Epidemiologic aspects of exogenous progestagens in relation to their role in pathogenesis of human breast cancer.Acta Endocrinol (Copenh). 1991; 125 Suppl 1:13-26.AE
This review focuses on epidemiologic studies of the relationship between breast cancer risk and exogenous progestagens, as present in oral contraceptives, injectable contraceptives and hormone replacement therapy. Subsequently, it will be discussed whether the present findings are consistent with one of the hypotheses that have been postulated for the role of hormones in breast cancer pathogenesis. The relationship between oral contraceptives and breast cancer is still controversial. Several studies have found that prolonged oral contraceptives use at young ages is associated with increased risk to develop breast cancer at an early age, i.e. before age 35. None of these studies, however, has been able to attribute the increased risk to specific formulations of oral contraceptives, or to the progestagen content of the preparations. This may be largely due to the fact that there is no good method to calculate progestagen potencies of different formulations. There are no reliable data regarding the effect of progestagen-only oral contraceptives on breast cancer risk. Studies conducted so far included only few women who used these preparations exclusively and for an extended period. Use of injectable contraceptives, mainly depot-medroxy-progesterone acetate, may slightly increase breast cancer, but current findings are inconclusive. There is suggestive evidence that the addition of progestagens to estrogen replacement therapy may increase breast cancer risk over that associated with exposure to estrogens alone. However, the data are not sufficient to warrant any recommendation about changes in clinical practice, and more studies of estrogen-progestagen replacement therapy are needed to settle this issue. It is argued that the "unopposed estrogen" hypothesis for breast cancer is not consistent with the known effects of oral contraceptives and estrogen replacement therapy. The "estrogen plus progestagen" hypothesis seems to be more consistent with current epidemiologic findings. This hypothesis predicts that prolonged exposure to estrogens alone carries a slightly increased breast cancer risk, whereas the combination of estrogens and progestagens increases the risk much more. Future studies will show whether the "estrogen plus progestagen" hypothesis can indeed explain the effects of oral contraceptives and estrogen-progestagen replacement therapy on breast cancer risk.