Background:  Breast cancer (BC) is the most frequently diagnosed type of cancer among women in Canada. BC is defined by different molecular subtypes; by the expression of estrogen receptors (ER), progesterone receptors (PR) and the human epidermal growth factor receptor-2 (HER2). Currently, Canada has no national cancer registry with epidemiology data by subtype. Thus, we conducted a study to determine the incidence, survival and clinicopathological characteristics of BC subtypes (triple negative breast cancer (TNBC), HER2+, and hormone receptor positive (HR+/HER2-) in Canadian women newly diagnosed with BC. Methods: Female patients diagnosed between April 1, 2012 and March 31, 2016 (fiscal 2012-2015) were identified in the Ontario Cancer Registry and individual patient data were linked to data in provincial health administrative databases. Descriptive statistics and Kaplan-Meier curves were generated. Results: In this cohort, 3,277 (9.5%) patients had TNBC, 4,902 (14.3%) had HER2+, and 22,247 (64.8%) had HR+ breast cancer. The annual incidence was 15 per 100,000 for the TNBC group, 21–23 per 100,000 for the HER2+ group, and 97–105 per 100,000 for the HR+ group. The lowest median overall survival (mOS) of only 8.9 months was observed among patients with clinical stage IV TNBC; in comparison, the median OS was 37.3 months and 35.2 months among women with HER2+ and HR+/HER2- metastatic BC, respectively. Conclusions: This study, the most recent and largest administrative database analysis of a Canadian population to date, reveals a subtype distribution consistent with previously reported data along with comparable annual incidence and overall survival patterns.

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