Contributed by Su Zheng, MD, PhD and Uma Rao, MD
Published on line in August 2002
This was a 74-year-old gentleman, who presented with a painless mass in his right medial thigh. There was no history of trauma. The patient was otherwise asymptomatic with no fever or lymphadenopathy. Various blood tests were unremarkable. Biopsy was performed and diagnosed as "inflammatory pseudotumor". The possibility of extranodal Rosai-Dorfman disease was raised because of the presence of a few S100-positive histiocytes. The mass was excised surgically and the specimen was sent to the pathology department for further evaluation.
Grossly, the specimen consisted of a skin ellipse, 4.0 x 0.8 cm, covering a non-encapsulated but well-circumscribed subcutaneous ovoid firm mass measuring 6.5 x 4.5 x 2.5 cm. The cut surfaces of the mass were yellow-tan, glistening and lobulated (Figure 1). Touch preparations stained with routine hematoxylin and eosin revealed numerous inflammatory cells including lymphocytes and plasma cells. A number of large cells with round or oval, often indented, vesicular nuclei were also present; some of which contain small lymphocytes within their cytoplasm (Figure 2). Flow cytometric immunophenotypic studies were performed and demonstrated heterogeneous T cells and polyclonal B cells.
Microscopically, the lesion consisted of loosely arranged sheets of large pale cells intermingled with collagen bundles, fibroblasts, small lymphocytes, plasma cells and lymphoid aggregates. An occasional storiform pattern was observed in the intervening stroma (Figure 3 and 4). These large cells had round or indented vesicular nuclei and small but distinct nucleoli, with abundant pale cytoplasm and occasional dendritic processes. Some of them contained intact lymphocytes (emperipolesis) in the cytoplasm (Figure 5). Mitotic figures were absent. Immunohistochemical stains demonstrated that the large cells were positive for S100 (Figure 6) and CD68, and negative for CD1a, desmin, T- and B-cell markers.