HISTIOCYTIC/DENDRITIC-CELL NEOPLASMS AND RELATED DISORDERS
Malignant Histiocytosis (MH)
Malignant histiocytosis of true histiocytic origin is a rare disorder, and should be diagnosed only in
patients with fever, severe constitutional symptoms, pancytopenia, and hepatosplenomegaly. Most authors
have relied on bone marrow findings to diagnose MH. An association with mediastinal non seminomatous germ
cell tumors has been recognized since 1985 (Nichols et al, 1985). The bone marrow biopsy in cases of MH
shows a large number of atypical or frankly malignant looking histiocytes, often showing hemophagocytosis,
diffusely infiltrating the marrow cavity. The histiocytes are usually reactive with monoclonal antibodies
to monocytes/macrophages such as KP-1, PG-M1, and HAM56. In true MH, CD30 and other lymphoid markers, as
well as CD1a are negative. S-100 may be positive in some cases. The diagnosis is better supported by
genotypic demonstration of a germline configuration of the immunoglobulin and T-cell receptor genes.
Langerhans Cell Histiocytosis (LCH)
The initial diagnosis of LCH is rarely made from a bone marrow biopsy. However, a positive marrow biopsy is
not uncommonly seen in patients with multifocal eosinophilic granuloma. The bone marrow lesions, which
appear focal or diffuse, may consist predominantly of Langherans cells (LC) or be polymorphous with a
mixture of LC, eosinophils, neutrophils, plasma cells and lymphocytes. Immunohistochemically, the LC
expresses CD45, CD1a, S-100, CD74, vimentin , and HLA-DR . A significant proportion of cells express CD68
(KP-1) but not PG-M1. They do not express B and T cell antigens. According to the Histiocyte Society
guidelines a definitive LCH diagnosis is established only when the typical histology is present and either
Birbeck granules are found by EM or CD1a is detected by immunohistochemistry.
We have never encountered cases of interdigitating or follicular dendritic cell sarcoma in the bone marrow.
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