If there is HER2-low discordance between primary and subsequent breast cancer biopsies, in which scenarios would you choose to use trastuzumab deruxtecan?
Because of the potential heterogeneity of HER2 expression, I would still offer T-DXd for these patients. In addition, HER2 IHC 0 may not exclude T-DXd activity based on the DAISY trial.
Discordance in HER2 receptor status between primary and metastatic deposits can occur in up to 1/3 of samples. As with hormone receptor status, specimens collected via fine needle aspirate or on decalcified bone may produce inconsistent results for HER2 IHC.
I always recommend biopsy of a metastatic...
In patients with Her-low disease (IHC 1+ or 2 + with negative Her 2 FISH), I would favor obtaining new tissue from recurrent or metastatic site before considering drugs like Trastuzumab deruxtecan. Repeat biopsy will not only help verify the contemporary Her 2 status, but also allow testing for othe...