Concomitant T790M mutation and small‐cell lung cancer transformation after acquired resistance to epidermal growth factor receptor‐tyrosine kinase inhibitor

A 70‐year‐old man was admitted to our hospital with an abnormal chest X‐ray shadow. Bronchoscopy revealed an adenocarcinoma tumour with an epidermal growth factor receptor (EGFR) exon 19 deletion. Positron emission tomography–computed tomography scanning and magnetic resonance imaging showed advanced stage IV lung cancer. He was treated with erlotinib as a first‐line drug, which maintained a clinical response for 16 months. After disease progression, a re‐biopsy was done from the tumour in the right lower lobe. The obtained specimen harboured both small‐cell lung cancer (SCLC) transformation with retention of the EGFR 19 deletion and the development of an EGFR T790M mutation. We came across a very rare condition of concomitant T790M mutation and SCLC transformation after acquired resistance to EGFR‐tyrosine kinase inhibitor.