Malignant histiocytosis in the leukaemic stage: a new entity (M5c-AML) in the FAB classification?

Malignant histiocytosis (MH) is a rare, rapidly fatal disorder, characterized by systemic proliferation of abnormal histiocytes. Most patients present with pancytopenia. We report the case of a patient with MH in the leukaemic stage, who presented extremely pronounced general symptoms and multisystemic involvement. Determination of serum cytokines showed high levels of tumor necrosis factor (TNF), interleukin 6 (IL-6) and interleukin 1 receptor antagonist (IL-1ra). Cytogenetic studies proved the monoclonality of the histiocytic proliferation. These findings strongly suggest that we are dealing with a proliferation of activated macrophages (= histiocytes). By analogy with the monoblastic (M5a) and monocytic (M5b) acute leukaemia of the French-American-British (FAB) classification, we propose a new entity, 'M5c', designating acute histiocytic leukaemia.