Follicular dendritic cell sarcoma is a neoplastic proliferation of spindle to ovoid cells with a phenotype similar to that of follicular dendritic cells.

Malignant neoplasms showing follicular dendritic cell differentiation are uncommon. Most reported cases have involved lymph nodes of the neck, mediastinum, and axilla.^1,2,3,4,5,6 The patients are adults with a mean age of 46 years, and there is a slight female predominance.^1,2,3,4,5,6 The patients may be asymptomatic or present with cough, hemoptysis,
or chest discomfort. 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.⁷

Follicular dendritic cell sarcomas are formed by oval to spindle cells with eosinophilic cytoplasm growing in sheets and fascicles, with a focal storiform pattern and whorls reminiscent of those seen in meningioma.^{4,5,6,7,8,9,10,11,12,13} The nuclei are oval or elongated with thin nuclear membranes, inconspicuous or small eosinophilic nucleoli, and clear or dispersed chromatin. Nuclear pseudoinclusions are common. Typically, the tumor cells are intimately admixed with small lymphocytes, with a prominent perivascular cuffing. Occasional multinucleated tumor cells can be seen. Some cases can exhibit necrosis, marked cellular atypia, high mitotic rate, and/or abnormal mitoses.

A diagnosis of follicular dendritic cell sarcoma should be confirmed by immunohistochemical studies demonstrating expression of CD21, CD23, clusterin, and CD35 and by ultrastructural studies showing numerous long slender cytoplasmic processes and mature desmosomes.¹³