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Tamoxifen and contralateral breast cancer: the other side.

Tamoxifen has been administered to thousands of women during the last 30 years. Initially, it was used for treatment of metastatic breast cancer, next for adjuvant treatment of breast cancer, and most recently for reduction in the incidence of breast cancer in high-risk women. Tamoxifen is one of the first molecularly targeted therapies to be successful in improving survival in a large number of patients with a common cancer. It is thought that tamoxifen functions as an antiestrogen by structurally altering the estrogen receptor (ER), which thereby disrupts the ER–estradiol complex (1). An overview of randomized trials of adjuvant hormonal therapy with tamoxifen included information on 37 000 women, of whom 18 000 were ER positive and 12 000 had unknown ER status (2). This overview showed that the 10-year survival of women who had ER-positive tumors was increased substantially by tamoxifen treatment. The absolute increase in survival was 10.9% for women with lymph nodepositive disease and 5.6% for women with lymph node-negative disease. The mortality reduction had a statistically significant trend, with a greater effect with longer treatment. The proportional mortality reductions after 1, 2, and 5 years of treatment were 12%, 17%, and 26%, respectively. Updated results presented at the most recent Early Breast Cancer Trialists’ Collaborative Group meeting in Oxford, U.K., in September 2000 revealed similar results. Tamoxifen use clearly improved the survival of women with ER-positive breast cancer or breast cancer of unknown ER status, with 17% and 34% proportional reductions in mortality, respectively. There were no decreases or increases in mortality in women with ER-negative breast cancer who were treated with tamoxifen. The public health implications of this mortality reduction in women treated with tamoxifen are substantial. One of the other reasons for administering tamoxifen is for the reduction of breast cancer in the contralateral breast. In this issue of the Journal, Li et al. (3) report that they did not find a statistically significant reduction in contralateral breast cancer in women treated with adjuvant tamoxifen, with a hazard ratio of 0.9 (95% confidence interval [CI] 0.7 to 1.2). This negative finding seriously calls into question the generalizability of the results of the subset analyses presented by Li et al. The overview