Tamoxifen has been found to have estrogen-like effects on the uterus, which may account for women's increased endometrial cancer risk. Researchers believe obesity may cause greater exposure to estrogen in the uterus, as well. Endometrial cancer occurs far less frequently than breast cancer, and decisions about whether to prescribe tamoxifen should be considered in light of this differential in risk, Bernstein said. In the United States, recent population statistics show that 110 of every 100,000 women develop breast cancer annually. This is five times greater than the rate of endometrial cancer, which is 21 cases in every 100,000 women.
Women also are more likely to die from breast cancer than from endometrial cancer.
The chances that a woman will die of breast cancer each year are 26 per every 100,000 women in the United States, far higher than the three per every 100,000 women who die each year of endometrial cancer. Our results suggest that physicians should be particularly vigilant in monitoring tamoxifen-treated patients with these additional risk factors. Bernstein noted that the risk of endometrial cancer must be balanced against tamoxifen's proven benefits in breast cancer treatment and its effectiveness in reducing the incidence of breast cancer among women at high risk for that cancer. The researchers conducted the study with 324 patients with endometrial cancer who had previously been treated for breast cancer and 671 similar patients with breast cancer who did not develop endometrial cancer.