugs increased risk for venous thromboembolic disease and hot flashes; tamoxifen increased risk for endometrial cancer and stroke. Tamoxifen and raloxifene reduce the incidence of estrogen receptor-positive breast cancer in women. The relative risk reduction seems similar across all breast cancer risk groups. The absolute risk reduction varies by risk factors for breast cancer, however, and must be balanced against the potential harms to judge the appropriateness of treatment for individual women.