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A discrete signal is counted as a single copy of HER2 or

Chr17. SISH signals (black) are typically smaller and more dis-

crete in appearance than Red ISH signals (red), due to differ-

ences in target sizes (Fig. 3 ). A cluster is defined as multiple

overlapping SISH signals in the nuclei that cannot be individu-

ally discerned. During SISH interpretation process, a small clus-

ter may be counted as six signals, and larger clusters as 12 signals

or more (Fig. 6 ). Based on the recent evidence HER2 copy

number differentiated between IHC scores better than the

ratio, consideration should be given to the copy number (≥6)

when IHC-negative cases appear amplified by the ratio only.

IHC score = “3+” cases with a copy number > 6 will essentially

HER 2 assessed by a validated IHC assay

Controls and test slides show appropriate staining

(at least 5 cohesive

tumour cells in

biopsies)

Moderate/weak

complete or

basolateral

membranous staining

(at least 5 cohesive

tumour cells in

biopsies)

Strong complete or

basolateral

membranous

staining of at least

No membranous

staining or staining

only

in rare cells

(at least 5 cohesive

tumour cells in

biopsies)

Staining is weak or

detected in only one

part of the

membrane

Fig. 5 An algorithm and a practical guide for interpretation of immunohistochemical staining for HER2 in
esophageal adenocarcinoma

HER2