Abstract 257: Estrogen receptor positive and negative breast cancer secular trends in Ireland (2004-2013)

Background: Estrogen receptor (ER) status in breast cancer is strongly associated with etiologic factors, prognosis and treatment response. Data suggest that population-based incidence rates of ER-positive (ER+) tumors are increasing and ER-negative (ER-) cancers have fallen over time in the United States (US), Denmark and Scotland. Whether these trends are consistent in other populations with different etiologic exposures and across ER±/HER2± subtypes is unknown. We, therefore, analyzed breast cancer secular trends in Ireland by ER expression; and for the first time, by joint expression of ER±/HER2±, a well-established clinical marker of uncertain etiological significance. Methods: This study included invasive breast cancers (n=24,845; 2004-2013) among women aged 20-84 years within the population-based National Cancer Registry of Ireland (97% coverage). The general population at risk (n=10,401,986) was obtained from the Irish Central Statistics Office. Using a validated approach to account for missing ER/HER2-status, we assessed ER± and ER±/HER2± secular trends with age-standardized incidence rates (ASR) and estimated annual percentage change (EAPC) with 95% confidence intervals (95%CI). Results: Over the 10-year follow-up period, 19,264 ER+ and 4,161 ER- breast cancers were diagnosed (1,420 were ER-unknown). ASRs increased significantly for ER+ (EAPC: 2.20%/year (95%CI: 0.97, 3.45%/year)) and decreased significantly for ER- cancers (EAPC: -3.43%/year (95%CI: -5.05, -1.78%/year)). The ER-specific trends were qualitatively similar among three age groups (