TRUE MALIGNANT HISTIOCYTOSIS

Objective: To attempt to distinguish cases of true malignant histiocyt osis from the clinical syndromes of so-called malignant histiocytosis with use of recent methods. Design: We retrospectively studied the lab oratory data and clinical course of Mayo patients who had clinical syn dromes of so-called malignant histiocytosis and reviewed available par affin-embedded tissue specimens to identify the nature of the malignan t cells. Material and Methods: After elimination of cases of infection -associated hemophagocytic syndrome, we reviewed and studied seven cas es of so-called malignant histiocytosis in patients who had undergone assessment at Mayo Clinic Rochester between 1973 and 1993. We identifi ed histiocytes by using current morphologic, cytochemical, and immunoh istochemical methods. The clonal nature of the malignant cells was ide ntified with morphologic, cytogenetic, and molecular genetic studies. Results: Only one of the seven cases had a true histiocytic origin. Th e malignant cells were T cells in three other cases (the cells were al so CD30(+) in two cases), CD30(+) cells only in one case, epithelial c ells in one case, and an undetermined cell type (stained positively on ly with antitrypsin) in one case. Conclusion: True malignant histiocyt osis is an exceedingly rare disease, and only a few reports have clear ly identified the histiocytic origin of the malignant cells. Previousl y, the lack of monoclonal antibodies specific to histiocytes and the a bsence of techniques for performing molecular genetic studies on paraf fin-embedded tissue prevented the study of such cases. With newer tech niques, cases of true malignant histiocytosis can now be identified.