HER2 in gastric cancer: an immunohistochemical study on tissue microarrays and the corresponding whole-tissue sections with a supplemental fish study.

Since focal HER2 expression is an issue in GC, TMA construction from the paraffin-embedded surgically-obtained tissue may not reflect its real status. The aim of this study was to assess the HER2 status in tissue microarrays (TMAs) and the corresponding whole sections using HercepTest immunohistochemistry (IHC), and to correlate it and to assess the concordance of HER2 IHC and fluorescence in situ hybridization (FISH) in TMAs. Concordance of the HER2 expression status for 302 cases of gastric cancer using 9 paired TMAs was evaluated using a 2-mm core size and 305 corresponding whole sections. Concordance of the IHC and FISH HER2 status was compared. In addition,, the HER2 status was compared to clinicopathological characteristics and patients' survival. Using the whole-section approach, HER2 over-expression was found in 25.2 % (HER2 3+ 6.6 %, HER2 2+ 18.7 %) of tumours. The overall concordance of IHC between the cores and the whole section was 84.9 %; 15.1 % of the tumours showed HER2 amplification. The overall concordance of IHC and FISH on cores was 75.7 %. The level of amplification correlated with the IHC score. Relationship between the intestinal and papillary types and tumour grade was observed for tumours with over-expression and amplification, whereas tumour location was related only to over-expression. There was a statistically significant difference in the overall survival of the patients, which was related to HER2 amplification. In conclusion, good concordance of the IHC HER2 results between tissue cores in TMA and whole sections, and excellent concordance of the IHC and FISH results on tissue cores was found. At least a part of the observed IHC HER2 heterogeneity could very likely be explained by fixation artifacts. With adequate fixation, a higher concordance of IHC HER2 between the cores and the whole sections can be expected. The TMA approach could enable an easier analysis of more than one representative tumour block.