T790M mutation in stage IV EGFR-mutated NSCLC patient with acquired resistance reverted to original 19Del mutation after administration of a series of precision treatments: a case report

Existing studies have yet to elucidate clearly the mechanisms of secondary resistance to third generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), neither is there any established standard therapy for patients resistant to third generation EGFR-TKIs. This case report demonstrates a rare mutation pattern in a male patient with a pathologic diagnosis of non-small cell lung cancer (NSCLC) harboring an EGFR exon 19 deletion (19Del) mutation, who then acquired an EGFR-T790M mutation after developing resistance to the first generation EGFR-TKI (gefitinib). The mutation reverted to the original EGFR-19Del mutation after the patient developed secondary resistance against the third generation TKI (osimertinib). This patient eventually achieved partial response (PR) with second generation TKI (afatinib) as a fourth-line treatment.