Comparison of automated silver enhanced in situ hybridisation (SISH) and fluorescence ISH (FISH) for the validation of HER2 gene status in breast carcinoma according to the guidelines of the American Society of Clinical Oncology and the College of American Pathologists

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Abstract
HER2 is an important tumour marker in breast cancer. However, there is controversy regarding which method reliably measures HER2 status. This study evaluates the concordance between HER2 gene amplification in invasive breast cancer determined by fluorescence in situ hybridisation (FISH) and a new silver enhanced in situ hybridisation (SISH) technique. Ninety-nine cases were analysed by direct-labelled manual FISH (PathVysion®, Abbott/Vysis) and bright field automated SISH (INFORM®, Ventana). For comparison, all specimens were stained by immunohistochemistry (Dako-HercepTest™ and Ventana-PATHWAY®4B5). Evaluation was performed by five pathologists following the algorithms of the manufacturers and the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines. Concordance was calculated and the value of κ statistics estimated. Overall concordance between FISH and SISH was 96.0% (κ = 0.754, 95%CI). Discrepancies were mostly seen in tumours with intra-tumoural heterogeneity of HER2 amplification. In conclusion, HER2 gene copy status can be reliably determined by SISH. The 96% concordance with FISH fulfils the ASCO/CAP requirement of greater than 95% concordance for amplified vs non-amplified cases. There was a low inter-observer variability in the interpretation of SISH, suggesting that SISH is equally reliable in determining HER2 amplification as FISH. Because SISH combines bright field microscopy with molecular analysis and full automation, it appears to be particularly suited for routine application in surgical pathology.