Hormone Replacement Therapy in Relation to Breast Cancer

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Abstract
The possible association between use of estrogen replacement therapy or combined estrogen-progestin replacement therapy and the incidence of breast cancer has been assessed in numerous studies.1-28 The Collaborative Group on Hormonal Factors in Breast Cancer pooled and reanalyzed the data from most of these (52 705 women with breast cancer, 108 411 women without breast cancer).29 They reported a modest increase in the risk of breast cancer associated with ever use of estrogen replacement therapy (relative risk [RR], 1.14; P<.001), with evidence of an increasing RR with increasing duration of use (P = .003). The risk of breast cancer was increased among current users (RR, 1.21; P<.001), but not among past users (RR, 1.07; P = .10). They also found that, among women whose duration of current combination therapy was more than 5 years, the risk appeared to be increased relative to never users, but the estimate was imprecise due to small numbers (RR, 1.52; 95% confidence interval [CI], 0.80-2.92). In addition, there is recent evidence that use of hormone replacement therapy (HRT) may differentially affect the incidence of lobular cancer relative to other types of breast cancer.30,31