Next‐Generation Assessment of Human Epidermal Growth Factor Receptor 2 (ERBB2) Amplification Status in Invasive Breast Carcinoma: A Focus on Group 4 Using the 2018 ASCO/CAP HER2 Testing Guideline

AIMS The American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) updated the testing guideline in 2018 to address issues from uncommon HER2 fluorescence in situ hybridization (FISH) results according to the 2013 guideline. Next-generation sequencing (NGS) may be used to better classify patients. We aim to assess ERBB2 amplification status of invasive breast carcinoma with equivocal HER2 immunohistochemistry (IHC) results using NGS, focusing on Group 4 (HER2/CEP17 Ratio<2.0; Average HER2 Signals/cell ≥4.0 and <6.0). METHODS AND RESULTS We retrospectively reviewed HER2 FISH and NGS data of HER2 IHC-equivocal breast carcinomas at our center between January 2009 and September 2019, wherein all three assays were performed on the same tissue block, and compared HER2 FISH results, according to 2018 ASCO/CAP guideline, and ERBB2 amplification status by NGS. A total of 52 HER2 FISH and NGS results from 51 patients with HER2 IHC-equivocal breast carcinomas were reviewed. The cohort included 8 cases classified as 2018 ASCO/CAP ISH Group 1, 3 as Group 2, 3 as Group 3, 14 as Group 4 and 24 as Group 5. Thirteen of 14 (92.9%) Group 4 (HER2 negative) cases were classified as ERBB2 nonamplified using NGS; the discordant case was later classified as Group 1 with alternate sample FISH testing. NGS revealed no significant difference in somatic mutations or copy number alternations between Groups 4 and 5. CONCLUSIONS Our NGS findings support the reclassification of HER2 FISH-equivocal cases as HER2 negative under the 2018 ASCO/CAP guideline.