Abstract
PURPOSE: This study aimed to assess the concordance of human epidermal growth factor receptor 2 (HER2) expression scoring by immunohistochemistry (IHC) among practicing pathologists in Japan, given the challenging nature of scoring and the critical role of HER2 status in breast cancer management. METHODS: Whole slide images (WSI) from 20 invasive breast cancer cases (1 representative WSI per case) selected to represent a diverse IHC scores and staining patterns were used in an online survey involving seven reference pathologists who established consensus HER2 IHC scores (0 to 3 +) decided by majority interpretation. Participating pathologists nationwide scored the same 20 WSI cases online using the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) 2018 guidelines. Deidentified case metadata were registered in the uPath system. RESULTS: A total of 144 participating pathologists responded. The scoring results of the participating pathologists most commonly agreed with the consensus IHC score, followed by a ± 1 point deviation and no survey responses with > 1 point deviation. The mean percentage of agreement with the consensus score for all 20 cases was 63.4%. In cases where the reference pathologists' scores were discordant, the participating pathologists also showed a lower concordance rate. CONCLUSION: This study highlighted the current status of HER2 expression scoring by IHC for breast cancer among pathologists in Japan. These findings underscore the challenges in HER2 IHC scoring cases and emphasize the need for improved standardization and training, especially in the evolving landscape of HER2-targeted therapies.