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may be influenced by pre-analytical issues as previously dis-
cussed. Hence, it is imperative that validation and optimization
are carried out in individual laboratories with strict quality con-
trol. Every test result should be interpreted with parallel positive
and negative controls. Figure 5 is an algorithm and a practical
guide for HER2 IHC interpretation.- Interpretation of in situ hybridization (ISH)
Absolute HER2 gene copy number and/or the ratio between
the HER2 gene copy number and centromeric probe 17 deter-
mine HER2 amplification (CEP17). The Trastuzumab for
Gastric Cancer (ToGA) study defined ISH-positive status for
gastric/esophagogastric junction adenocarcinoma as
HER2:CEP17 ratio 2.0 irrespective of the copy number [ 11 ].
The Gastric HER2 Testing Study (GaTHER) demonstrated
that gene amplification assessment based on HER2 copy num-
ber was more reproducible than assessment based on the
HER2/CEP17 ratio, concluding that single- probe CISH or
SISH is the optimal method for HER2 ISH testing in gastric/
esophagogastric junction carcinomas in Australia [ 47 ]. This
study provides evidence that HER2 copy number differentiated
between immunohistochemistry scores better than the ratio.
Others have again emphasized the importance of considering
the HER2 gene copy number in the final consideration of the
HER2 status [ 48 ].
Fig. 4 Lateral membranous staining with linear staining at contact sites between
two cells and basolateral membranous staining creating a U-shaped staining
pattern (×400)Duminda Subasinghe et al.