Follicular Dendritic Cell Sarcoma of the Parapharyngeal Space Arising in Association With Castleman Disease.

Follicular dendritic cell sarcoma (FDCS) is a rare malignant neoplasm of follicular dendritic cell origin. It is occasionally associated with unicentric Castleman disease (UCD), particularly the hyaline-vascular (HV) variant (HV-UCD). We report a 56-year-old woman with FDCS arising in the parapharyngeal space in the background of HV-UCD. The patient presented with a painless right neck mass and extensive cervical lymphadenopathy without systemic symptoms. Surgical resection of the parapharyngeal mass revealed FDCS in the lymph nodes with features of HV-UCD. Immunohistochemistry confirmed the follicular dendritic cell lineage of the lesional cells, with positivity for CD21 and CD23. Subsequent lymph node dissection from levels 2A, 2B, 3, and 4 showed features of HV-UCD without residual FDCS. This case highlights the diagnostic challenge FDCS represents, particularly when arising in an unusual location. While complete surgical excision remains the standard of care, long-term follow-up is necessary to monitor for recurrence or metastasis.