Localization

 lymph nodes
 mediastinum (#11745075#)
 lungs (#11486174#)
 breast (rare) (#10196389#)

Microscopical synopsis

 patterns of growth: diffuse, myxoid and fascicular
 neoplastic cells large, polygonal, with a slightly eosinophilic cytoplasm and oval or convoluted nuclei
 neoplastic cells intermingled with small lymphocytes, plasma cells and a few bizarre multinucleated giant cells.
 spindled tumoral cells (+/-)

Variants

 myxoid variant

Immunochemistry

 S-100+
 CD21+
 EMA+ (epithelial membrane antigen)
 CD21+/- (complement receptor)
 CD35+/- 9complement receptor)
 CD68+/-
 cytokeratins+/-
 clusterin+

Electron microscopy

 numerous interdigitating cytoplasmic processes with intercellular junctions of the serrated
 immature desmosomal or undifferentiated types

Differential diagnosis

 inflammatory pseudotumour
 interdigitating dendritic cell sarcoma
 true histiocytic lymphoma
 malignant Langerhans cell histiocytosis
 anaplastic large-cell lymphoma
 melanoma
 sarcomas

References

 Perez-Ordonez B, Rosai J. Follicular dendritic cell tumor: review of the entity. Semin Diagn Pathol. 1998 May;15(2):144-54. Review. PMID: #9606805#