The First Case of Clear Cell Sarcoma of Soft Tissue with Purely Plasmacytoid Morphology: A Case Report

Clear cell sarcoma of soft tissue (CCSST) is a rare malignant tumor occurring in the extremities of young adults. The tumor cells arranged in nests and sheets, with clear to eosinophilic cytoplasm separated by fibrous septa. CCSST has been documented to have atypical histopathological features, such as epidermotropism or myxoid differentiation, which may set pitfalls in differential diagnosis. Herein, we report a case of CCSST with purely plasmacytoid morphology which has never been described. The patient, a 15-year-old male, presented with a 5-cm mass in his left inguinal area. He received cryotherapy for a “black mole” on his back two years ago without pathology record. Positron emission tomography-computed tomography examination showed nodules in the left groin and the lung, the latter was considered as metastasis. Needle biopsy of inguinal mass showed tumor cells were uniformly sized, with eosinophilic cytoplasm and eccentric, basophilic nuclei, resembling plasmocytes. Immunostaining showed strong expression of S-100, HMB45, Melan-A, and MUM-1 in all tumor cells, while CD79a and CD38 were negative. Genetic analysis revealed the tumor harboring no BRAF mutations but EWSR1 rearrangement. Accordingly, the diagnosis of CCSST with plasmacytoid morphology was made. This is the first case of CCSST with purely plasmacytoid morphology which should be aware during evaluating and differentiating CCSST from tumors with similar morphology, such as plasmacytoid melanoma and extramedullary plasmacytoma, etc.