Extranodal follicular dendritic cell sarcoma of the gastrointestinal tract. Morphologic, immunohistochemical and ultrastructural analysis of two cases.

Tumors showing differentiation toward follicular dendritic cells are rare and usually occur in lymph nodes. Their occurrence at extranodal sites was recognized recently. This report documents two further extranodal cases, one arising in the small intestine of a 23-year-old woman and the second in periduodenal retroperitoneal soft tissue of a 63-year-old man. Both neoplasms displayed a typical biphasic morphologic pattern, composed of large cells with indistinct cytoplasmic borders creating a syncytial appearance, admixed with spindle cell areas resembling malignant fibrous histiocytoma, and infiltrated by numerous small T lymphocytes. Expression of CD21 and CD35, together with ultrastructural demonstration (in one case) of long cytoplasmic processes connected by desmosomes, confirmed follicular dendritic cell differentiation. One case also expressed HLA-DR, and both showed aberrant epithelial membrane antigen staining. Neither tumor showed CD45 expression. In both cases the tumors were treated by local excision. Case 1 has shown an unusually aggressive course with extensive intraperitoneal recurrence. The biphasic morphologic pattern and tumor immunophenotype are discussed in relation to increasing evidence of immunophenotypic and morphologic heterogeneity of normal follicular dendritic cells.