RESUMEN
We describe a 63-year-old man who, while under treatment as an outpatient for adult onset Still's disease (AOSD), developed edema of the extremities and mediastinal tumor was observed on a chest X-ray film and a chest CT scan. He was not pathologically diagnosed at first and received radiation therapy with a total dose of 30 Gy. Transbronchial biopsy was carried out because the tumor enlarged, and the tumor invasion was observed in the left lumen of the main bronchus. Histological findings suggested a diagnosis of carcinoma with sarcomatoid elements (CSE). Further radiation therapy with a dose of 20 Gy was unsuccessful; his condition gradually worsened and the patient died. The autopsy findings demonstrated that CSE developed in the middle mediastinum, and the other organs were not involved. From an embryologic standpoint, there seemed to be some possible differential diagnoses, such as a pleomorphic carcinoma as a subtype of lung cancer, and CEA as a metastatic mediastinal lymph node cancer of unknown origin or a primary mediastinal lymph node cancer. Like the present case, tumors developed in the middle mediastinum with rapid progression are rare. We report a case, that was difficult to diagnose and treat.