A Cytological Review of Follicular Dendritic Cell-Derived Tumors with Emphasis on Follicular Dendritic Cell Sarcoma and Unicentric Castleman Disease.

Follicular dendritic cells (FDCs) are antigen-presenting cells located in the germinal centers of the lymph nodes. Among the few tumors showing FDC differentiation are follicular dendritic cell sarcoma (FDCS) and Castleman disease (CD), more precisely the unicentric hyaline vascular (HV) variant. Both are relatively rare tumors, and the diagnostic cytological experience is limited to descriptions of isolated cases or small series. The purpose of this review is to bring together all the available cytological published information, and our personal experience, in order to obtain a global idea of the cytological features of these peculiar FDC-derived tumors. The different descriptions of FDCS are very similar, reflecting a tumor that shows repetitive and characteristic cytological features. It shows a dimorphic population of mature lymphocytes and large tumoral cells with partial spindle morphology. Most cases of HV variant of CD can be recognized as benign upon cytology, however a precise diagnosis seems more difficult. It is characterized by reactive lymphocytes mixed with vessels and FDCs, either single or forming syncytial aggregates. Both, FDCS and CD are challenging for cytological diagnosis in which a high index of suspicion is necessary for a correct preoperative assessment. Cytology is very useful for follow-up of recurrences and metastases.