Histological transformation from non-small cell to small cell lung carcinoma after treatment with epidermal growth factor receptor-tyrosine kinase inhibitor

Several cases of acquired resistance in patients with activating epidermal growth factor receptor (EGFR) mutation have been reported. However, rare clinical cases exist of a transformation to small cell lung cancer (SCLC) following treatment with EGFR-tyrosine kinase inhibitors (TKIs). We report a case of non-small cell lung cancer (NSCLC) with L858R mutation at the time of diagnosis. After failure of EGFR-TKI therapy, we performed additional histopathologic examinations. We confirmed that the patient had a histological transformation from NSCLC to SCLC. We performed chemotherapy with etoposide and cisplatin against the SCLC and radiologic findings were improved.