Answer: Angiofollicular lymphoid hyperplasia (Castleman’s Disease)

Histology: The mass at low power appears to be lymphoid in nature. It has a vaguely nodular pattern of growth. The nodules have within the center of them a small germinal center-like structure. The germinal centers are often surrounded by small lymphocytes that palisade around the germinal center in an Indian file pattern. Many of the small germinal center-like structures have small vessels within them, some of which appear sclerotic. The germinal centers lack tingible body macrophages. The interfollicular area has a proliferation of small vessels and the lymphocytes surrounding the germinal centers appear to have a somewhat extended mantel zone.

Discussion: This is a classic example of the hyaline vascular type of Castleman’s Disease. The distinguishing feature is the presence of numerous small nodules which are composed of a hyalinized and vascularized germinal centers surrounded by small lymphocytes proliferating an Indian filing pattern. There is striking hyalinization and vascularization of the germinal centers as well as vascular proliferation in the interfollicular area. The lesion can be distinguished from follicular lymphoma because in follicular lymphoma the nodules are more proliferative and are composed of usually monotonous proliferation of small cleaved cells sometimes admixed with larger transformed lymphocytes. In addition, in follicular lymphoma, the neoplastic nodules are closer together in a back-to-back pattern and immunophenotypic studies would demonstrate a clonal proliferation of B-cells. The lesion can be distinguished from a non-specific reactive hyperplasia because in a non-specific follicular hyperplasia, one would expect to see larger germinal centers which would have more proliferative activity associated with tangible body macrophages. Reactive germinal centers often demonstrate polarization which is absent in the small germinal centers of Castleman’s Disease. The lesion can be also distinguished from a thymoma in that a thymoma would contain spindle cells or an epithelial proliferation which would also stain positive with keratin. The hyaline vascular type of Castleman’s Disease is the most common type of Castleman’s Disease and often presents in the mediastinum. These masses are often identified on routine chest x-ray in an otherwise asymptomatic patient. In addition to the localized Castleman’s disease, there are also cases of more generalized disease. Some of the patients with more generalized disease have a different histologic variety of Castleman’s Disease which is called plasma cell type of Castleman’s disease. In contrast to hyaline vascular Castleman’s disease, the plasma cell variant has rather large proliferative appearing germinal centers and the interfollicular area contains a sheet-like dense proliferation of plasma cells often associated with a vascular proliferation.