How do you approach a breast cancer patient with outside pathology returning as HER2 positive on FISH but internal pathology review showing conflicting results and HER2 negative on FISH on same sample?

I will give some thoughts but appreciate other opinions as well. First, it is important to consider the labs performing the assessment and their level of comfort/expertise in this assay (i.e. which do you trust).

Next, it's important to recognize that especially in borderline cases of Her2 positivity, there can be discrepancies in assessment. A Japanese study was published on this experience (Umemura et al., PMID 19019764). It is also possible that your patient may have Her2 heterogeneity. There are multiple studies on this, here is one: Marchiò et al., PMID 32112814.

You can call your pathologist (essential regardless) and ask them what they think is going on. You can ask them how much of the sample was assessed, and whether there were some ...