The prevention of breast cancer through reduced ovarian steroid exposure.

Analysis of epidemiologic data on cancers of the breast, ovary and endometrium; the effects of endogenous hormones on cell proliferation; and current carcinogenesis concepts, suggest that hormonal contraceptives can be developed that will reduce lifetime risk of all 3 cancers. The 'unopposed-estrogen hypothesis' accounts for endometrial cancer risk factors. Ovarian cancer risk is closely related to the total frequency of ovulation. The risk of breast cancer can be explained by an 'estrogen-plus-progestogen hypothesis'. On the basis of this analysis an hormonal contraceptive regimen has been developed consisting of a gonadotropin-releasing hormone agonist (GnRHA) plus continuous low-dose add-back estrogen and a short course of progestogen every fourth month. The total dose of add-back estrogen is estimated to be approximately 38% that in present-day low-dose combination-type oral contraceptives (COCs). The total dose of progestogen is approximately 15% that in COCs. This regimen prevents ovulation and should thus reduce ovarian cancer risk. It also reduces the exposure of the endometrium to unopposed estrogen, and the exposure of the breast to estrogen-plus-progestogen. It is estimated that use of such a regimen for 10 years will only reduce lifetime risk of endometrial cancer by one-sixth, but lifetime risk of ovarian cancer is estimated to be reduced by two-thirds, and lifetime risk of breast cancer is estimated to be reduced by one-half.