Interdigitating cell sarcoma: a morphologic and immunologic study of lymph node lesions in four cases.

Interdigitating cell sarcoma is an extremely rare tumor. Its presentation and histologic appearance has varied among the reported cases. In this study, the authors investigated four cases of the hematolymphoid malignancy arising within lymph nodes, which were considered to be of interdigitating cell origin. All patients presented in the 6th to 8th decade of life with peripheral lymphadenopathy, and had a relatively indolent clinical course, without bone marrow or skin involvement. Carcinomas were observed as a second neoplasm in two of four patients. Distinctive morphologic features are proliferation of histiocyte-like cells with nuclear pleomorphism and occasionally multinucleated, paracortical distribution sparing of B-cell regions, fibrosis, sinus infiltration, and a prominent eosinophil/plasma cell infiltrates. The combination of light microscopic, fine structural, and immunohistochemical features suggested that these tumors derive from interdigitating cells; these tumor cells expressed CD68 (KP1), S-100 protein and HLA-DR, but lack CD21 (1F8), desmosomes and Birbeck granules. The diagnosis of interdigitating cell sarcoma should be considered on any pleomorphic tumor with the features described in this report.