Answer: Rosai-Dorfman Disease

Histology: The deep dermis and subcutaneous tissue is involved by a process which is characterized by small lymphocytes, fibrosis, and larger epithelioid cells with vesicular chromatin and prominent nucleoli. These cells have pale cytoplasm and focally have lymphocytes and other inflammatory cells within their cytoplasm (emperipolesis). These cells are immunoreactive for S100 protein and CD68 but not for CD1a. There is a dense background of fibrosis and plasma cells.

Discussion: A fibrohistiocytic lesion is suggested by the focal storiform pattern; however, the presence of emperipolesis and the strong S100 immunoreactivity of the large histiocytes argue against fibrous histiocytoma. Balloon cell melanomas do not have the growth pattern of the current lesion, though they do share the strong S100 positivity. Balloon cell melanomas are typically more superficially located, and lack the dense inflammatory infiltrate seen in this case. Juvenile xanthogranulomas typically occur in a younger age group, are associated with Touton giant cells, and are S100 protein negative.

Rosai-Dorfman disease (sinus histiocytosis with massive lymphadenopathy, SHML) may be entirely extranodal as it is in the current case. These extranodal cases tend to have a more storiform pattern, be more spindled, have more collagen deposition, and have less emperipolesis, so the diagnosis is more difficult. The main differential here is with fibrohistiocytic soft tissue tumors. SHML may have a protracted course or may regress. Some patients have died due to extensive disease comprising vital organs or underlying immune abnormalities.