Atypical generalized eruptive histiocytosis associated with acute monocytic leukemia.

Histiocytoses are diseases caused by proliferation of either dendritic cells/Langerhans cells or of monocytes/macrophages. Generalized eruptive histiocytosis belongs to the cutaneous non-Langerhans cell histiocytoses and is a rare monocyte-macrophage proliferative disorder that usually follows a benign clinical course. We present the case of a 59-year-old man who presented with a 7-month history of progressively developing erythematous macules and slightly elevated papules widely distributed over the trunk, neck, face, and thighs. Ultrastructurally, no Birbeck granules were observed, and immunochemistry did not reveal any S-100 protein or CD1a antigen in any of the lesional cells, excluding Langerhans cell histiocytosis. In addition, the histiocytic infiltrate in the skin of our patient was shown to strongly express MS-1 high molecular weight protein, a marker described as highly characteristic for cutaneous non-Langerhans cell histiocytoses. Bone-marrow smear examination and flow cytometric analysis revealed monocytic leukemia. This is the second report of generalized eruptive histiocytosis associated with acute monocytic leukemia. We discuss the differential diagnoses of the clinical picture and stress that this benign cutaneous disorder may indicate an underlying hematologic malignancy.