Table 3.

Assessment of HER2 heterogeneity in breast cancer

Suggestions
The pathologist should scan entire HER2 ISH slide before counting.
Review of HER2 IHC slide is helpful to find areas with potential HER2 amplification
From this point of view, CISH or SISH has an advantage to evaluate HER2 heterogeneity, because it can be easily matched with HER2 IHC slide under light microscope.
If there is a subpopulation of tumor cells with HER2 amplification comprising > 10% of tumor cells on the slide, a separate counting should be performed within the subpopulation.
The HER2/CEP17 ratios or HER2 gene copy number should be calculated for both amplified and non-amplified areas separately.
If possible, it is recommended that in situ hybridization report includes proportion of amplified cells within a tumor.

HER2, human epidermal growth factor receptor 2; ISH, in situ hybridization; IHC, immunohistochemistry; CISH, chromogenic in situ hybridization; SISH, silver in situ hybridization; CEP17, chromosome enumeration probe 17.