[Hormone substitution therapy in menopause and risk of breast cancer]

If the administration of an estrogen alone increases the risk of breast and endometrium cancer, the addition of a progesterone derivative does not seem to change completely the risk of breast cancer whilst it clearly decreases that of the uterine one. One must be careful in making a large extrapolation of the data on the endometrium (progestogens inhibit the mitotic action of estrogens) to that on the mammary epithelium (inhibition of the estrogen activity at the canal level but synergy with estrogens at the lobe level). The high tissue concentrations of oestradiol at the mammary gland level after menopause could partly explain the small influence of exogenous estrogens administered alone and/or in combination with progestogens on the transformation of a gland already oversaturated with estrogens.