Conjugated estrogen use and risk of endometrial cancer.

The relation between use of conjugated estrogens and the risk of uterine cancer was examined among 188 white women with newly diagnosed endometrial cancer and 428 controls hospitalized for nonmalignant conditions requiring surgery at the Boston Hospital for Women-Parkway Division, Boston, Massachusetts, in January 1970-June 1975. As in prior studies, the greatest increases in risk were associated with dosages of 0.625 mg or greater (relative risk (RR) = 3.8, 95% confidence interval (CI) = 2.2-6.6) and duration of use of 10 or more years (RR = 7.6). Risk was elevated whether or not use was cyclic. Cyclic use was associated with a higher risk (RR = 3.6, 95% CI = 2.2-6.6) than continuous use (RR = 2.4, 95% CI = 1.3-4.1), but the difference between these risk estimates was not statistically significant. Risk remained increased even among women who had discontinued use of conjugated estrogens five or more years previously (RR = 4.5). Cases who were previous users had less advanced lesions at diagnosis than had never users. The highest risk associated with use of conjugated estrogens was that for stage I, grade 1 disease with no myometrial invasion. However, increases in risk of more advanced disease were seen among long-term users.