Endometrial Cancer Incidence in Breast Cancer Patients Correlating with Age and Duration of Tamoxifen Use: a Population Based Study

Background: Our study aimed to assess the endometrial cancer risk after tamoxifen adjuvant treatment for female breast cancer patients in Taiwan. Materials and Methods: A total of 74,280 breast cancer patients between January 1997 and December 2004 were included in the study; 39,411 received tamoxifen treatment and 34,869 did not. Tamoxifen-associated endometrial cancer was defined as endometrial cancer that occurred in patients at least 6-month after the diagnosis of breast cancer, who underwent tamoxifen treatment. Results: A total of 222 patients developed endometrial cancer, and of these, 153 (69 %) were seen in patients with tamoxifen treatment, and 69 (31%) were seen in patients without the use of tamoxifen. The incidence of endometrial cancer was 0.388% (153/39,411) in patients with tamoxifen treatment, while was 0.198% (69/34,869) in patients without tamoxifen treatment. Logistic regression analysis demonstrated that tamoxifen use and age over 35 years were significantly correlated with development of endometrial cancer (p<0.001 and p=0.002, respectively). The odds ratio was 2.94 (95% CI, 2.13-4.06) for 3 years or longer tamoxifen use. The odds ratio was 4.08 (95% CI, 1.67-9.93) for women older than 35 years compared to those 35 or younger than 35 years. There were no significant differences in prior hormone exposure, hypertension and diabetes. Conclusions: To the best of our knowledge, this is the largest population based study that shows in patients with breast cancer, tamoxifen use for more than three years or patients older than 35 years was associated with a significantly increased risk for developing endometrial cancer.