Concordance between human epidermal growth factor receptor 2 (HER2) status by immunohistochemistry (IHC) and next-generation sequencing (NGS) in tissue and blood samples from gastric and gastroesophageal junction cancers (GC/GEJC).
Authors: Steven Brad Maron, Kathleen Burke, Pooja Gupta, Kristen Sarlo, Marija Tesic-Schnell, Michele Sue-Ann Woo, Samuel J. Klempner
Journal: Journal of Clinical Oncology (2025)

478 Background: An estimated 18% of patients (pts) with GC/GEJC have HER2-positive (HER2+; IHC 3+ or 2+/in situ hybridization–positive [ISH+]) tumors. Although IHC/ISH are the standard testing methods for determining HER2 positivity, NGS can identify HER2 ( ERBB2 ) amplification alongside other biomarkers. This study assessed the concordance of detecting HER2 amplification by tissue/plasma NGS with IHC/ISH HER2 status in pts with GC/GEJC. Methods: Pts were retrospectively identified in the Tempus database. The primary analysis assessed tissue and plasma NGS HER2 copy number (CN) concordance with a HER2+ (IHC 3+ or 2+/ISH+) or HER2− (IHC 0, 1+, or 2+/ISH−) status; NGS samples were collected in ≤30 days of the IHC sample collection date. HER2 amplification was defined as tissue CN ≥8 or plasma log2(CN) ≥0.5; additional amplifications were captured by manual inspection. A secondary analysis assessed the correlation between paired tissue and plasma HER2 log2(CN) in pts with plasma and tissue NGS samples collected ≤30 days apart. The secondary analysis included pts irrespective of IHC sample collection date. Results: Of 1578 pts with GC/GEJC and a curated IHC score evaluated in the primary analysis, 1507 and 305 pts underwent tissue and plasma NGS testing, respectively. Among pts with tissue and plasma NGS results, 902 (59%) and 201 (66%) had primary tumors of the stomach, respectively; all remaining pts had primary tumors of the cardia.According to IHC/ISH, 229 (15%) and 40 (13%) pts were HER2+ among those with tissue and plasma NGS testing, respectively. Of the pts identified as HER2 amplified by tissue and plasma NGS,95% and 92% were HER2+ by IHC/ISH, respectively. Positive percent agreement (PPA) of HER2 amplification by tissue and plasma NGS with IHC/ISH HER2+ status was 70% and 58%, respectively. Concordance rates by PPA, negative PA (NPA), and overall PA (OPA) are summarized in Table. In the secondary analysis, a positive correlation in NGS HER2 log2(CN) was observed in pts with paired tissue and plasma NGS (mutant variant allele frequency &gt;5%; R=0.85; P&lt;2.2e−16; n=307). Conclusions: Based on PPA, we observed moderate concordance between NGS HER2 amplification detection (plasma or tissue) and standard HER2 IHC assessment. NGS is not sufficient to detect all pts with HER2+ GC/GEJC. Further testing is warranted to determine if NGS testing could complement IHC/ISH in identifying pts who may benefit from HER2-directed therapies. IHC 3+ or 2+/ISH+ (HER2+), n IHC 0, 1+, or 2+/ISH− (HER2−), n Total Total 229 1120 Tissue HER2 amplified, n 160 8 168 Tissue HER2 not amplified, n 69 1112 1181 PPA: 69.9% NPA: 99.3% OPA: 94.3% IHC 3+ or 2+/ISH+ (HER2+), n IHC 0, 1+, or 2+/ISH − (HER2 − ), n Total Total 40 233 Plasma HER2 amplified, n 23 2 25 Plasma HER2 not amplified, n 17 231 248 PPA: 57.5% NPA: 99.1% OPA: 93.0%