Thoracic -deficient undifferentiated tumor.

Thoracic -deficient undifferentiated tumor (-UT) is characterized by an undifferentiated rhabdoid morphology and 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 -UT is crucial for early diagnosis and treatment planning.