Comparison of four immunohistochemical tests and FISH for measuring HER2 expression in gastric carcinomas

: Accurate testing for HER2 is essential to identify patients who are likely to benefit from trastuzumab therapy. Although comparative studies on the performance of commercially available immunohistochemistry (IHC) antibodies for HER2 have been performed in breast cancer, similar studies have not been done in gastric carcinoma (GC) thus far.

: Quadruplicate tissue microarrays (TMAs) of 289 advanced GCs were immunostained with HercepTest, A0485, 4B5 and CB11, and the results were compared to those of gene amplification by fluorescence in situ hybridisation (FISH).

: HER2 was amplified by FISH in 38 (13.2%) GCs. HER2 overexpression (2+ or 3+) was observed in 40 (13.8%) cases by HercepTest, 46 (15.9%) by A0485, 40 (13.8%) by 4B5 and 27 (9.3%) by CB11. The sensitivity and specificity of IHC compared to FISH was 78.9%/96% for HercepTest, 86.5%/94.4% for A0485, 76.3%/95.6% for 4B5 and 60.5%/98.4% for CB11. Focal HER2 gene amplification was more common in GCs with no (0 and 1+) or low (2+) HER2 expression. Discordant IHC results were seen in 23 cases (8.0%) with the four antibodies. All HER2 3+ cases (n = 22) by HercepTest were positive with A0485 and 4B5, while one was negative with CB11. CB11 was found to be negative in 10 HercepTest 2+ and FISH+ cases.

: Focal HER2 gene amplification is more common in GCs with no or low expression of HER2. Although the specificity and positive predictive value of CB11 for FISH is highest, it lacks sufficient sensitivity to be used as a first line screening test to assess HER2 status in GCs.