Answer: Histiocytosis X

Histology: The lymph node demonstrates a sinusoidal proliferation of pale cells with pink to clear cytoplasm. The cells are associated with a prominent eosinophilic infiltrate as well as patchy areas of necrosis. Occasional giant cells are observed. No Reed-Sternberg cells are identified.

Discussion: Although histiocytosis X commonly presents in skin and/or bone, lymph node involvement can be the primary mode of presentation or it can be associated with more extensive disease. The characteristic finding in lymph nodes is the presence of a sinusoidal infiltrate of pale cells with small, oval nuclei that sometimes appear coffee bean in shape. The proliferation is almost always associated with numerous eosinophils and there may be associated eosinophilic abscesses with foci of necrosis. Cytologic atypia and mitoses are not prominent. This lesion needs to be distinguished from routine sinus histiocytosis. In this entity, the histiocytes are somewhat vacuolated in appearance and lack the coffee bean nuclear features of histiocytosis X. Immunohistochemical studies can be extremely helpful in this differential diagnosis. The Langerhan’s cells are positive for S-100 and also stain for CD1a. The latter is more specific for Langerhan’s cells than S-100, which will stain other cells such as interdigitating dendritic reticulum cells. Over the years, there has been controversy as to whether this histiocytic proliferative process is reactive or neoplastic in nature. Recently, multiple groups have utilized molecular analysis to demonstrate the clonal nature of the cells in Langerhan’s cell histiocytosis.