Hormone-replacement therapy may be associated with increased incidence of luminal breast cancer: Results from a large-scale health maintenance organization (HMO) analysis.

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e13599 Background: Trends in breast cancer (BC) incidence may be impacted by potentially competing variables (e.g., mammography rates, hormone-replacement therapy [HRT] use). Methods: This observational retrospective study examined trends/associations between BC incidence, mammography rates, and HRT use among female members of the Maccabi Healthcare Services (the second largest HMO in Israel).BC subtypes were determined based on therapies received by patients diagnosed after 2006 (following trastuzumab approval in the adjuvant setting). Results: Between 2002 and 2014, 14,092 BC cases (88% invasive, 12% in-situ) were identified. The age-adjusted incidence rate of invasive BC peaked in 2005, consistent with increased mammography screening that year, and decreased thereafter. HRT use among all female members aged≥45 years decreased from 13.2% in 2002 to 4.6% in 2014, consistent with the global trend after the Women's Health Initiative publication. Analysis by BC subtype involved 6,218 invasive BC patients diagnosed between 2007 and 2014 (luminal A, 47.5%; luminal B1 without human epidermal growth factor receptor 2 [HER2] over-expression, 25.7%; luminal B2 with HER2 over-expression, 7.7%; estrogen receptor [ER]-negative/HER2+, 4.9%; triple negative, 8.3%; unknown, 6.0%). Overall, 75-86% of patients across all subtypes did not have any HRT exposure vs 14-25% who were current users (within 1 year before the BC diagnosis), recent users (within 2-5 years), or past users ( > 5 years). Current and recent use of HRT was statistically significantly higher in luminal BC vs ER-negative tumors: rates in luminal A/B1/B2, 15.3%/12.1%/11.1% vs ER-negative HER2+/triple-negative/unknown, 8.9%/9.7%/7.7% ( P< 0.001). In BC patients (≥45 years) with HRT exposure, the preparations used were estrogen plus progesterone (62%), estrogen alone (24%), and tibolone (14%). In non-BC cases (≥45 years), the respective values were similar: 61%, 26%, and 13%. Conclusions: HRT current/recent exposure may contribute to increased incidence of luminal BC tumors.