(008) Breast cancer incidence by subtypes - a Danish register-based study from 2012 to 2022.

Background: Breast cancer is a heterogeneous disease with multiple intrinsic tumor subtypes commonly grouped into four major subtypes; ER+/HER2-, ER+/HER2+, ER-/HER2+, and ER-/HER2-. Each subtype has a different prognosis, choice of treatment, and response to treatment. To our knowledge there is no existing overview of the annual incidence of breast cancer by subtype in Denmark based on register data. Information on the number of patients by subtype is crucial to understand specific treatment trajectories and predicted survival.

Objectives: The objective was to evaluate the extent to which Danish nationwide register data are applicable for identifying breast cancer incidence by subtype and switches between subtypes among all Danish women diagnosed with breast cancer from 2012 to 2022.

Methods: Breast cancer diagnosis and information on hormone receptors were retrieved from The Danish National Patient Register and The Danish National Pathology Register. Based on the hormone receptors, we operationalized the following four subtypes of breast cancer: ER+/HER2-, ER+/HER2+, ER-/HER2+, and ER-/HER2-. All others were categorized as Inconclusive. To do this, we applied a methodological approach that considered the structure of the registries whilst aiming at representing the clinical settings.

Results: For the patients’ first subtype we identified 3.4% ER-/HER2+, 63.9% ER+/HER2-, 7.5% ER+/HER2+, 8.4% ER-/HER2-, and 13.3% as Inconclusive. This distribution diverged from the existing literature. We identified switches for all subtypes between the first and the second pathological measurement within the first 60 days, particularly from Inconclusive to ER+/HER2-.

Conclusions: This study provides an understanding of the distribution of subtypes for incident breast cancer patients in Denmark using the nationwide registers. Performing an exact calculation of the number of incident patients by subtypes is complicated due to the heterogenetic nature of the disease. Exploring additional methodological approaches are needed to enhance the identification of breast cancer patients by subtypes.