A case of EGFR-positive lung adenocarcinoma that once transformed into small cell carcinoma but worsened into adenocarcinoma again

Epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors (TKIs) have a crucial role in the treatment of advanced EGFR mutated non-small lung cancer (NSCLC); however, most patients with EGFR-mutated NSCLC eventually develop acquired resistance to EGFR-TKIs. Small cell lung carcinoma (SCLC) transformation accounts for about 10 % of the mechanisms of acquired resistance of EGFR-TKIs, and the type of anticancer drugs used for treatment must change drastically when transformation occurs. We herein report a case of a patient with EGFR-positive lung adenocarcinoma that transformed into small cell carcinoma but worsened into adenocarcinoma again, by accurately identifying the lung cancer stage and selecting appropriate treatment thorough repeated re-biopsy. In this report, the accurate diagnosis through repetitive re-biopsy and tumor marker measurements in different histologic types was critical.