For patients with EGFR-mutant NSCLC who progress on osimertinib, do you repeat tumor molecular profiling?   

I do favor repeating a tumor biopsy in patients that progress on osimertinib. If I send cfDNA that would be concurrently not instead of tumor tissue. The mechanisms of resistance to osimertinib are many including the emergence of C797S mutation, MET amplification, HER2 amplification, BRAF 600E mutat...

I agree with Dr. Jalal that tissue is important in the EGFR space for small cell transformation documentation as a resistant mechanism to TKI's and is also interesting that  p53 and RB mutations in ctDNA can be a clue to SCLC transformation.