Purpose: There is ongoing debate about whether tamoxifen for breast cancer is associated with a risk of endometrial cancer in Japanese women. We conducted a study to investigate this further.
Methods: We conducted a retrospective hospital-based cohort study. A total of 674 consecutive patients underwent surgery for primary breast cancer between January 1989 and December 1998. By December 2003, endometrial cancers had been diagnosed in six of these patients. Based on medical records, we evaluated the potential risk factors for endometrial cancer, including age, menopausal status, obesity, parity, diabetes mellitus, hypertension, and tamoxifen. The 674 patients were divided into three groups based on the cumulative duration of tamoxifen use (A, <2 years vs B, 2-5 years vs C, >5 years). To examine the relationships between endometrial cancer and tamoxifen (and other factors), the hazards ratio (HR), 95% confidence interval (CI), and two-sided P value for endometrial cancer associated with each variable were calculated by the Cox regression method.
Results: Endometrial cancer was found in 1/318 (0.31%) patients in group A, 3/247 in group B, and 2/109 in group C. In a multivariate analysis no variable was significant, but tamoxifen use for longer than 5 years (group C) was closely correlated with endometrial cancer (HR = 7.92, CI = 0.69-90.89, P = 0.096).
Conclusion: Although our data did not reach significance, they support a link between long-term tamoxifen and the development of endometrial cancer in Japanese women with breast cancer.