A bottom-up initiated digital external quality assessment scheme for the state-of-the-art pathology in Sweden: reduced variability between pathology departments

External quality assessment (EQA) schemes for pathology are essential, yet large/international programmes do not assess morphology-based biomarkers or address local/regional needs. This study outlines bottom-up initiated, flexible Swedish Digital Pathology EQA rounds for breast pathology, and presents results from the 2021 and 2023 rounds. Six breast carcinoma cases were selected for each EQA round by the Swedish Breast Pathology Expert Group (KVAST Breast). Whole tissue slides stained with HE, IHC, and ISH were anonymized, digitized, and uploaded to the digital EQA platform. Biomarkers were selected based on national registry data analysis and pathologist and clinician feedback. The 2021 round assessed Nottingham grade (NHG), oestrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), while the 2023 round focused on NHG, HER2-low, and global Ki67. Twenty-seven pathology departments participated. From 2021 to 2023, the variability of NHG assessment on digital slides improved from moderate to substantial (kappa 0.50; 95% CI 0.45-0.55 to 0.64; 95% CI 0.60-0.68), with better agreement for NHG3 than NHG1. Participants showed substantial and excellent agreement in ER (kappa 1) and PR (0.75 (95% CI 0.69-0.82). We found similar agreement in distinguishing HER2 IHC 0 (0.78; 95% CI 0.72-0.82) and HER2 IHC 3 + (0.94; 95% CI 0.88-1.00) from other HER2 IHC scores. Participants showed substantial agreement in detecting Ki67 high and Ki67 low cases (kappa 0.65; 95% CI 0.60-0.71 and 0.69; 95% CI 0.64-0.74, respectively). This digital EQA identifies local issues and complements large international EQAs to address challenges in the rapidly changing biomarkers of cancer therapy.