DECLINE IN INCIDENCE OF ENDOMETRIAL CANCER FOLLOWING INCREASE IN PRESCRIPTIONS FOR OPPOSED CONJUGATED ESTROGENS IN A PREPAID HEALTH PLAN

During the 1980s, the ecologic association of conjugated estrogens wit h endometrial cancer changed from positive to negative in a prepaid he alth plan. During the same period, use of progestins increased dramati cally. We investigated whether the latter increase could explain the r eversal of the estrogen-cancer association. Endometrial cancer inciden ce was estimated from cases recorded in the health plan registry divid ed by the number of women over age 45 years. Conjugated estrogens usag e was measured as milligrams prescribed per woman per year and progest in was measured as tablets per woman per year, both based upon pharmac y records. Graphical and regression methods were used to analyze the r esulting ecologic data, In a log-linear regression of incidence on con jugated estrogens and medroxyprogesterone usage, estrogen usage had a strong positive association with incidence, while medroxyprogesterone had a strong negative association with incidence. The change in the di rection of the ecologic association between estrogen and endometrial c ancer that occurred in 1984 continued until 1993, suggesting that the decline in endometrial cancer incidence and concomitant increase in co njugated estrogens usage since 1984 is explained by the increasing use of progestins, The data are entirely consistent with the hypothesis t hat progestins can protect against most of the excess risk conferred b y conjugated estrogens, although the ecologic nature of the data prohi bits drawing further inferences. (C) 1998 Academic Press.