Concurrent epidermal growth factor receptor T790M secondary mutation and epithelial‐mesenchymal transition in a lung adenocarcinoma patient with EGFR‐TKI drug resistance

Almost all epidermal growth factor receptor (EGFR)‐mutant lung cancers develop resistance to EGFR‐tyrosine kinase inhibitors. Several mechanisms for this acquired resistance have been identified, including development of an EGFR T790M mutation, MET amplification, hepatocyte growth factor overexpression, loss of phosphatase and tensin homolog expression, epithelial–mesenchymal transition, and transformation to small cell lung cancer. Herein, we report a case of a lung cancer patient with EGFR exon 19 deletion who was resistant to EGFR‐tyrosine kinase inhibitor treatment during disease progression. Using histological and gene sequencing analysis, we observed that the primary adenocarcinoma acquired T790M mutation in EGFR exon 20, and a secondary sarcomatoid carcinoma developed in the vicinity. Assessment of E‐cadherin and Vimentin expression confirmed that the sarcomatoid carcinoma had undergone an epithelial–mesenchymal transition. Therefore, it is important to perform a tissue re‐biopsy after the development of resistance to identify the best treatment options. Surgical resection might be a better “salvage” treatment in cases of oligometastatic progression.