Abstract
A large number of EGFR mutant non-small cell lung cancer patients primordially benefit from first-line treatment with first-generation EGFR-tyrosine kinase inhibitors (TKIs), such as gefitinib and erlotinib. However, multiple acquired resistance mechanisms have been described that limit the clinical efficacy of first-generation EGFR-TKIs. Herein, we report a rare case of lung adenocarcinoma harboring an EGFR exon 19-deletion mutation before the administration of target therapy. This patient acquired resistance to first-generation EGFR-TKIs through small cell lung cancer (SCLC) transformation accompanied by the T790M mutation. Unexpectedly, this SCLC patient maintained a sensitive response to the third-generation EGFR-TKI osimertinib. This special case may indicate that osimertinib represents an effective target drug for SCLC patients who harbor an EGFR T790M mutation.