Castleman disease (CD, also known as angiofollicular lymph node hyperplasia) is a heterogeneous group of lymphoproliferative disorders with common histopathologic features. The disease is subclassified based on the number of regions of involved lymph nodes (unicentric vs multicentric), and whether the pathology is associated with Human herpesvirus 8. Subdividing cases as either unicentric CD, HHV-8 associated multicentric CD, or idiopathic multicentric CD is important due to its implications for treatment and clinical outcomes. 

The current case is an example of unicentric Castleman disease affecting the perirenal/periadrenal lymph node region. Although there is a histopathologic spectrum in CD, ranging from a hyaline vascular phenotype to a plasma cell phenotype, most cases of unicentric CD are of the hyaline vascular subtype. At low-power the lymph node is characterized by follicular hyperplasia, however many of the atretic germinal centers are atretic and frequently oppose one another (so called germinal center "twinning"). Mantle zones often show a distinctive "onion-skin appearance" with concentric arrangements of lymphocytes.  Hyalinized blood vessels may be seen penetrating germinal centers,  termed as a "lollipop sign." Additionally, there is typically increased hyalinization and vascularity in the lymph node paracortex, as well as the presence of high endothelial venules (HEVs) within interfollicular areas. 

The exact etiology and cell type driving unicentric CD pathogenesis is unclear, although there is evidence suggesting that at least a subset of cases may be neoplastic in nature. Complete surgical resection is almost always curative in unicentric CD, whereas multicentric CD requires chemotherapy.