Clear Cell Sarcoma masquerading as Malignant Melanoma: A Case Report

Abstract

Abstract Introduction/Objective Clear cell sarcoma (CCS) is a rare and diagnostically challenging soft tissue tumor that can mimic other malignancies with melanocytic differentiation. Histologically it shows diffuse sheets of clear cells with scattered melanin pigment in a subset of cases. Other morphologies have been described but contain atleast a portion of clear cell features. We report a case of CCS with unusual histomorphology, highlighting the importance of incorporating cytogenetic and immunohistochemical studies to accurately diagnose this tumor. Methods/Case Report We present a case of a 49-year-old male with a left lateral knee mass associated with pain and swelling for one year. A core biopsy revealed a diffuse proliferation of pleomorphic spindle and atypical epithelioid cells. These atypical cells exhibited intranuclear vacuoles, variable prominent nucleoli, abundant dusky cytoplasm, and increased mitotic activity. Immunohistochemical studies were positive for S100, SOX10, and Melan-A (patchy), suggesting a melanocytic differentiation. However, no expression of Preferentially Expressed Antigen in Melanoma (PRAME) or BRAF V600E mutation was noted. Given the clinical history and lack of cutaneous melanocytic lesion, clear cell sarcoma was considered in the differential diagnosis. Fluorescence in situ hybridization (FISH) studies revealed Ewing sarcoma region 1 (EWSR1) gene rearrangement in neoplastic cells, confirming the diagnosis of CCS. Results (if a Case Study enter NA) NA Conclusion The tumor lacked definitive histologic features including clear cell features, demonstrating the variability in histomorphology that can be encountered with CCS. Distinction of CCS from melanoma is of clinical significance as treatment significantly differs. Accurate diagnosis requires the incorporation of cytogenetic and immunohistochemical studies to differentiate it from melanocytic neoplasms. Variability in histomorphology of CCS can pose, as is evident in our case, as a diagnostic pitfall, and CCS should be considered in the differential diagnosis of unusual non-cutaneous melanocytic proliferations that lack clear cell features and show compelete spindle and epithelioid morphology.