Decreased incidence of breast cancer in postmenopausal estrogen-progestogen users.

In a prospective study at Wilford Hall USAF Medical Center from 1975 to 1981, 5563 postmenopausal women were followed for a total of 37,236 patient-years of observation. During these seven years, 53 patients were found to have breast cancer, for an incidence of 142.3:100,000 women per year. The mean age (+/- SD) of the patients with cancer was 56.9 +/- 8.24 years, and the mean age of the entire patient population was 56.8 +/- 6.75 years. The expected incidence of breast cancer in this age group, according to the Third National Cancer Survey (1975), is 188.3:100,000 women, and for ages 55 to 59, according to the National Cancer Institute Surveillance, Epidemiology, and End-Result Reporting (NCI SEER) data (1980), is 229.2:100,000. The lowest incidence of breast cancer (67.3:100,000) was observed in the estrogen-progestogen users and was significantly lower than that of the untreated group (342.3:100,000), with P less than or equal to .01. The incidence of the estrogen-progestogen users was also significantly lower than that expected from the NCI SEER data, with a relative risk of 0.3 (95% confidence interval, 0.1 to 0.8). The incidence of mammary malignancy in the estrogen users (141.0:100,000) was significantly lower than in the untreated group (342.3:100,000), with P less than or equal to .01. Although the incidence in the estrogen users was not significantly lower than that expected according to the NCI SEER data (relative risk = 0.7, 0.5 to 1.1), there was a trend in that direction. These data indicate that estrogen therapy for postmenopausal women does not increase the risk of breast cancer and may afford some protection. Added progestogen to postmenopausal estrogen therapy significantly decreases the risk for this malignancy.