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 result, especially if the ratio is inflated by hypodiploidy of chromosome 17. Another consideration if she is ER-positive, which is particularly important since it would determine whether or not she gets chemotherapy, would be to send tissue for Oncotype testing - the single gene HER2 score may be helpful (if positive they won't complete the testing but you'll still get an answer), as would a low Oncotype RS, suggesting a biologically less aggressive ...

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