Evaluation of analytical accuracy of HER2 status in patients with breast cancer

Human epidermal growth factor receptor 2 (HER2) gene status and overexpression, occurring in ~ 13.6% of primary breast cancers, is essential for identifying patients likely to benefit from biological treatment. In this method of evaluation study, we tested and compared the HER2 gene–protein assay (GPA) with routine HER2 immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). The GPA was evaluated using 67 formalin-fixed paraffin-embedded (FFPE) HER2 equivoval IHC (2+) breast cancer tissue samples. Overall, agreement between GPA silver in situ hybridization (SISH) and FISH was 91.9% (57/62). Regression analysis revealed slightly higher, but non-significant difference in HER2/chromosome enumeration probe 17 (CEP17) ratio for GPA as compared to FISH (p = 0.074). Intraclass correlation coefficients (ICCs) of 0.94 and Spearman´s rank correlation coefficients of 0.93 (p < 0.0001) for FISH and GPA SISH suggested strong inter-observer association for methods with one observer counting on average 0.23 significant higher for GPA SISH (p = 0.014). Intra-observer IHC method reproducibility was 52.6% (κ = 0.3122, p = 0.004) and 79.7% (κ = 0.6428, p = 0.9197), suggesting fair significant and substantial non-significant difference between tests for reviewers. Inter-observer reproducibility for IHC methods was 53%. While inter-observer reproducibility for experienced IHC interpretation suggested significant differences (κ = 0.3636, p = 0.0332), unexperienced interpretation of IHC GPA suggested fair non-significant difference between reviewers (κ = 0.3101, p = 0.0747). Using FISH as reference, the diagnostic indices for GPA SISH were as follows: sensitivity 100%, specificity 95% and accuracy 92%. Inaccuracy between tests was in 80% of cases due to ISH categorization as equivocal by one of the methods. IHC results highlight that it may be beneficial with a method for simultaneously visualization of HER2 gene and protein status.