Interdigitating Dendritic Cell Sarcoma/Tumor Leticia Quintanilla-Martinez Ingolstaedter Landstraße 1 Oberschleissheim, Germany

Case History: Cervical lymph node in a 68-year-old male patient. The lymph node showed partial effacement of the architecture with involvement mainly of the paracortical region with the presence of residual follicles. Sheets of spindle cells with a storiform pattern and marked cytologic atypia were identified. The tumor cells were positive for S100 and negative for CD21 and CD1a. Slide 1 Click to view with ImageScope Click to view with a Web-Based Viewer Diagnosis: Interdigitating dendritic cell sarcoma/tumor. The morphology and immunophenotype are typical in this tumor. Discussion: Neoplasms of histiocytes and dendritic cells are rare. Immunophenotypically and morphologically 5 different categories are recognized (Pileri) Follicular dendritic cell (FDC) tumor/sarcoma (21% of the cases) CD21+, CD35+, CD1a- and S100-/+, CD68+/- lysosyme – Utrastructure: Presence of desmosomes Langerhans cell (LC) tumor /sarcoma (38% of the cases) CD1a+, S100+, CD68+, lysosyme-/+, CD21-, CD35- Ultrastructure: Birbeck granules Interdigitating dendritic cell (IDC) tumor/sarcoma (7% of the cases). S100+, CD68+/-, lysosyme-, CD1a-, CD21-, CD35- Ultrastructure: Complex interdigitating cell processes. Histiocytic sarcoma (27% of the cases) CD68+, lysosyme+, CD1a-, S100-/+, CD21-, CD35- Ultrastructure: Numemrous lysosomes Unclassifiable (7% of the cases) IDC tumor/sarcoma is a rare neoplasm affecting mainly elderly patients with a median age of 71 years, although rare cases have been described in pediatric population (Pillay). The patients present with lymph node enlargement or less frequent extranodal disease, particularly the skin, intestine, soft tissue, spleen, liver, kidney and lung. Microscopically, IDC tumors reveal a spectrum of morphological features: the tumor cells are usually medium to large-spindle shaped with a fascicular or whorled growth pattern, similar to identical to FDC tumors. Cytologic atypia varies from case to case and the mitotic rate is usually low. The key morphological finding suggesting the diagnosis of IDC tumor is the paracortical location of the process in the lymph node and appropriate phenotype. In contrast to FSC tumors, where there is a predominance of small B-lymphocytes interspersed within the large tumor cells, in IDC tumors, the admixed small lymphocytes are almost always of T-cell lineage, with a near-absence of B-cells. Recently, has been shown that the strong expression of clusterin in FDC tumors can also help in the differential diagnosis in cases with overlapping morphologic features (Grogg). Some IDC tumors are composed of large and round to oval cells with abundant cytoplasm and vesicular nuclei raising the differential diagnosis of LC tumor, true histiocytic sarcoma or even anaplastic large cell lymphoma. In these cases the most important criteria for the diagnosis of IDC tumor are: strong S100 protein positivity, negativity for FDC markers and CD1a. Other important differential diagnoses are malignant melanoma and fibroblastic reticular cell tumor. The latter, which can also show a paracortical location, is typically negative for S100 protein and positive for smooth muscle actin and desmin. The clinical course appears to be variable ranging from benign localized disease to widespread lethal disease. References: Pileri SA, et al. Tumours of histiocytes and accessory dendritic cells: an immunohistochemical approach to classification from the International Lymphoma Study Group based on 61 cases. Histopathology 2002, 41: 1-29 Pillay K et al. Interdigitating dendritic cell sarcoma: a report of four pediatric cases and review of the literature. Histopathology 2004, 44:283-291 Grogg KL, et al. Clusterin expression distinguishes follicular dendritic cell tumors from other dendritic cell neoplasms: report of a novel follicular dendritic cell marker and clinicopathologic data on 12 additional follicular dendritic cell tumor and 6 additional interdigitating dendritic cell tumors. Am J Surg Pathol 2004, 28: 988-998.