Results of HER2 Change After Surgery?

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Hi all,

My mom was diagnosed with stage I triple positive (ER+ PR+ HER2+) breast cancer in September. She just had a bilateral prophylactic mastectomy. During the biopsy and clinical staging, the ICH test came back equivocal, so it was sent out for FISH, which tested positive.

After the mastectomy, they retested the FISH and it was determined negative. I know tumors are non-homogenous, however, I also know if any portion of the tumor tests positive for HER2, Herceptin should be given. I don't want undertreatment given and neither does my mom-- at all!

I just feel weird about how it was once positive, and now negative. She also had interlaced DCIS component, which I know is more likely to be HER2+ than IDC itself. Is it possible that that is what was biopsed and initially tested positive for HER2 over-expression?

I just find it odd that something so crucial could vary so much and would like insight if something similar has happened to other women (or men) on here.

Thank you!