HER2 status by standardized immunohistochemistry and silver-enhanced In Situ hybridization in Korean breast cancer

Purpose: Amplification of the human epidermal growth factor receptor 2 (HER2) gene occurs in 18% to 20% of breast cancers, and it is recognized as a prognostic and predictive marker. We investigated the HER2 status in Korean breast cancer by immunohistochemistry (IHC) and silver-enhanced in situ hybridization (SISH), as the first step toward building a nationwide quality assurance program for HER2 testing. Methods: A total of 1,198 breast carcinoma samples were collected from six institutions and IHC and SISH were performed using tissue microarrays in central laboratories. The results were compared to those of local laboratories. Results: Available data were obtained from 959 samples. Central IHC results were negative, equivocal, and positive for 756 (78.8%; range among institutions, 76.8-81.8%), 37 (3.9%; 1.9-6.2%), and 166 (17.3%; 13.6-20%), respectively. SISH results were negative, equivocal, and positive for 756 (78.8%; 77.4-79.9%), 2 (0.2%; 0-0.7%), and 201 (21%; 20.1-22.2%), respectively. HER2 gene amplification was observed in 4.4%, 19%, and 73.9% of the negative, equivocal and positive groups stratified by local IHC results, respectively. When central SISH was considered to be the gold standard method for measuring HER2 status, the false-negative and false-positive rates of local IHC were 14.4% (29/201) and 7.1% (54/756). The concordance rate between central IHC and SISH was 98.4%. Conclusion: Central IHC and SISH markedly decreased the interlaboratory variability of HER2 status and the results of the two were highly concordant. The quality control program for HER2 testing must be focused on decreasing both the false negativity and positivity of IHC in local laboratories.