Thoracic SMARCA4-deficient undifferentiated tumor

Thoracic <i>SMARCA4</i>-deficient undifferentiated tumor (<i>SMARCA4</i>-UT) is characterized by an undifferentiated rhabdoid morphology and <i>SMARCA4</i> deficiency pathologically. In the 2021 revision of the WHO classification, it was categorized as an epithelial tumor because of its shared genetic background with smoking-related lung cancer. We describe the case of a 44-year-old man with a rapidly enlarging mediastinal mass observed on chest radiography and CT. The tumor was resistant to radiation therapy, chemotherapy, and immune checkpoint inhibitors, and the patient's survival time was approximately 5 months. Familiarity with the clinical background and imaging findings of thoracic <i>SMARCA4</i>-UT is crucial for early diagnosis and treatment planning.