Castleman's disease and related disorders.

Three disorders that bear the eponym of Castleman's disease (CD) are discussed. The localized CD of hyaline-vascular (HV) type features an architecturally abnormal, hypervascular lymphoid tissue with burned-out germinal centers, and presents as an asymptomatic, slowly growing mass. It may represent a lymphoid hyperplasia associated with excessive angiogenesis. The localized CD of plasma cell (PC) type, instead, features an architecturally recognizable lymph node with solid sheets of PCs and presents with systemic manifestations of inflammation and B cell hyperreactivity. It appears as a localized chronic reaction to unknown antigens. "Multicentric" CD indicates a clinicopathologic entity characterized by the histology of CD of "mixed" type, a predominantly lymphadenopathic presentation consistently involving peripheral nodes, manifestations of multisystem involvement, and an idiopathic nature. It is best considered as a systemic B cell lymphoproliferation, which probably arises in a setting of immunoregulatory deficit, and may result in the outgrowth of clonal B cell populations. An attempt is presented to place the latter two forms of CD in the larger perspective of idiopathic PC disorders, by taking into account both the nature of the diseases (hyperplastic, dysplastic, neoplastic) and the lymphocyte traffic system that is involved (bone marrow-bound, mucosa-associated or peripheral-node-bound). In such a scheme, localized CD, PC type, and multicentric CD appear as a hyperplastic and a dysplastic disorder, respectively, of peripheral-node-bound B cells, related to, and often associated with, primary nodal plasmacytoma and osteosclerotic myeloma (so-called POEMS, Takatsuki's or Crow-Fukase's syndrome).