Follicular dendritic cell sarcoma of the head and neck expressing thyroid transcription factor-1: a case report with clinicopathologic and immunohistochemical literature review.

Follicular dendritic cell sarcoma (FDCS) is a rare neoplasm of follicular dendritic cells, most commonly affecting the lymph nodes and extranodal soft tissues of the head and neck, but also potentially arising in any visceral organ. FDCS with its diverse morphologies raises an occasionally challenging differential of primary and metastatic tumors with overlapping histologic and immunohistochemical features. When involving the head and neck, FDCS may be confused with squamous cell carcinoma, undifferentiated carcinoma, extracranial meningioma, and variants of papillary thyroid carcinoma. We describe here a case of FDCS showing nuclear grooves, intranuclear pseudoinclusions, diffuse epithelial membrane antigen and focal cytokeratin staining, and the first documented report of positivity for thyroid transcription factor-1. A discussion of the differential diagnosis and potential diagnostic pitfalls in FDCS brought forth by thyroid transcription factor-1 immunoreactivity and a full review of clinicopathologic and immunohistochemical features of head and neck FDCS are presented.