In 1998, the Breast Cancer Prevention Trial found that women at increased risk of breast cancer who took tamoxifen for 5 years had a 49% reduction in risk of invasive breast cancer compared with high-risk women who took a placebo. The FDA subsequently approved tamoxifen for breast cancer chemoprevention for women 35 years old or older with a 5-year breast cancer risk of 1.67% or higher. However, tamoxifen also has been associated with an increased risk of endometrial cancer, pulmonary embolism, stroke, and deep vein thrombosis, and not all eligible women will benefit from the treatment.
Using national data on cancer risk factors and screening practices, Andrew N. Freedman, Ph.D., and his colleagues at the National Cancer Institute calculated the number of women in the United States who, according to FDA criteria, would be eligible for tamoxifen chemoprevention. They then estimated the percentage of women who would likely have a favorable balance of risks and benefits.
Of the 65,826,074 women in the United States who are between the ages of 35 and 79, Freedman and his colleagues estimated that 10,232,816 (or 15.5%) would be eligible for tamoxifen chemoprevention based on breast cancer risk. The percentage of women eligible for tamoxifen chemoprevention increased with age and varied dramatically by race, with 18.7% of white women, 5.7% of black women, and 2.9% of Hispanic women eligible to take the drug to prevent breast cancer.
Among white women, the authors estimated that 2,431,911 (or 4.9%) would benefit from tamoxifen chemoprevention, compared with 42,768 (or 0.6%) black women. The authors explain that the difference in net benefit between the two groups may be because the estimated risk of breast cancer among black women is relatively low and because the risk of stroke, pulmonary embolism, and deep vein thrombosis is higher among black women than among white women. It was not possible to calculate a reliable estimate of the number of Hispanic women who would benefit from tamoxifen.
The results also suggest that, although the percentage of women eligible for tamoxifen is highest among women in the 60 to 79 year age group, the proportions of white women who may benefit are greatest in the 40 to 49 year and 50 to 59 year age groups.
Among the more than two million white women who would likely benefit from taking tamoxifen, 58,148 invasive breast cancers would be expected to develop in the next 5 years, the authors say. If all of these women took tamoxifen over the next 5 years, based on the 49% risk reduction reported in the Breast Cancer Prevention Trial, the authors estimate that 28,492 of these breast cancers could be prevented or deferred.
Although the findings suggest an overall benefit of tamoxifen for certain women, the authors note that each woman should be counseled on the appropriate therapy for her situation. "Counseling individual women about tamoxifen chemoprevention must involve both fully informing a woman of her disease risk and benefits and considering her comorbidities, personal values, preferences, lifestyle, and specific medical situation," the authors write.
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Freedman AN, Graubard BI, Rao SR, McCaskill-Stevens W, Ballard-Barbash R, Gail MH. Estimates of the number of U.S. women who could benefit from tamoxifen for breast cancer chemoprevention. J Natl Cancer Inst 2003;95:526–32.
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