Effect of osimertinib treatment on lung adenocarcinoma with squamous cell transformation harboring the T790M mutation: A case report and literature review.

We herein report a case of squamous cell transformation combined with the epidermal growth factor receptor (EGFR) mutation T790M associated with acquired resistance to EGFR tyrosine kinase inhibitors (EGFR-TKIs) in a 73-year-old male patient diagnosed with stage IVA lung adenocarcinoma. Gene alterations were analyzed by collecting and studying pleural effusion at the time of diagnosis. Examination revealed an exon 19 deletion in the EGFR gene. Following treatment with the second-generation EGFR-TKI afatinib, squamous cell carcinoma was identified by performing a re-biopsy of the recurrent site. Although the levels of cytokeratin 19 fragment, which is a tumor marker for the follow-up of squamous cell carcinoma, were increased at that point, the levels of carcinoembryonic antigen, a marker particularly associated with adenocarcinoma, remained within normal limits. The T790M mutation and the original exon 19 deletion were detected simultaneously. The patient received treatment with the third-generation EGFR-TKI osimertinib, achieving a good clinical response. These findings suggest that osimertinib is beneficial for lung adenocarcinoma patients with squamous cell transformation harboring the T790M mutation.