Castleman Disease and Rosai-Dorfman Disease.

This chapter describes the main features of two different diseases, Castleman Disease (CD) and Rosai-Dorfman Disease (RDD). Castleman disease (CD) is a clinical and histopathologically heterogeneous lymphoproliferative disorder that encompasses at least three distinct entities with some common overlapping morphological features: Hyaline Vascular CD (HVCD), Unicentric Plasma Cell CD and Multicentric CD. The most important feature of HVCD is the presence of abnormal germinal centers with hyaline-vascular transformation, sometimes showing multiple germinal centers within a single reactive lymphoid follicle, this outlining HVCD as a disorder of follicular dendritic cells. Unicentric and multicentric CD are, in contrast, lymphoproliferative lesions. Proinflammatory hypercytokinemia is an essential feature of multicentric CD, distinguished by a florid clinical presentation. Rosai-Dorfmann Disease is a histiocytic proliferative disorder diagnosed by the presence of tissue infiltration by S100-positive CD1a-negative histiocytes and plasma cell aggregates, often with Russell bodies. A typical, though not specific, characteristic of the disease is emperipolesis. Initially considered to be an inflammatory/reactive condition, molecular studies suggest that at least some cases of RDD could be considered as a low-grade histiocytic neoplastic process.