Transformation of epidermal growth factor receptor T790M mutation‐positive adenosquamous carcinoma of the lung to small cell carcinoma and large‐cell neuroendocrine carcinoma following osimertinib therapy: an autopsy case report

A 59‐year‐old man with relapsed epidermal growth factor receptor (EGFR) exon 19 deletion‐positive stage IA adenosquamous carcinoma after lobectomy was treated with erlotinib and bevacizumab for 1 year followed by erlotinib alone for 1 year. Because of mediastinal and supraclavicular lymphadenopathy and a nodule on the left anterior chest wall, the patient underwent repeat biopsy from the supraclavicular lymph node. Pathological analysis demonstrated adenosquamous carcinoma harbouring EGFR exon 19 deletion and T790M mutation. Osimertinib treatment was therefore started. Six months later, the patient underwent a second‐repeat biopsy from the mediastinal lymph nodes and liver metastases. Both specimens showed small cell lung carcinoma (SCLC). After chemotherapies for SCLC, he died from lung cancer. An autopsy demonstrated tumour heterogeneity, including histological types of adenosquamous, SCLC, and large‐cell neuroendocrine carcinoma. Repeat biopsies at the time of disease progression are useful to choose subsequent treatment for patients with EGFR mutation‐positive lung cancer.