Difficult Diagnosis of Interdigitating Dendritic Cell Sarcoma of the Retroperitoneum: A Case Report and A Brief Review of the Literature.

Interdigitating dendritic cell sarcoma (IDCS) is a rare and aggressive neoplasm classified within the M-group of malignant histiocytoses. Its diagnosis poses a significant challenge. This article aims to describe a rare clinical case of IDCS and to illustrate the differential diagnostic process undertaken by the authors in establishing this diagnosis. A 60-year-old woman was admitted for the resection of a retroperitoneal mass discovered via CT scan. Morphological examination revealed a 7.5×5.5×5.0 cm tumor, encapsulated by a thin fibrous capsule. The tumor was composed of 90-95% inflammatory infiltrate with lymphocyte-like cells showing mature nuclear morphology (CD3+ and CD20+ cells) mixed with histiocytes and plasma cells, and 5-10% large polymorphic spindle-shaped cells expressing expression of CD45, CD68, CD1a, CD21, CD35, CD31, and CD34. An extensive immunohistochemical panel was performed to exclude various other tumors. Based on the morphology and immunophenotype, a diagnosis of IDCS was established. Further literature analysis indicated the nonspecificity of symptoms in patients with this tumor localization and variability in CD45 and CD68 staining in tumor cells, with consistent lack of expression of CD21, CD23, CD35, CD1a, and specific T- and B-cell antigens. IDCS is a rare and poorly understood tumor with a poor prognosis. The nonspecificity of clinical symptoms and the need for extensive morphological differential diagnosis render this entity a diagnosis of exclusion, requiring significant diligence from the pathologist.