Interobserver and Interantibody Reproducibility of HER2 Immunohistochemical Scoring in an Enriched HER2-Low-Expressing Breast Cancer Cohort.

Our results demonstrated notable interobserver and interantibody variation on evaluating HER2 immunohistochemistry, especially in cases with scores of 0-1+, although the performance was much more improved among breast-specialized pathologists with the awareness of HER2-low concept. More accurate and reproducible methods are needed for selecting patients who may benefit from the newly approved HER2-targeting agent on HER2-low breast cancers.