Histiocytic neoplasias: Immunohistochemical evaluation of their frequencies among malignant lymphoma and related conditions in Japan

Through histologic review of 1,766 cases with malignant lymphoma and related conditions, 35 cases (2%) were selected as probable histiocytic neoplasias. Proliferating cells in these cases had voluminous, granulated cytoplasm, and round to irregularly shaped nuclei often with bi‐ or multinucleated forms showing monomorphous or polymorphous proliferation accompanying small lymphocytes, plasma cells, and, less frequently, eosinophils. Cases showing proliferation of convoluted cells with numerous benign‐appearing histiocytes or large cells with clear cytoplasm were excluded under a diagnosis of T‐cell lymphoma. To evaluate the immunologic character of proliferating cells, immunohistochemistry using antibodies Mx‐Pan B, MB‐1, MT‐1, UCHL‐1, lysozyme, α1‐arantitrypsin, α1‐antichymotrypsin, S‐100α, S‐100α, Leu M1, epithelial membrane antigen, and Ki‐1 were carried out in 23 cases. Naphthol‐ASD‐chloroacetate reaction and toluidine blue stain were also performed. These procedures revealed that 12 cases (52%) were B‐cell type, three cases (13%) T‐cell type, six cases (26%) true histiocytic type, and two cases null type. Therefore, the frequency of cases with true histiocytic neoplasias among cases with malignant lymphoma and related conditions in Japan may be 0.5%.