“M” GROUP: MALIGNANT HISTIOCYTOSES
Synonym: Histiocytic, interdigitating cell, Langerhans cell or indeterminate cell sarcomas
The malignant histiocytoses represent a group of very rare undifferentiated tumors with an anaplastic histology. Diagnosis is mainly one of exclusion and is based on extensive immunohistochemical analysis and expression of at least two of the following histiocyte/dendritic cell markers: CD68, CD163, CD4, and lysozyme. These tumors develop primarily in adults and are often aggressive, with the most common sites being the lymph nodes, skin, soft tissue, and GI tract.
Miscellaneous ALK+ histiocytosis
A few cases of histiocytoses, including cutaneous histiocytoses, have been noted to express anaplastic lymphoma kinase (ALK). ALK+ histiocytosis affects primarily young children and there is usually systemic involvement including hepatosplenomegaly and dramatic hematologic abnormalities. However, lesions may be restricted to the skin.
In ALK+ histiocytosis, the histiocytes are very large with irregularly folded nuclei and abundant eosinophilic cytoplasm, sometimes with emperipolesis. Over time, there may an increase in the number of foamy histiocytes and Touton giant cells, thereby resembling a JXG. By immunohistochemistry, the histiocytes are positive for ALK, histiocytic markers (CD68, CD163) and variably S100; they are negative for CD1a, CD207, and BRAF V600E. Gene fusions can be present, in particular KIF5B–ALK and COL1A2–ALK. In unresectable or disseminated disease, ALK inhibitors may be tried.
The authors wish to thank Drs. Warren T. Goodman and Terry L. Barrett for their valuable contributions to this chapter in the previous editions.
Histiocytic Syndromes: A Dermatological Perspective. London: Martin Dunitz; 1998:39–58.58. Durham BH, Lopez Rodrigo E, Picarsic J, et al.