Follicular dendritic cell tumor: review of the entity.

Malignant neoplasms showing follicular dendritic cell differentiation are uncommon. Most reported cases have involved lymph nodes of the neck, mediastinum, and axilla. Approximately 30% of the cases were located in extranodal sites, such as liver, tonsil, and intra-abdominal soft tissue. Microscopically, follicular dendritic cell tumor is composed of oval to spindle cells with eosinophilic cytoplasm arranged in sheets, fascicles, and whorls, sometimes admixed with foci showing a storiform pattern of growth. The tumor cells are characteristically admixed with small lymphocytes. The tumor nuclei are oval to spindle, with thin nuclear membranes, small basophilic nucleoli, and clear or dispersed chromatin. Scattered multinucleated tumor cells are frequently seen. Necrosis, marked cellular atypia, high mitotic rate, and abnormal mitoses may occur and are harbingers of an aggressive behavior. The tumor cells typically express CD21, CD35, Ki-M4p, Ki-FDRC1p, and vimentin, with occasional positivity for S-100 protein, muscle-specific actin, and epithelial membrane antigen. Ultrastructural examination shows complex interdigitating cytoplasmic processes joined by desmosomes. The behavior of these tumors is more akin to that of a low-grade soft tissue sarcoma than a malignant lymphoma and is characterized by local recurrences in 36% of cases and metastases in 28%. A small proportion of cases have arisen against a background of Castleman disease of the hyaline-vascular type and others in association with Epstein-Barr virus infection. Complete surgical resection is the treatment of choice whenever feasible. Adjuvant radiotherapy or chemotherapy appears indicated in cases having adverse pathological features and in recurrent or incompletely resected lesions. Much still needs to be learned about the most effective adjuvant therapy and the molecular biology of these tumors.