The study also found that prognosis is similar for combined hormone therapy's users and nonusers, suggesting that death from breast cancer may be higher for hormone therapy users as well.
In the Women's Health Initiative (WHI) randomized trial, estrogen plus progestin was associated with an increase in both breast cancer incidence and mortality.
In order to determine the differences between the WHI trial and other observational studies, Rowan T. Chlebowski, M.D., Ph.D., Los Angeles Biomedical Research Institute (LA BioMed) lead researcher and colleagues, looked at postmenopausal women, who had no prior hysterectomy with negative mammograms within two years and were either users or non-users of estrogen and progestin combined therapy.
They found that breast cancer incidence was higher in estrogen plus progestin users than incidence in nonusers.
Women who started hormone therapy closer to menopause had a higher breast cancer risk with a weakening influence as the time from menopause increased.
The researchers wrote that because survival after breast cancer diagnosis did not differ between estrogen plus progestin users and nonusers, the higher breast cancer incidence of those using estrogen plus progestin may lead to increased breast cancer mortality on a population basis.
The study has been published in the Journal of the National Cancer Institute.