Background: The concordance rate between immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) for human epidermal growth factor receptor-2 (HER2) is still a major clinical concern. We describe a new criteria evaluating HER2 IHC.
Materials and methods: We expanded the equivocal category to include the American Society of Clinical Oncology and College of American Pathologists equivocal category and cases with 3+ between 30% to 90% of tumor cells and 2+ in ≥1% of tumor cells. Then, we prospectively evaluated 106 cases with FISH.
Results: There were 2 categories that were not required to be tested by FISH based on American Society of Clinical Oncology and College of American Pathologists criteria. The first category, cases with 2+staining between ≥1% and <10%. In this category, there were 15 cases, 2 (13.3%) had HER2 amplification by FISH. The second category, cases with 3+ staining between 30% and 90%. In this category, there were 16 cases, 5 (31.3%) were not amplified and 1 (6.3%) was borderline.
Conclusions: Expanding the equivocal category could increase the concordance rate between IHC and FISH with minimal additional cost.