Answer: Castleman’s disease

Histology: The mass is a lymphoid proliferation that is characterized by the presence of numerous small hyaline germinal centers. In the interfollicular areas there is rather extensive vascular proliferation. The most distinctive aspect of this lesion is the small germinal centers that demonstrate extensive vascularization as well as hyalinization. In some areas it is possible to see the vessel entering into the small germinal center. Another feature typical of Castleman’s disease is the Indian filing of the small lymphocytes around the germinal centers.

Discussion: This lesion is very distinctive and would raise a differential diagnosis primarily between a reactive hyperplasia vs. Castleman’s disease. Two major categories of Castleman’s disease have been described: the hyaline vascular type or the plasma cell type. The vascularization and hyalinization of the germinal centers is typical of the former and more common type. This lesion often presents as a mediastinal mass. In this setting, this lesion must be distinguished from a thymoma or lymphoma. The second morphologic category of Castleman’s disease, plasma cell type, is much less common. It can be distinguished from the hyaline-vascular variety by the presence of large, reactive germinal centers and a striking proliferation of plasma cells in the interfollicular regions. The plasma cell variant of Castleman’s disease has histologic features similar to those seen in the hyperplasia associated with HIV infection and rheumatoid arthritis and must be distinguished on clinical grounds from these two entities. Patients with the hyaline vascular variety of Castleman’s disease usually lack B symptoms, whereas patients with the plasma cell type often have associated fever, elevated sedimentation rate and hypergamablobulin anemia. Lastly, a multi-centered or systemic form of Castleman’s disease has been described. These patients often have generalized lymphadenopathy and laboratory abnormalities.