Intra-abdominal Follicular Dendritic Cell Sarcoma With Marked Pleomorphic Features and Aberrant Expression of Neuroendocrine Markers: Report of a Case With Immunohistochemical Analysis Alvaro Lezid Padilla-Rodrı´guez, MD,*y Miguel Bembassat, MD,w Miguel Lazaro, MD,z and Carlos Ortiz-Hidalgo, MD*y Abstract: Follicular dendritic cell sarcoma (FDCS) is a very rare malignant tumor arising most frequently in lymph nodes with only few reports of extranodal locations. We report the case of a 35-year-old man with a large retroperitoneal mass. Histologi- cally the tumor was composed of highly pleomorphic cells exhibiting some uncommon features such as an epithelioid appearance, cystic spaces, and multinucleated cells with morphologic features of emperipolesis. Immunohistochemically the neoplastic cells were immunoreactive for CD21, CD23 and CD35. A previously unreported expression of neuroendocrine markers (Synaptophisyn and Neuron-Specific-Enolase) was present. Ultrastructurally no neuroendocrine secretory granules were detected. FDCS can mimic a wide variety of other malignant tumors, and a correct diagnosis requires exclusion of other neoplasms and immunohistochemical confirmation. Key Words: follicular dendritic sarcoma, pleomorphic, neuro- endocrine (Appl Immunohistochem Mol Morphol 2007;15:346–352) F ollicular dendritic cell sarcoma (FDCS) is a rare malignant neoplasm, arising from follicular dendritic cells of the lymphoid tissue that affects mainly lymph nodes; however, approximately one third of the reported cases are extranodal, involving many different sites including gastrointestinal tract, retroperitoneum, media- stinum, lung, pancreas, liver, spleen, thyroid, breast, intracranial, and head and neck region. 1–12 The classic histologic feature of this neoplasm is a spindle to ovoid cell proliferation growing in a storiform pattern. The neoplastic cells are characteristically im- munoreactive with follicular dendritic cell markers including CD21, CD35, and CD23. 13,14 However in rare cases, the morphologic spectrum of the tumor cells can be widely variable and the immunohistochemical findings are mandatory for a definitive diagnosis. We present a case of a dendritic cell sarcoma with extensive pleomorphic features and an aberrant expres- sion of neuroendocrine markers, not described before. CASE REPORT The patient was a 35-year-old man who presented with a lower abdominal mass, which had been present for approximately 1 month. A firm, painless apparently fixed tumor was palpated, predominantly in the right inferior abdominal quadrant. The patient did not refer any other symptoms. CAT scans revealed a right retroperitoneal, multilobated mass displacing the bladder, sized 20 cm in the largest dimension (Fig. 1). The mass was surgically removed. HISTOPATHOLOGIC FEATURES On gross examination the neoplasm weighed 653.5 g and measured 21.0  13.8  6.0 cm. The tumor was well circumscribed, multilobated, spherical to ovoid, gray to yellow, and focally hemorrhagic. The sectioning surface of the mass was of firm consistency, heterogeneous in color, from light gray to brown-yellow, and exhibited focal irregular hemorrhagic and necrotic areas (Fig. 2). Histologic sections revealed the tumor surrounded by an incomplete fibrous capsule, growing in a diffuse pattern with multiple areas of coagulative necrosis and interspersed areas of small pseudovascular cystic spaces filled with eosinophilic proteinaceous material and/or erythrocytes (Fig. 3A). The neoplastic cells showed a wide cytologic spectrum. The classic storiform pattern was present only focally (Fig. 3B), but a fascicular configuration and a rather loose array of spindle cells with focal circular whorls centered on a blood vessel were observed. The main cellular pattern was composed of sheets of ovoid, polygonal cells that usually displayed a diffuse or occasionally trabecular configuration. The neoplastic cells were characterized by a moderate amount of slightly eosinophilic cytoplasm and indistinct cell borders resulting in a syncytial appearance. Copyright r 2007 by Lippincott Williams & Wilkins Received for publication September 14, 2005; accepted March 10, 2006. From the Departments of *Pathology, wSurgery, and zMedical Oncology, The American British Cowdray Medical Center; and yDepartment of Cell and Tissue Biology, Universidad Panamericana, Mexico City, Mexico. Reprints: Alvaro Lezid Padilla-Rodriguez, MD, Department of Pathol- ogy, ABC Medical Center, Calle Sur 136 No.116, Col Las Ame´ricas, Mexico DF 1120, Mexico (e-mail: apadillar@abchospital.com). ORIGINAL ARTICLE 346 Appl Immunohistochem Mol Morphol  Volume 15, Number 3, September 2007