Do you repeat molecular testing at progression in gastric/GEJ tumors or is the initial pathology sufficient for entire treatment course?  

As I have addressed elsewhere, yes, I would rebiopsy at the time of progression on any anti-Her2 therapy to confirm persistent Her2 positivity. I try very hard to biopsy a progressive tumor. If this is not possible, ctDNA to assess for persistent ErBB2 amplification is an option, albeit one that is ...

If it is feasible to do so, then yes, I will try to arrange for a repeat biopsy and retesting Her2 (usually reasonably efficient in patients with intact primary tumor). Or, alternatively, liquid biopsy. I will try to anticipate setting up a biopsy when I suspect there may be progression. It is gener...

Yes, I do my best to repeat molecular testing at progression. Change in HER2 status or loss of HER2 status following HER2-directed therapy in gastroesophageal cancer has been consistently reported.