[INTERDIGITATING DENDRITIC CELL SARCOMA]. Interdigitating dendritic cell sarcoma (IDCS) is an extremely rare hematopoietic tumor representing neoplastic counterpart of interdigitating dendritic cells of lymph nodes and other hematopoietic tissues. The tumor grows in fascicles and bundles (arrow) in the interfollicular zone with residual lymphocytes and follicles around (arrowhead).
[INTERDIGITATING DENDRITIC CELL SARCOMA]. The tumor growth of IDCS can replace most of the lymphoid tissue by large bundles and fascicles simulating a variety of spindle cell sarcomas, melanoma and spindle cell carcinoma. In the absence of any significant amount of lymphoid tissue the true nature of the neoplasm may only be determined by immunohistochemical evaluation.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. Typically these tumors grow in bundles and fascicles (arrow) in the interfollicular zone dissecting the lymphocyte population of the paracortex. The morphology may be indistinguishable from leiomyosarcoma, spindle cell synovial sarcoma, solitary fibrous tumor, spindle cell melanoma, palisaded myofibroblastoma, and a host of other neoplasms with similar histologic features. The key to suspect this neoplasm is based on the clinical history of solitary node enlargement, the interfollicular growth pattern and the quality of individual cells.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. A storiform growth pattern (arrow) may also dominate the node with lymphocytes cuffing around the curved bundles of tumor cells (arrowhead).
[INTERDIGITATING DENDRITIC CELL SARCOMA]. These tumors can have a range of histologic differentiation and grades. The well differentiated tumors show little morphologic atypia and are called “tumors” whereas the high grade tumors are called “sarcomas” in keeping with degree of nuclear plemorphism, as shown in this case. Low grade “tumors” show rare mitoses whereas “sarcomas” shows 5-10 mitoses in 10 HPF. A mitosis is shown with a big arrow. Note fine fibrillary collagen in the background (arrowhead).
[INTERDIGITATING DENDRITIC CELL SARCOMA]. Note nuclear pleomorphism in this case (arrows) with irregular and large nuclei very similar to nuclei seen in certain cases of large cell lymphomas. However, the spindle cell nature of tumor cells and growth pattern practically excludes a diagnosis of lymphoma.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. The individual cells of an interdigitating dendritic cell sarcoma do not show distinct cell borders but the fine fibrillary collagen in the stroma may simulate an appearance of distinct cell membranes. Also note abundant pale eosinophilic nature of the cytoplasm and large vesicular nuclei with one of a few distinct nucleoli.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. In cross sections of the tumor bundles individual tumor cells may appear “epitheliod” or “histiocytoid” with abundant pink to pale cytoplasm and round to pleomorphic nuclei with vesicular chromatin and distinct nucleoli. Note the presence of collagen “strings’ around many individual tumor cells (arrows). The presence of such collagen deposition is not seen with metastatic spindle cell melanomas.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. The true identification of the neoplasm requires immunohistochemical staining. A fairly consistent immunophenotype of these tumors have emerged including positive staining for vimentin, S-100, fascin, CD68, and negative staining for CD45 (LCA), CD21, CD23, CD35, CD1a, podoplanin, cytokeratins, and HMB45, and Melan-A. This photomicrograph shows positive staining for fascin which is an excellent marker for dendritic cells.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. Staining for S-100 is consistently seen in these cases and can be very useful in differentiating these tumors from follicular dendritic cell tumors/sarcomas. However, positive staining for S-100 may mislead one to mistake this neoplasm for a melanoma.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. Expression of CD68 is variably seen in this tumor but can be helpful in the differential diagnosis when positive.
[INTERDIGITATING DENDRITIC CELL SARCOMA]. Expression of smooth muscle actin (SMA) can rarely be seen in these tumors. However, when positive other muscle markers must be performed to exclude a diagnosis of smooth muscle tumor. Desmin and muscle-specific antigen (HHF-35) were negative in this case.
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