Discordance in HER2 status between primary gastric adenocarcinoma tumors and cells from the corresponding malignant effusions

Metastasis of gastric cancer commonly manifests as a malignant effusion, which presents an alternative cell source for human epidermal growth factor receptor 2 (HER2) status identification. This study aimed to compare HER2 status in primary gastric adenocarcinoma tumors and corresponding cell blocks prepared from malignant effusions (CB-MEs). HER2 status was retrospectively evaluated by immunohistochemistry (IHC) in primary gastric adenocarcinomas and paired pathologically confirmed CB-MEs of 45 patients. Silver in situ hybridization (SISH) was also performed in cases with IHC 2+ for primary gastric adenocarcinomas and above IHC 1+ for CB-MEs. HER2 positivity was observed in 4.4% (2/45) of primary gastric adenocarcinomas and 6.7% (3/45) of CB-MEs. The HER2 concordance rate between primary gastric adenocarcinomas and CB-MEs was 88.9% (40/45) (κ = − 0.056). All five patients with HER2 positivity in the primary tumor or a CB-ME had a negative result in the corresponding paired sample. Of the 15 patients with two or more serially sampled CB-MEs, HER2 expression determined by IHC differed between each CB-ME in six (40%) patients, and all three patients with HER2 positivity in CB-MEs exhibited HER2 positivity in one of the serially sampled CB-MEs. The HER2 positivity rate was very low in gastric cancer patients with malignant effusions. Our results suggest that HER2 positivity was discordant between the primary gastric adenocarcinoma and corresponding CB-MEs and among serially sampled CB-MEs. The possibility of detecting HER2 positivity can be improved if the primary gastric adenocarcinoma tumor as well as all the available CB-MEs from each patient are analyzed.