Unicentric Subcutaneous Hyaline Vascular Castleman Disease With Concurrent Features of Calcifying Fibrous Tumor

A calcifying fibrous tumor (CFT) is a rare benign soft tissue lesion characterized by dense hyalinizing fibrosis and psammomatous calcifications. Chronic inflammatory infiltrates are described but never emphasized as being a conspicuous aspect of the pathology. Initially described as a reactive fibrosing inflammatory lesion, CFTs are typically asymptomatic and have been reported in various anatomical locations but rarely manifest in subcutaneous tissue. Unicentric Castleman disease (CD) is a rare lymphoproliferative disorder whereby the initial description of the disease emphasized atrophic germinal centers and hypervascularity within the germinal centers, first described in the context of mediastinal nodular unicentric CD (UCD). Since that original description, a multicentric variant has been observed. Furthermore, the histopathology of both the unicentric and multicentric variants has been expanded to include many plasma cells, with the potential for lambda light chain restriction, and hyperplastic germinal centers, which fall under the histologic designation of the plasma cell variant of CD. Extranodal UCD is uncommon, with its presentation in the skin being especially rare. There are cases of CFT with concomitant features of CD either in the same lesion or at contiguous sites. In this study, we report the first pediatric case of subcutaneous CFT that exhibited concurrent features of UCD showing a mixed hyaline vascular and plasma cell pathology. In particular the sample demonstrated nodular hyalinizing fibrosis, small psammomatous calcifications, and atrophic hypervascular lymphoid follicles with characteristic "castlemanoid" architecture characterized by atrophic germinal centers with increased vascularity. There were also plasma cells that exhibited a polytypic profile. The germinal center foci exhibited a prominent CD21 follicular dendritic cell network. IL-6 immunohistochemical staining revealed expression localized predominantly to benign macrophages and reactive CD8 + cytotoxic T cells at the periphery of germinal centers. Hyaline vascular CD and mixed plasma cell and hyaline vascular CD encompass a morphologic spectrum that includes a predominantly fibrotic phase that could potentially obscure the primary diagnosis.