Abstract
Thymoma is an epithelial neoplasm of the thymus, which commonly lies in the anterior mediastinum and recurrences of thymoma generally are locally, and retroperitoneal recurrence is considered to be rare. A 46-year old Asian woman with invasive thymoma had undergone thymectomy 10 years ago. Computed tomography demonstrated a wellcircumscribed mass in the left retroperitoneal space. The patient had not any symptom including myasthenia gravis. Because on the anterior mediastinum area shows no sign of tumor recurrence and the mass adjacent to the vertebral body, neurogenic tumor was suspected. Surgical resection was performed using a retroperitoneal approach, which revealed the tumor adhering neighboring diaphragm. The tumor was histologically diagnosed to be type B1 thymoma according to the World Health Organization classification. The retroperitoneal mass was an unusual local recurrence after thymectomy. The patients whose had under invasive thymectomy should be evaluated carefully when finding retroperitoneal mass during follow-up.