The risk of endometrial and breast cancer after estrogen treatment. A review of epidemiological studies.

Estrogen treatment of climacteric women has been found to be associated with a substantially increased risk of endometrial cancer and a possible slight excess risk of breast cancer. Numerous retrospective case-control studies, reported mainly in the United States, have provided evidence of a causal link between the use of estrogens and the development of endometrial cancer. The magnitude of the risk increase has been shown to be correlated with characteristics of the exposure, chiefly the duration of treatment and the presence of certain host factors in the patient, e.g. obesity and late menopause. Cases of endometrial cancer occurring after estrogen exposure were shown to have favorable tumor characteristics and excellent survival rates. The early results from a prospective cohort study have indicated that estrogen therapy, as practised in Sweden, is associated only with an excess risk of premalignant endometrial changes and that the addition of progestogens might exert a protective effect. The risk of breast cancer after estrogen therapy has been studied in both retrospective and prospective investigations. In the majority of these studies no evidence of an increased risk has been found. However, in two case-control and two follow-up studies the risk estimates were slightly but significantly raised in association with long-term and high-dose exposure.