A Case of Metaplastic Thymoma

Abstract

Metaplastic thymoma is a rare thymic epithelial tumor that shows a biphasic structure with a mixture of polygonal and spindle cell components and accounts for 1-2% of all thymomas. In this report, we describe a case of metaplastic thymoma and its bibliographic consideration. The patient was a 58-year-old man. A computed tomography (CT) scan performed upon the patient's visit to our Emergency Outpatient Department for urinary calculus indicated an anterior mediastinal tumor. A chest plain CT scan revealed a 68×40mm, well-circumscribed, internally homogenous tumor in the anterior mediastinum. Fluorodeoxyglucose (FDG)-positron emission tomography (PET)-CT revealed hyperaccumulation of FDG with a maximum standardized uptake value (SUVmax) of 8.37, coinciding with the anterior mediastinal tumor. The patient was diagnosed as having a thymic tumor, and thymo-thymectomy including the surrounding lymph nodes was performed via a median sternotomy approach. No pericardial, mediastinal or pleural infiltration was observed. The excised specimen was 77×57×32 mm in size and had formed from the lower pole of the left lobe in the thymus. The section was white and sharply well-circumscribed, with no obvious invasion of the capsule. Pathological findings indicated a biphasic structure with a mixture of polygonal cell components forming a solid nest and bundle-like spindle-shaped cellular components surrounding the nest. Immunostaining showed strong positivity for CK (AE1/AE3) in the polygonal cells and for vimentin in the spindle cells, leading to the diagnosis of metaplastic thymoma.