A case of large "thymic granuloma". Neoplasm of T-zone histiocyte.

An unusual case of classic thymic granuloma is reported. A 16-year-old boy presented with a huge mediastinal mass and the superior vena caval syndrome. Irradiation and combination chemotherapy induced complete remission, despite several episodes of lymph node recurrence. Examination of the first biopsy specimen was considered to indicate a histiocytic tumor or malignant form of eosinophilic granuloma, because of the diffuse proliferation of histiocytes and marked infiltration of eosinophils. Late involvement in the lymph node appeared to have the same histologic characteristics as the mediastinal tumor. Immunohistochemically, the proliferating histiocytes were stained with anti-S100 protein IgG but not with anti-lysozyme or anti-NCA (nonspecific cross-reacting antigen with carcinoembryonic antigen) antibody. These characteristics were similar to those of interdigitating cells in the lymph nodes, thymus, and other lymphoid tissues. The relationship of this case to mediastinal Hodgkin's disease or proliferative disorders of histiocytes is discussed.