Malignant lymphoma of true histiocytic (monocyte/macrophage) origin

Since the advent of the newer classifications of non-Hodgkin's lymphoma and the realization that the majority of tumors classified as histiocytic under the Rappaport classification were in fact of lymphocytic origin, there have been remarkably few reports of true histiocytic (monocyte/macrophage) tumors and it has been suggested by some that such tumors should not be considered as a variety of malignant lymphoma. This article describes five patients with malignant lymphoma of neither B- or T-lymphocyte origin in whom the malignant cells could be characterised immunologically, cytochemically, and immunohistochemically as of true histiocytic derivation. The cases showed considerable morphologic diversity but there were shared characteristics at both light microscopic and ultrastructural levels. Positive immunohistochemical staining for alpha 1-antitrypsin was the single most useful criterion in classifying these tumors. Without the use of special techniques there were no clinical or pathologic features that reliably distinguished these cases from non-Hodgkin's lymphomas of lymphocytic derivation. Tumors of histiocytic origin are, therefore, inevitably being included among the non-Hodgkin's lymphomas and are most appropriately classified as such. Identification of histiocytic lymphomas should be encouraged so prognosis and optimum treatment can be established.