Estrogen and combined estrogen-progestogen therapy in the menopause and breast cancer

Most of the data on menopausal hormone therapy (HT) and breast cancer risk available up to the mid-1990s were included in a collaborative reanalysis based on over 52,000 women with and 108,000 without breast cancer. HT increased the risk of breast cancer by about 2.3% per year of use. Subsequent studies have confirmed that breast cancer risk is elevated in current and recent (but not past) HT users and that the relative risk (RR) is higher for users of combined estrogen-progestin treatment than for users of estrogen only, and this higher RR is seen with various types of preparations and different routes of administration. With reference to intervention studies, information on combined HT derives from the Women's Health Initiative (WHI). After 7 years of follow-up, 166 breast cancer cases were recorded in the HT group, as against 124 in the placebo group, corresponding to a RR of 1.24. Data from two other, smaller, randomized studies are available. In a combined analysis of the three randomized trials, 205 cases of breast cancer were observed in the treated groups as against 154 in the placebo groups, corresponding to a pooled RR of 1.27. However, in the estrogen-only component of the WHI population, at 8 years of follow-up 94 cases were observed in the estrogen group, opposed to 124 in the placebo group (RR=0.77). The results recorded in the WHI and the Million Women Study do not confirm the suggestion that breast cancers in women using HT have a more favorable prognosis. HT has also been related to an increased risk of recurrent breast cancer.