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


Interventions Associated With Decreased Risk of Breast Cancer: Benefits and Harms

Selective estrogen receptor modulators (SERMs)

Data from adjuvant breast cancer trials using tamoxifen have shown that tamoxifen not only suppresses the recurrence of breast cancer but also prevents new primary contralateral breast cancers.[74] Tamoxifen also maintains bone density among postmenopausal women with breast cancer.[75,76,77,78,79] Adverse effects include hot flashes, venous thromboembolic events, and endometrial cancer.[80,81,82]

These adjuvant trial results were the basis for the Breast Cancer Prevention Trial (BCPT) that randomly assigned 13,388 patients at elevated risk of breast cancer to receive tamoxifen or placebo.[83,84] The independent Endpoint Review, Safety Monitoring, and Advisory Committee closed the study early because of a 49% reduction in the incidence of breast cancer for tamoxifen-treated versus placebo-treated participants. After about 4 years of follow-up, placebo-treated women had 154 cases of invasive breast cancer compared with 85 cases in women who received tamoxifen. Noninvasive breast cancers were also reduced, with 59 cases in the placebo group versus 31 in the tamoxifen-treated group. Another benefit of tamoxifen use was a reduction in fractures, with 47 occurring in the tamoxifen-treated women compared with 71 in the placebo group. These benefits were accompanied by an increased incidence of endometrial cancer and thrombotic events in women aged 50 years and older. There were 33 endometrial cancers and 99 vascular events (including 17 cases of pulmonary embolism and 30 cases of deep vein thrombosis) in women who received tamoxifen compared with 14 endometrial cancers and 70 vascular events (including 6 cases of pulmonary embolism and 19 cases of deep vein thrombosis) in women who received a placebo.[84]

An update of the BCPT results after 7 years of follow-up demonstrates results similar to those in the initial report.[85] Follow-up was more complete for the tamoxifen group than for the placebo group because of a greater drop-out rate among women in the placebo group after early termination of the study. In addition, women who received a placebo were given the option of taking tamoxifen or participating in the Study of Tamoxifen and Raloxifene (STAR), and 32% did so. Breast cancer rates decreased among women in the placebo group from year 6 to year 7 of follow-up. A statistically significant RR of 43% for invasive breast cancer persisted at follow-up despite the addition of women to the placebo arm. The rate of invasive breast cancer among women in the placebo group was 6.29 per 1,000 women versus 3.59 per 1,000 women for women in the tamoxifen group, for a risk reduction of 0.27%. Benefits and risks of tamoxifen were not significantly different from those in the original report, with persistent benefit of reductions in fracture and persistent risks of endometrial cancer, thrombosis, and cataract surgery. No overall mortality benefit was observed after 7 years of follow-up (RR = 1.10; 95% CI, 0.85–1.43).

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