Histiocytic malignancies. Morphologic, immunologic, and enzymatic heterogeneity.

We have studied 14 hematopoietic malignancies with histologic features of histiocytic differentiation, using frozen section immunologic stains, plastic section enzyme histochemistry, and paraffin section immunocytochemistry. There was morphologic, immunologic, and enzymatic heterogeneity, including findings in seven cases that suggested differentiation toward specialized subsets of histiocytes. Four cases expressed a mature monocyte/macrophage phenotype by frozen section monoclonal antibody staining and three of these had histologic patterns diagnostic of malignant histiocytosis; two other cases had ATPase and S100 protein reactivity and morphologic features consistent with interdigitating (reticulum) cell proliferations; and one case was alkaline phosphatase positive, suggestive of differentiation toward fibroblastic reticulum cells. Four cases had histologic findings consistent with malignant histiocytosis, but weak or unreactive staining patterns and were considered poorly differentiated histiocytic or primitive hematopoietic malignancies. Three other cases, also morphologically consistent with malignant histiocytosis, were identified as probably T-cell lymphomas. The morphologic and phenotypic characteristics of non-neoplastic histiocytes and dendritic cell types and their related neoplasms are discussed. Histiocytic malignancies comprise a diverse group that can be identified and subclassified by immunologic and enzymatic techniques.