Use of menopausal hormone therapy and risk of ductal and lobular breast cancer among women 55-74 years of age.

The Women's Health Initiative (WHI) randomized trials found that use of combined estrogen and progestin menopausal hormone therapy (CHT) increases breast cancer risk, but use of unopposed estrogen hormone therapy (EHT) does not. However, several questions regarding the impact of hormone use on risk of different types of breast cancer and what thresholds of use confer elevations in risk remain. We conducted a population-based case-control study among women 55-74 years of age to assess the association between menopausal hormone use and risk of invasive ductal and invasive lobular breast carcinomas. Associations were evaluated using polytomous logistic regression and analyses included 880 ductal cases, 1,027 lobular cases, and 856 controls. Current EHT and CHT use were associated with 1.6-fold [95 % confidence interval (CI): 1.1-2.2] and 2.3-fold (95 % CI: 1.7-3.2) increased risks of lobular breast cancer, respectively, but neither was associated with risk of ductal cancer. Lobular cancer risk was increased after 9 years of EHT use, but after only 3 years of CHT use. Evidence across more than a dozen studies indicates that lobular carcinoma is the type of breast cancer most strongly influenced by menopausal hormones. Here, we characterize what thresholds of duration of use of both EHT and CHT that confer elevations in risk. Despite the rapid decline in hormone therapy use the WHI results were published, study of the hazards associated with these medications remains relevant given the estimated 38 million hormone therapy prescriptions that are still filled in the United States annually.