ENDOMETRIAL CANCER RISK AFTER DISCONTINUING USE OF UNOPPOSED CONJUGATED ESTROGENS (CALIFORNIA, UNITED-STATES)

To examine the decline in risk of endometria cancer after discontinuat ion of use of conjugated estrogens, we conducted a case-control study in a prepaid health plan. We identified 318 patients who had endometri al cancer but had no history of bilateral oophorectomy and had been in the Southern California (United States) Raiser Foundation Health Wan for more than 10 years. For each patient, one or two control members w ere selected, 599 in all, matched for age and duration of membership a t the time of cancer detection and who had had neither hysterectomy no r bilateral oophorectomy, A history of prescriptions for conjugated es trogens and of potential confounders was obtained for each subject by reviewing outpatient medical records. Rate ratios (RR) contrasting use rs with nonusers were estimated by time of latest prescription. We fou nd that estrogen-induced risk of endometrial cancer decreases rapidly as the estrogen-free interval increases. The RR estimates, adjusted fo r duration of use and potential confounding factors, declined from 5.0 for those receiving their latest prescription within 24 months (95 pe rcent confidence limits [CL] = 2.6-9.8), to 1.8 for those receiving th eir latest prescription within 24 to 48 months (CL = 0.9-3.7), to valu es near one for each latest prescription interval earlier than 48 mont hs ago (P for trend = 0.00004), For those who used conjugated estrogen s extensively (five or more prescriptions, five to 10 years ago), the RR estimate declined from 5.1 for those whose latest prescription was within two years to 0.6 yr for those whose latest prescription was fou r to five years previously (P for trend = 0.05).