BReast cancer and benefits of estrogen therapy
Observational epidemiological studies have consistently reported increased breast cancer risk associated with postmenopausal hormone therapy (HT) use. However, Women’s Health Initiative (WHI) results indicate that breast cancer risks differ between estrogen plus progestin, and estrogen alone. There is limited research on the relationship between estrogen dose, route of delivery, and formulation (bio-identical vs synthetic) and the risk of breast cancer.
In the WHI clinical trials (WHI-CT), breast cancer risk during the intervention phase (ended March 31, 2005) was increased in the estrogen plus progestin arm (hazard ratio [HR] 1.25. 95% confidence interval [CI] 1.07-1.46), but not in the estrogen-alone arm (RR 0.77, 95% CI 0.57-1.01) In follow up analysis of the health outcomes from the estrogen-alone trial after 10.7 years (including a median 5.9 years for the intervention phase), women on estrogen therapy alone had a significantly lower breast cancer risk compared with those on placebo (HR 0.77, 95% CI 0.62-0.95) Furthermore, in women who developed breast cancer, the risk of dying was lower in women who took estrogen compared with those on placebo. More recently, breast cancer mortality results for women randomized to conjugated equine estrogen (CEE)-alone were found to have statistically significant lower mortality rates after 18 years in follow-up versus placebo (HR 0.55, 95% CI 0.33-0.92). It is Unknown whether the lower breast cancer risk in the estrogen-alone trial extends to other doses, routes of delivery