True histiocytic lymphoma

T RUE HISTIOCYTIC lymphoma (II-IL) is an uncommon type of non-Hodgkin’s lymphoma composed of malignant cells with enzymatic and immunologic features of phagocytic histiocytes.lm4 Since effective means of consistently identifying neoplasias of macrophagehistiocyte lineage have been elusive, the incidence of THL is uncertain, but it is thought to be less than 1% of all non-Hodgkin’s lymphoma.ss6 A full appreciation of THL, as a recognizable disease, has been complicated by its rarity and the diverse and inconsistent terminology used to characterize various histiocytic processes. In 1939 Scott and Robb-Smith described four cases of “atypical Hodgkin’s disease” that were fatal; they termed this disorder histiocytic medullary reticulosis because of the systemic proliferation of erythrophagocytic histiocytes within Iymphoid tissue.’ Rappaport subsequently coined the term “malignant histiocytosis” in 1966.* Patients affected with malignant histiocytosis have an aggressive systemic disease and their survival is usually limited to months.“” The majority have fever, weight loss, night sweats, lymphadenopathy, which may be generalized, and hepatosplenomegaly. They also often develop anemia, leukopenia, and/or thrombocytopenia. The diagnosis of THL, on the other hand, is usually reserved for those cases that present in a more localized, nodal, or extranodal, manner. Whether malignant histiocytosis and THL are part of the same spectrum of histiocytic and reticulum/dendritic cell malignancies, which have been given arbitrary names and definitions, is not known at present. Other terms used for malignant histiocytic processes include “histiocytic sarcomas,” “reticulum cell sarcomas,” and “interdigitating reticulum cell sarcomas.4,1’ This article will describe (1) the morphologic, cytochemical, and immunologic considerations necessary to establish the diagnosis of THL, (2) the clinical characteristics of “true histiocytic lymphoma” as defined in the literature, and (3) the clinical characteristics and course of seven patients from the University of Minnesota whose tumors have been especially well characterized.