Would you offer adjuvant TDM-1 to a patient with residual disease after neoadjuvant treatment for biopsy proven HER2 positive breast cancer, although residual tumor biopsy shows discordance with HER2 negative status?  

Yes, I would offer T-DM1 in this case since such patients would have been included in the KATHERINE trial that showed a clear benefit in patients with residual invasive disease, unselected for repeat staining (which is not the standard of care) (1).  It is plausible that the HER2 status of resi...

Just curious if this patient is negative by FISH on residual disease? I would treat this patient, irrespective of FISH with TDM-1. In the pivotal trial (von Minckwitz et al., PMID 30516102) of TDM-1 versus trastuzumab, HER2 status was primarily tested on the pretreatment biopsy or if not available, ...

It is not our standard practice to repeat HER2 testing on the surgery specimen. However, this has occurred a few times in my practice. Usually, I would recommend continued treatment for a HER2+ breast cancer. Based on the improved iDFS in KATHERINE, it is standard of care to change to T-DM1...

Yes. Trastuzumab and other anti-HER2 neu drugs may have rendered the results from residual tumor falsely negative.