Regression of thymoma associated with a multilocular thymic cyst: report of a case.

A 28-year-old male was diagnosed with acute pericarditis after presenting with acute chest pain, fever and an abnormality in an electrocardiogram. No symptoms suggestive of myasthenia gravis were observed. Although the symptoms were alleviated by antibiotics, computed tomography (CT) showed an anterior mediastinal mass with bilateral pleural effusion. He was, therefore, diagnosed with thymoma and referred to our hospital. Surgery was performed, since the pleural effusion disappeared. The pathological examination revealed the mass to be a type B2 thymoma classified as pathological stage I (Masaoka's classification) with a multilocular thymic cyst.