Case 1: Molecular Testing for HER2+ Breast Cancer
Ruta Rao, MD: Let’s talk about a few points from this case that you presented. This tumor is HER2 [human epidermal growth factor receptor 2]–positive by immunohistochemistry [IHC], but HER2 amplified by FISH [fluorescence in situ hybridization]. I will start by asking Dr Sadimin to comment on this. How frequently do we see this, and what are the current testing guidelines that speak to this?
Evita Sadimin, MD: Let me start by explaining the difference of testing between IHC and FISH. In IHC, we stain tumor cells on the tumor tissue for the expression of the quantity of HER2 protein in the tumor cells. The scoring is based on the staining that we see, whether membrane staining is present, and then the intensity and the amount of tumor cells that are staining. For the grading, if we have more than 10% of tumor cells that are staining with a strong and complete membrane staining, then we call it 3+ positivity. If we have more than 10% of tumor cells showing weak to moderate complete membrane staining, then we grade it as 2+, which we refer to as equivocal. If the staining is not fulfilling either 1 of those 2, we consider it negative, with grade 1+ having some staining and grade 0 having no staining. As you can imagine, because the grading is the assessment of strength and amount of tumor cells staining, it is somewhat subjective. The strength of the staining itself can also be affected by fixation time and the clone of the antibody being used, with some antibodies being better than others.
On the other hand, doing the FISH test is a more objective process. We count fluorescent signals that correspond to the HER2 gene copy number and the number of the centromere CEP17, the gene copy number. We then take the ratio of the 2 numbers. While it is more objective, the guidelines, as we know, have changed. That refers to the cutoff of the ratio, and it continues to evolve.
The 2007 guidelines were different from the 2013 guidelines, and both were different from the 2018 guidelines. The interpretation of a majority of the cases remains the same, but we do have certain types of cases that fully change. For example, we have a small percentage of cases that, following the 2013 guidelines, we have interpreted as positive. But now following the 2018 guidelines, we have to interpret the case as negative. There is a lot of communication with the clinicians and with the treating team in terms of these results and how they have changed. If we have different results compared with previously, then we will discuss some retesting. If the results are different, it may be the difference of the guidelines that we are using.
Because IHC and FISH are testing different things, we use them together. Some institutions perform IHC first followed by FISH in equivocal cases, and others perform FISH first. The guidelines report as 5 different groups. If it’s a straightforward positive, or group 1 case, or a straightforward negative, or group 5 case, then we don’t do anything. If the results fall into groups 2, 3, or 4, then we refer to IHC. In our institution [Rutgers Cancer Institute of New Jersey], we’re now doing IHC first followed by FISH for equivocal cases. But we used to do FISH first followed by IHC several years ago.
Transcript edited for clarity.
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