Mimics of carcinoma: pancytokeratin positive follicular dendritic cell sarcoma

Introduction/Objective Follicular dendritic cell sarcoma (FDC-S) is a malignant neoplasm of follicular dendritic cells that can present with variable morphology including epithelioid and spindle cell components. Given its morphological diversity, FDC-S can mimic neoplasms including poorly differentiated carcinomas. Here we present a rare case of FDC-S in which immunohistochemical staining for pancytokeratin was focally positive, potentially leading to an erroneous diagnosis if a full spectrum of immunohistochemical stains was not considered. Methods/Case Report A 34-year-old man presented with an enlarging left neck mass, night sweats and fatigue that began approximately three months ago. CT scan showed a 5.6 x 4.2 x 3.7 cm necrotic mass in the left upper lateral carotid space. Histologic examination of the incisional biopsy specimen demonstrated portions of soft tissue comprised of pleomorphic and plump spindle cells forming whorls with no discernable node architecture (Panels A-B). The tumor cells are large with oval nuclei, vesicular chromatin, distinct nucleoli, and a moderate amount of eosinophilic cytoplasm. Occasional binucleated and multinucleated tumor cells, rare tumor cells with nuclear pseudoinclusions and atypical mitotic figures, and focal necrosis are identified. Scattered small mature lymphocytes are present. By immunohistochemistry, tumor cells are positive for pancytokeratin (focal) (Panel C), CD21 (panel D), CD23 (panel E), D2-40 (panel F), CD35 and clusterin, and negative for CD3, CD20, CD30, CD45, ALK-1, EMA, PAX5, PDL1 and S100 stains. The overall morphology and immunostaining pattern are compatible with follicular dendritic cell (FDC) sarcoma. Results (if a Case Study enter NA) NA Conclusion FDC sarcoma rarely occurs as the solitary mass in the head and neck region. The unusual focal positivity for pancytokeratin highlights the importance of comprehensive immunohistochemistry stains to confirm the diagnosis of FDC sarcoma since this case is focally positive for pancytokeratin, mimicking other neoplasms such as poorly differentiated carcinomas