Follicular dendritic cell tumor and vascular neoplasm complicating hyaline-vascular Castleman's disease.

The localized form of hyaline-vascular Castleman's disease may rarely be complicated by a vascular neoplasm, which appears to arise in a background of vascular hyperplasia. We report a case complicated by follicular dendritic cell tumor, a hitherto undescribed occurrence that is expected from the presence of a "dysplastic" component of follicular dendritic cells in Castleman's disease. The patient had a large tumor mass in the mesocolon, associated with multiple small peritoneal deposits. The lesion showed typical histologic features of follicular dendritic cell tumor with storiform arrangement of spindly cells and admixture of small lymphocytes. The follicular dendritic cell nature of the tumor was confirmed by immunoreactivity with CD21 and CD35 antibodies and by ultrastructural demonstration of cell processes connected by desmosomes. Hyaline-vascular follicles were identified at the peripheral portion or occasionally scattered in the center of the tumor. Rarely, follicle-like structures were formed by tumor cells, possibly representing in situ malignant change. In addition, there was a small focus of vascular neoplasm comprising well-formed pericyte-rich small blood vessels devoid of cellular atypia. This report of the development of follicular dendritic cell tumor in Castleman's disease therefore completes the picture in that tumors corresponding to each cell type that proliferates in Castleman's disease have now been documented.