There is a known increase in risk of endometrial cancer among women who have been treated with tamoxifen for breast cancer. Some studies have indicated that that risk is substantially higher for rare, aggressive forms of uterine tumors. In a Brief Communication published in the Journal of the National Cancer Institute, Rochelle E. Curtis and colleagues from the Division of Cancer Epidemiology and Genetics at the National Cancer Institute report their calculated estimates of that risk.

Using data from NCI's Surveillance, Epidemiology, and End Results (SEER) program, the authors calculated that among breast cancer patients diagnosed from 1980 through 2000 who were initially treated with tamoxifen, the overall risk of subsequent uterine corpus cancer was increased more than twofold relative to the general SEER population.

The relative risk was substantially higher for malignant mixed mullerian tumors (MMMTs) than for endometrial adenocarcinomas, although the excess absolute risk was smaller-an additional 1.4 versus 8.4 cancers per 10,000 women per year, respectively.

Among those who survived for 5 years or longer, there was an eightfold relative risk for MMMTs and a 2.3-fold risk for endometrial adenocarcinomas, with patients developing MMMTs having a worse prognosis.

According to the authors, these findings indicate that tamoxifen may have delayed effects, such as the increased risk of MMMTs, rare but aggressive tumors of unclear pathogenesis.

SOURCE:
Journal of the National Cancer Institute, January 7, 2004