Endometrial cancer risk in estrogen users after switching to estrogen-progestin therapy.

Objective: It is unknown whether postmenopausal unopposed estrogen users are better off, in terms of endometrial cancer risk, switching to a combined estrogen-progestin regimen or stopping hormone use altogether.

Methods: We analyzed data from a series of three population-based case-control studies in western Washington state during 1985-1999, comparing proportions of "switchers" and "stoppers" in cases and controls. We also assessed whether the risk of endometrial cancer in either group of former unopposed estrogen users returned to that of never users.

Results: After multivariate adjustment using unconditional logistic regression, women who switched to a combined regimen with a progestin added for at least ten days/month (37 cases, 47 controls) had half the risk of endometrial cancer of women who stopped hormone use altogether (86 cases, 78 controls) (adjusted odds ratio = 0.5, 95% confidence interval: 0.3-1.1). Most subgroups of former users, whether they switched or stopped, had some increased risk of endometrial cancer compared to never users.

Conclusions: Results from this study suggest that unopposed estrogen users may reduce their risk of endometrial cancer more by switching to a combined regimen with progestin added for at least ten days/month than by stopping hormone use altogether.