Although we have known that endometrial or uterine cancer develops in a tiny proportion of women taking tamoxifen, we have not known which particular groups of women are at greatest risk of this disease. The researchers found not only that women whose breast cancer is treated with tamoxifen face a 50 percent greater risk of getting endometrial cancer, but that the risk is compounded in women who also have received estrogen replacement therapy or who are obese. Estrogen therapy and obesity already are known to increase the chance a woman will get endometrial cancer, without taking tamoxifen-but the effect is even stronger in women taking tamoxifen. Scientists have found that tamoxifen acts somewhat similar to estrogen on the uterus, which may account for women's increased endometrial cancer risk. Obesity may cause greater exposure to estrogen in the uterus, as well.
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 of every 100,000 women in the United States, far higher than the three of every 100,000 women who die each year of endometrial cancer. But nobody really knows what the answer is. That can better guide treatment after surgery and give women more detailed information about the extent of their cancer. The findings may also be used to help identify those women with breast cancer who can be spared the side effects and expense of chemotherapy. During most breast cancer surgeries, surgeons remove breast tissue as well as several lymph nodes, called axillary nodes, to analyze them for signs of cancer.