Analytical validation of human epidermal growth factor receptor 2 immunohistochemistry by the use of the A0485 antibody versus the 4B5 antibody and breast versus gastric scoring guidelines in ovarian clear cell carcinoma.

AIMS: The aim of this study was to evaluate human epidermal growth factor receptor 2 (HER2) immunohistochemistry (IHC) in ovarian clear cell carcinoma (OCCC) by using two antibodies and two scoring guidelines in correlation with HER2 amplification and clinicopathological features. METHODS AND RESULTS: A tissue microarray was constructed by use of a total of 71 OCCC cases for IHC (the A0485 antibody and the 4B5 antibody) and dual-colour silver in-situ hybridisation (DISH). Two pathologists independently scored the IHC according to the 2018 American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) breast cancer guidelines (breast guidelines) and the 2016 ASCO/CAP gastro-oesophageal adenocarcinoma guidelines (gastric guidelines). IHC concordances between A0485 and 4B5 were 87.3-93.0%. Three to 16 (4.2-22.5%) cases had an IHC score of 2+/3+ with frequent basolateral/lateral membranous staining. The 4B5 antibody yielded fewer IHC 2+ cases than the A0485 antibody (n = 2-6 versus n = 5-12). Five (7.0%) cases had HER2 amplification as determined with DISH. Cases with papillary-predominant growth patterns were significantly more likely to have HER2 amplification (P = 0.0051). In predicting DISH results, IHC scored according to the gastric guidelines yielded 100%/100% sensitivity and 83.3-95.5%/98.2-100% specificity, and IHC scored according to the breast guidelines yielded 60-80%/33.3-66.7% sensitivity and 95.5-100%/100% specificity (including/excluding IHC 2+ cases). One case had intratumoral heterogeneity, with discordant results between primary and metastatic tumour specimens. CONCLUSION: We demonstrated HER2 amplification in 7% of OCCC cases, and the molecular change is significantly associated with papillary-predominant growth patterns. In predicting HER2 amplification, a combination of 4B5 IHC and gastric guidelines provides the best sensitivity and fewer equivocal (IHC 2+) cases. Given the intratumoral heterogeneity, assessment of HER2 status on whole tissue sections and on both primary and metastatic tumour specimens is recommended.