Dr. Press: A HER2-equivocal result isn't very helpful to either the pathologist or especially the clinician or the patient, because they are faced with a situation where they have to either treat with a HER2-targeted therapy or not treat with a HER2-targeted therapy. So these cases need to be resolved.
One of the ways of resolving it is to send the case out to a reference laboratory that routinely deals with these kinds of issues. Oftentimes, academic laboratories do so, as do many commercial laboratories. In the end, though, this has to be addressed.
The IHC 2+ [cases], of course, can be tested by FISH [fluorescence in situ hybridization] to determine whether they are HER2-amplified or not.
The equivocal range for FISH ratios between 1.8 and 2.21 is going to be revised to the FDA-approved criteria of a ratio of 2.0—either greater than or less than—to determine the HER2 status, in the ASCO-CAP [American Society of Clinical Oncology–College of American Pathologists] guidelines that will be coming out later this year. So this equivocal range for FISH is going to be removed, as will the FISH ratio of 2.2 that has been recommended for HER2 gene amplification.
Now those cases that show aneusomy—where there is an increase in the average copy of HER2, let's say above 4 or even in the greater than 6 copies of HER2 per tumor cell nucleus, but also an increase in the chromosome 17 centromere count of a similar nature, so that the ratio winds up being less than 2—suggests that this is an example of chromosome 17 aneusomy and not HER2 gene amplification. And of course, this also will depend a little bit on the way the fluorescent signal appears in each [of the] nuclei. Usually in HER2-amplified cases, the HER2 signals are grouped, even if they are relatively small amplicons of three signals, for example. They are often relatively geographically circumscribed in the tumor cell nuclei, so that this assessment can be made of a low-end amplified case, whereas in most cases it's a chromosome 17 aneusomy. [With chromosome 17 aneusomy,] usually the green chromosome 17 centromere signals are in proximity to the orange or red HER2 signals by FISH, and they are often randomly distributed throughout the nuclei so that the HER2 FISH signals tend not to be grouped.And in that setting, it's relatively easy and straightforward to make the distinction between aneusomy and low-end amplified cases. Our experience, which was published in 2010 demonstrated that those cases that had chromosome 17 aneusomy did not respond to HER2-targeted therapy, and we did not consider these cases to be candidates for this type of [HER2-targeted] treatment.2