Answer: Thymoma

Histology: This is an intrathymic lymphoid-rich lesion. The lesion is fairly well delineated, and has a rounded countour. While the majority of the lesion consists of small lymphocytes, there is a background of thymic epithelial cells that are evenly dispersed throughout the tumor In other parts of this lesion, perivascular serum lakes are identified, as well as areas of medullary differentiation. The normal thymic architecture of cortex and medulla is effaced.

Discussion: Thymic lymphic hyperplasia would retain the normal thymic architecture, and often contains intrathymic lymphoid follicles. Castleman’s Disease is rarely centered in the thymus, and instead affects mediastinal lymph nodes. This lesion is characterized by “onionskinning” of mature lymphoid cells around sclerotic, hypervascular germinal centers. Lymphoblastic Lymphoma typically affects younger patients, and lacks the evenly dispersed epithelial component of a lymphocyte-predominant thymoma. These lesions may have an identical immunophenotype by Flow Cytometry, which is a recognized pitfall.