How do you approach a women with localized HER2+ breast cancer that was HER2+ by FISH but negative (0+) by IHC?

This is challenging, but since HER2 amplification by FISH is defined as HER2 positive by ASCO/CAP guildeines, if the patient is fit for consideration of HER2-directed therapy with chemotherapy, I would still proceed with those regimens.

I agree with Dr. @Dr. First Last that you should repeat both tests. The other variable is how 'positive' the FISH was - I'd be much more inclined to consider her truly HER2-positive if it demonstrated a high level of amplification by HER2/CEP17 ratio and HER2 copy number than a barely positive resul...

Please repeat both tests in 2 different labs before making a final decision; lab error should be eliminated first.