Abstract LB-60: Long-term use of hormone therapy and breast cancer incidence and mortality

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Abstract Background: The risk of breast cancer associated with longer-term use of both estrogen (ET) or estrogen + progesterone (E+P) hormone therapy (HT) cannot be determine from existing randomized controlled trials. Methods: The Nurses’ Health Study included 121,700 female registered nurses aged 30-55 in 1976 who have been followed with biennial questionnaires to medication use, risk factors and cancer incidence. Only postmenopausal women were included in this analysis. The main outcome was incidence of invasive breast cancer. We additionally evaluated the risk of fatal breast cancer (breast cancer diagnosis that resulted in breast cancer death). Results: During 1.57 million person-years of follow-up from 1980 through 2008, 5631 incident invasive breast cancers were diagnosed and 884 died of breast cancer. Breast cancer risk increased with duration of current E+P use without evidence for a plateau (e.g. RR 1.88 (95% CI 1.64-2.16) for 10-14.9 years and RR 2.35 (95% CI 1.90-2.92) for 15-19.9 years of use). More than 10 years of current ET use was also associated with increased breast cancer risk (RR 1.22 (95% CI 1.06-1.40) for 10-14.9 years and RR 1.43 (95% CI 1.22-1.68) for 15-19.9 years of use). Past E+P was also weakly associated with breast cancer risk. When the population was restricted to the same entry criteria as the Women's Health Initiative trial, we also observed a non-significant deceased risk of breast cancer HT with current ET use similar in magnitude to that seen in the trial (RR 0.61 (95% CI 0.63-1.05) for < 5 years and RR 0.93 (95% CI 0.76-1.14) for 5-9.9 years), but still observed an increased risk with current ET users of 15-19.9 years (RR 1.30 (95% CI 1.07-1.59). Effects were stronger for ER positive tumors and among thinner women. Results were similar when limited to women who underwent regular screening. Although breast cancer incidence was increased in these groups, there was no increase in the risk of fatal breast cancer with either E+P or ET use (RR 1.05 (95% CI 0.67-1.68) and RR 1.08 (0.80-1.46) for 10-19.9 years of current use respectively). Conclusions: Both past and current E + P users and current long-term ET users were at increased breast cancer risk. We did not observe an increase in fatal breast cancer with HT use. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-60. doi:1538-7445.AM2012-LB-60