HER2 expression in breast cancer: Comparisons of fluorescence in situ hybridization and silver in situ hybridization, and immunohistochemical staining using monoclonal antibody and polyclonal antibody

Background and aim: The human epidermal growth factor receptor 2 (HER2) gene is an important tumor marker in breast cancer. The silver in situ hybridization (SISH) has been recently introduced for measuring the HER2 amplification status. We evaluated the concordance between HER2 gene amplification in invasive breast cancer as determined by the fluorescence in situ hybridization (FISH) and SISH techniques and we compared the results to that of immunohistochemical (IHC) staining with using polyclonal c-erbB-2 antibody and monoclonal HER2 antibody. Methods: A total of 90 cases were analyzed by direct-labeled manual FISH and bright field automated SISH. All the specimens underwent IHC staining using c-erbB-2 and PATHWAY 4B5. The evaluation was performed by following the recommendation of the manufacturer and the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines. Results: The overall concordance rate between FISH and SISH was 98.8% (Kappa index: 0.96). There was no significant difference in estimating the HER2 status using polyclonal and monoclonal antibody. Conclusions: The 98.8% concordance of FISH fulfills the ASCO/CAP requirement of greater than 95% concordance for the amplified cases versus the non-amplified cases. The HER2 gene copy status can be reliably determined by SISH.