Extranodal follicular dendritic cell sarcoma of the head and neck region: three new cases, with a review of the literature.

Extranodal follicular dendritic cell (FDC) sarcoma of the head and neck region is uncommon, with 16 well-documented cases previously reported (four in the tonsil, four in the pharynx, two in the palate, five in the soft tissue, and one in the thyroid). We here report an additional three cases of extranodal FDC sarcoma in the tonsil (two cases) and pharynx (one case). In these new cases, the neoplastic cells were arranged in diffuse, fascicular, and vaguely whorled growth patterns. A background lymphocytic infiltrate was sprinkled throughout the neoplasms, with focal prominent perivascular cuffing. Scattered multinucleated giant cells were present. Immunohistochemically, tumor cells were strongly and diffusely positive for follicular dendritic cell markers CD21 and CD35. Tumor cells were diffusely positive for fascin and negative for leukocyte common antigen, S-100 protein, cytokeratin, and Epstein-Barr virus (EBV) latent membrane protein-1 (EBV-LMP). EBV was also not detected in the tumor cells by in situ hybridization for EBV-encoded RNAs. FDC sarcomas are probably an underrecognized neoplasm, especially when they occur in extranodal sites in the head and neck region. Two of the three new cases we report were initially misdiagnosed, and five cases of extranodal FDC sarcoma in the head and neck region reported in the recent literature were initially misdiagnosed. Our aim is to complement the current understanding of this neoplasm and alert pathologists to this rare entity in this region to avoid misdiagnosis. Recognition of extranodal FDC sarcoma requires a high index of suspicion, but this tumor has numerous distinctive histological features that should bring the neoplasm into the differential diagnosis. Confirmatory immunohistochemical staining with follicular dendritic cell markers such as CD21 and/or CD35 is essential for the diagnosis. Correct characterization of this neoplasm is imperative given its potential for recurrence and metastasis.