A dramatic course after the resection of an SMARCA4‐deficient undifferentiated tumour

Thoracic SMARCA4‐deficient undifferentiated tumours are ordinarily found as a huge mass with systemic metastasis, and the prognosis is poor. The potential of immunotherapy for these unresectable tumours has been reported. An asymptomatic 68‐year‐old man with a smoking history had a left lung mass without distant metastasis and underwent complete resection. Two months after surgery, with no adjuvant therapy, he developed multiple distant metastases with aphasia and died 4 months after surgery. Adjuvant treatment may be necessary with immune checkpoint inhibitors, with a closer follow‐up to detect recurrence without symptoms.