Adenocarcinoma of the Lung Acquiring Resistance to Afatinib by Transformation to Small Cell Carcinoma: A Case Report

A 65-year-old woman visited our hospital due to right chest pain and dyspnea on exertion. Chest radiography revealed decreased permeability of the right lung. Computed tomography demonstrated a huge mass in the right upper lobe and right pleural effusion. Right pleural effusion cytology yielded a diagnosis of adenocarcinoma and was positive for mutation of epidermal growth factor receptor (EGFR; exon 21 L858R). Afatinib was selected for the initial treatment. Multiple tumors regressed remarkably, but then rapidly progressed 3 months later. We performed re-biopsy to detect the mechanism of resistance to afatinib. Histopathology revealed a mixture of small cell carcinoma (SCC) and adenocarcinoma harboring same EGFR mutation. To the best of our knowledge, this is the first report of transformation to SCC after treatment with afatinib.