Comparison of silver-enhanced in situ hybridization and fluorescence in situ hybridization for HER2 gene status in breast carcinomas

Purpose: Silver-enhanced in situ hybridization (SISH) is a newly developed method to evaluate HER2 gene amplification in invasive breast carcinomas. Most laboratories widely use fluorescence in situ hybridization (FISH) to evaluate the HER2 gene amplification status because FISH is a very sensitive and accurate technique. However, this technique is not the best because it requires specialized equipment and interpretation skills. We compared a new technique of SISH with FISH for assessing HER2 gene amplification in invasive breast carcinomas. Methods: HER2 gene amplification was assessed in 165 cases of invasive breast carcinoma by FISH and SISH with constructing a tissue microarray. The tumors were assessed by the guidelines of the American Society of Clinical Oncology/College of American Pathologists (ASCO/ CAP). Positivity was defined as a HER2/Chromosome 17 ratio greater than 2.2. Negativity was defined if the ratio was less than 1.8. The tumor was considered as equivocal for HER2 gene amplification if the ratio was between 1.8 and 2.2. The HER2 protein status was assessed. Immunostaining for HER2 protein was performed in a Benchmark automatic immunostaining device with using whole tissue sections. Results: There was agreement of the HER2 gene amplification status by SISH and FISH in 162 of 165 cases, which is a concordance rate of 98.2% (κ=0.94). There were three discrepant cases, with two of them being FISH positive and SISH negative (one case was IHC negative and one case was IHC positive) and one case was FISH negative and SISH equivocal. Conclusion: The 98.2% concordance between FISH and SISH meets the ASCO/CAP requirements for test validation of >95% concordance. These results indicate that SISH can be used as an alternative to FISH for assessing the HER2 gene amplification status in breast carcinomas.