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Case of the Month



Thymoma, WHO Type B2

A 33-year-old gentleman undergoes resection of a 5.0 cm mediastinal mass. The tumor is rubbery and encapsulated with a lobulated cut surface. Sections demonstrate lobules of tumor separated by fibrous septae. The tumor cells are large with abundant cytoplasm and prominent nucleoli and are surrounded by small lymphoid cells. Immunostain for Cam5.2 highlights the tumor cells. There is minimal invasion into the surrounding fatty tissue. Large nerve fibers are surrounded by tumor, further evidence of invasion into the surrounding soft tissue. The tumor is diagnosed as thymoma, WHO type B2. By definition, this thymoma is comprised of large polygonal epithelial cells with vesicular nuclei and prominent nucleoli. A roughly even admixture of lymphocytes is present. Clinical associations may include myasthenia gravis, pure red cell aplasia, and hypogammaglobulinemia