E. Strong (ed.). Gastric cancer. Principles and practice. Springer (2015)
HER2 testing is recommended in all patients with gastric cancers at the time of diagnosis. HER2 testing for gastric cancers is distinct from breast cancer immunohistochemistry (IHC) testing. Compared to breast cancer, esophagogastric cancers display unique immunostaining characteristics including a high incidence of tumor heterogeneity . In addition, because of the secretory nature of the gastric epithelium, intestinal type gland-forming carcinomas may show incomplete (basolateral or lateral only staining); and these are all considered as a positive result with IHC in addition to complete membrane staining. These differences have been taken into account when developing and validating the HER2 testing protocols specific for gastric cancers .
Hoffman et al. proposed the gastric cancer IHC scoring for gastric cancers, which was validated and subsequently used in the ToGA trial.
The current testing guidelines for esophagogastric cancers recommend that IHC should be the initial HER2 testing using validated assays [9, 10]. Samples with equivocal IHC scores of 2 + should be retested by fluorescence in situ hybridization (FISH) or other in situ methods. Cases with 3 + overexpression by IHC or FISH positive (ratio of HER2:CEP17 > 2.0) are considered positive. IHC 0–1 + are considered HER2 negative.
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