Background: Screening mammography and invasive breast cancer and ductal carcinoma in situ (DCIS) rates recently declined in the United States; screeningmammography declines among former hormone therapy (HT) users may be an important contributor. We longitudinally examined women and compared mammography use and cancer rates by HT use [current, former, and never users of estrogen + progestin (EPT) and estrogen only (ET)]. Methods:Westudied 163,490 unique women aged 50-79 years enrolled in Group Health (Washington State) between 1994-2009. Electronic data identified HT dispensing, mammography use and incident breast cancer diagnosis. We calculated age-adjusted screening compliance as a time-varying variable (screened-within-the-past-26 months, yes/no). Results: Before 2002, screening compliance differed significantly by HT with current EPT users having the highest rates (83%) followed by former EPT (77%), current ET (77%), former ET (72%), and never users (56%). After 2002, screening was high (∼81%) among current and former EPT and ET users and significantly increased among never users (∼62%). Invasive breast cancer rates significantly decreased over the whole study period (P trend≤0.05) for all HT users, except EPT current users (P trend=0.68); DCIS rates did not change in any group. Conclusions: Differential screening mammography rates by HT use do not explain invasive breast cancer incidence declines. Our data suggest discontinuing HT has an immediate effect on breast cancer rates, lending support to the mechanism that cessation leads to tumor regression. Impact: Studies examining the influence of a changing exposure in relation to outcomes should account for varying exposures, individuals' characteristics, as well as screening methods and frequency.
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