The updated 2018 ASCO/CAP guideline on HER2 interpretation in breast cancer: Comparison with previous guidelines and clinical significance of the proposed ISH groups

Summary The aim of the study was to evaluate the impact of the updated 2018 ASCO/CAP guideline on HER2 interpretation in breast cancer as compared to the previous guidelines and also the significance of in situ hybridization (ISH) groups proposed by the updated guideline. HER2 ISH reports and immunohistochemistry (IHC) data from 1,348 invasive breast cancers diagnosed at a single institution were included in this study. HER2 IHC was re-assessed using the 2018 guideline, and HER2 ISH status was determined by the 2007, 2013, and 2018 guidelines. When applying the updated guideline, most of the HER2 ISH equivocal cases by the previous guidelines were reclassified as ISH negative, and 0.8% of HER2 ISH positive tumors by the 2007 guideline and 2.5% of those by the 2013 guideline were changed to ISH negative. Accordingly, negative HER2 ISH results significantly increased in the 2018 guideline compared to the 2013 guideline. HER2 ISH positive tumors in ISH group 3 (HER2/CEP17 ratio