Tumor Heterogeneity Index to Detect Human Epidermal Growth Factor Receptor 2 Amplification by Next-Generation...

Intratumoral heterogeneity of human epidermal growth factor receptor 2 (HER2) is common in gastric cancer (GC). However, a direct comparison between the results of HER2 immunohistochemistry (IHC) and next-generation sequencing (NGS)-based cancer panel tests has not been explored in GC. We aimed to determine optimal thresholds of HER2 overexpression to be detected by NGS with the data of 168 metastatic GC cases with known expression levels of HER2 by IHC and the copy number alteration of ERBB2 obtained by NGS test by applying tumor heterogeneity index (THI) and receiver operating characteristic curve. GC tissues were obtained via biopsy (N = 103) and gastrectomy (N = 65). HER2 3+ by IHC was observed in 27 cases (16%), and 14 of 27 HER2 IHC 3+ cases (52%) showed intratumoral heterogeneity (<90% of tumor cells are HER2 3+). Of 27 HER2 3+ cases, 19 (70%) were detected by NGS. All eight cases with discrepant IHC and NGS results harbored intratumoral heterogeneity. The receiver operating characteristic curve analysis showed that the optimal value of THI to be detected by NGS was a score of 72 for biopsy specimens and 120 for resection specimens. In conclusion, intratumoral heterogeneity of HER2 expression is observed in 52% of metastatic GC cases. NGS laboratories should be aware that accuracy of ERBB2 copy number alteration detection by NGS is influenced by the THI.