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Computed tomography scan showed an approximately 6 cm sized cystic lesion at anterior mediastinum in a 75-year-old man. At thoracotomy, the cystic mass was covered by mediastinal pleura. Tumor adhered to sternum, ascending aorta, pericardium and contralateral mediastinal pleura. The tumor was extirpated totally. Pathological diagnosis was squamous cell carcinoma of the thymus that developed in a thymic cyst. Additional radiation therapy was done. In those cases, although surgical resection was done, there wasn’t sufficient data about addition therapy. In our study, we emphasized treatment for these rare lesions.