Commentary on the WHO 2008 classification of neoplasms arising from histiocytic and other accessory cells

Histiocytes and dendritic cells belong to a group of immunoregulatory cells that are responsible for antigen processing and presentation to lymphocytes. Neoplasms of histiocytic and dendritic origin are very rare [1], making up less than 1% of tumours in lymph nodes. Thus, information regarding epidemiology, prognosis and treatment [2, 3] is scarce. Table 1 summatizes the neoplasms derived from histiocytes and other accessory cells according to the 2001 and 2008 WHO classification. A new entity that has been added to the 2008 WHO classification of histiocytic dendritic cell neoplasms is disseminated juvenile xanthogranuloma (JXG). JXG is a benign proliferation of histiocytes similar to those that occur in the dermis. The disease that occurs in adults with bone and lung involvement is referred to as Erdheim– Chester disease. Although most JXG are benign, activation of macrophages can lead to cytopenias, liver damage and death. Neoplastic cells are composed of small and oval to spindled histiocytes with bland nuclear features without nuclear grooves. Dermal lesions tend to have foamy (xanthomatous) cytoplasm with Touton-type giant cells. Similar to macrophages, the tumour cells express CD14, CD68 and fascin. CD1a is negative and S100 is positive in less than 20% of the cases. Some are clonal but no cytogenetic or molecular changes have been identified. An association with neurofibromatosis type 1 has been made. Although the neoplastic cells are thought to be derived from dendritic cells, they were previously classified as soft tissue tumours. The recognition of their origin from dermal/interstitial dendritic cells and association with other hematologic malignancies such as juvenile myelomonocytic leukaemia and Langerhans cell disease warranted their classification within the category of histiocytic and dendritic cell neoplasms. It is unclear whether the solitary forms of JXG should be considered a benign counterpart of disseminated JXG. In addition, the range of cytologic atypia that is exhibited by these tumours is also unknown. J Hematopathol (2009) 2:75–76 DOI 10.1007/s12308-009-0035-y