Description:
This study examines the relationship between menopausal estrogen and estrogen-progestin replacement therapy and risk of breast cancer, focusing on whether associations differ according to whether the tumors are in situ or invasive. Data are from a prospective study conducted 1980-89 on 49,017 selected participants in the Breast Cancer Detection Demonstration Project, a five-year screening program conducted between 1973 and 1980 in the United States. Overall, the rate ratio for estrogen-only use compared with no-hormone use was 1.0, and that for the estrogen-progestin combination was 1.2 (95 percent confidence interval [CI] = 1.0-1.6). However, the associations differed according to whether the tumors were in situ or invasive. The rate ratios of in situ breast cancer associated with use of estrogens alone and the combination regimen were 1.4 (CI = 1.0-2.0) and 2.3 (CI = 1.3-3.9), respectively. Duration of estrogen-only use also was associated with risk of in situ tumors, with users for 10 or more years at twice the risk of nonusers (P-value for trend test = 0.02). Duration of use was not associated with risk of invasive cancer. Our results are consistent with the hypothesis that hormone replacement therapy is related to earlier-stage breast cancer; however, the possibility that the results reflect increased breast cancer surveillance among those taking hormones cannot be ruled out.