OBJECTIVETo study the clinicopathologic features and immunophenotype of intraabdominal extranodal follicular dendritic cell sarcoma (FDCS) and the relationship with Epstein-Barr virus (EBV).

METHODSThe clinical and histologic features of 4 cases of FDCS were evaluated. Immunohistochemical study was performed using standard EnVision method for CD21, CD23, CD35, S-100 protein, CD68, HLA-DR, vimentin, epithelial membrane antigen, desmin, CD34 and CD117. In-situ hybridization for EBV-encoded RNA (EBER) was carried out in 2 cases.

RESULTSThe age of patients ranged from 28 to 63 years (mean=42 years). The male-to-female ratio was 3:1. The clinical presentation was abdominal discomfort, pain or mass. Radiologic examination revealed concurrent lesions in stomach and left lobe of liver in 1 patient, while non-specific intraabdominal masses were detected in the remaining cases (in which the tumor was later found to be located in the appendix, mesentery of jejunum and omentum). Two cases were misdiagnosed as gastrointestinal stromal tumor before operation. Grossly, the tumors appeared as large solid nodules, with a mean diameter of 10.8 cm. Three of the cases showed areas of necrosis. Histologically, there were plump spindle, ovoid to epithelioid cells associated with scattered multinucleated giant cells. The tumor cells were arranged mostly in storiform pattern, whorls, fascicles or solid sheets. Lymphocytic infiltrates with perivascular cuffing were noted in all cases, resulting in a distinctive biphasic pattern. Two tumors showed significant cytologic atypia, with mitotic figures (including atypical mitotic figures) readily demonstrated. The remaining case (occurring in liver) was composed of scattered large atypical cells embedded in a dense inflammatory background, mimicking inflammatory pseudotumor. Immunohistochemical study showed that all cases were positive for CD21, CD23 and vimentin. There was focal expression of CD35, S-100 protein, CD68, HLA-DR and epithelial membrane antigen. The staining for CD34 and CD117 was negative. In-situ hybridization for EBER was negative in 2 cases tested.

CONCLUSIONSIntraabdominal extranodal FDCS is extremely rare. Familiarity with its characteristic histologic features and immunophenotype is important in distinguishing the tumor from other intraabdominal spindle cell lesions (such as gastrointestinal stromal tumor). Hepatic FDCS may show inflammatory pseudotumor-like features, resulting in misinterpretation. Non-hepatic intraabdominal FDCS seems to have little association with EBV infection.