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Thymoma arising in a thymic cyst.

We present a case of thymoma arising in the wall of the thymic cyst. A 77-year-old asymptomatic woman demonstrated an approximately 3.5 cm sized cystic mass in the left anterior mediastinum in the CT scan. The tumor was extirpated by a total thymectomy. Histologically, the cyst wall was lined by low cuboidal cells, but a markedly thickened wall was observed at the upper site of the mass, which was found to be thymoma. These findings may suggest that the thymoma originated from the wall of the thymic cyst.