Indian Data on HER2 Fluorescence In Situ Hybridization in Invasive Breast Cancer with Immunohistochemically Equivocal Results As Per 2018 ASCO/CAP Guidelines

Introduction Hormonal status and HER2 expression are valuable biomarkers and dictate the management of the patients diagnosed with invasive breast cancer (IBC). It is crucial to identify the patients who truly respond to anti-HER2 targeted therapy. Updated 2018 American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines has recommended certain modifications in HER2 interpretation by fluorescence in situ hybridization (FISH) with concomitant immunohistochemistry (IHC). Objectives We aimed to evaluate HER2 FISH interpretation in IBC with equivocal IHC results as per 2018 ASCO/CAP recommendations and compare FISH results with hormonal receptor status. Materials and Methods FISH results of 502 cases of IBC with equivocal IHC report between January 2016 to January 2022 were reviewed retrospectively. FISH results were categorized according to ASCO/CAP guidelines 2018 into five respective groups. Results FISH testing in IHC equivocal cases showed 219 (43.6%) cases were classic amplified (positive) belonged to group 1, 217(43.2%) cases were classic nonamplified (negative) fell into group 5, 39 (7.8%) and 02 (0.4%) patients were in group 2 (negative) and group 3 (positive), and 25 (5.0%) cases were in group 4 (negative). About 52.1 and 49.3% of cases with estrogen receptor and progesterone receptor positivity were reported as HER2 positive. Among 502 cases, 25 equivocal cases according to the 2013 guidelines were redefined as HER2 negative and 02 (0.4%) cases reported positive were classified negative as per updated 2018 guidelines. Conclusion Revised 2018 guidelines is helpful in accurate identification of HER2 status and in avoiding targeted therapy in unwarranted cases. Updated 2018 guidelines has removed equivocal HER2-FISH category that has eliminated management dilemma in these cases. Only long-term clinical follow-up will establish the validity of the updated guidelines.