An SS18::NEDD4 cutaneous spindled and epithelioid sarcoma: An hitherto unclassified cutaneous sarcoma, resembling epithelioid sarcoma with aggressive clinical behavior

SS18::SSX gene fusions as a result of t(X,18)(p11;q11) have only been described in synovial sarcoma (SS). Recently, an SS18::NEDD4 gene fusion was identified in a single case of primary renal SS exhibiting a hypocellular and myxoid morphology. Herein, we report a case of an unclassified malignant cutaneous spindled and epithelioid neoplasm in a 60‐year‐old female that resembled an epithelioid sarcoma (ES) and harbored a rare SS18::NEDD4 gene fusion. Briefly, the patient presented with a progressively growing cutaneous mass involving the volar aspect of right hand, warranting an amputation. Histologic sections revealed a cutaneous ulcerative neoplasm composed of spindled and epithelioid cells, bearing a certain semblance to ES, with diffuse invasion into the subcutis and skeletal muscle. Coagulation tumor necrosis and mitotic figures were present. By immunohistochemistry, the tumor cells were positive for keratins (AE1/3 and cam5.2), vimentin, CMYC, BCL2, p53, smooth muscle actin (focal), and TLE1 (multifocal) and negative for p40, p63, CK5/6, CK7, CK20, CD56, CD31, CD34, ERG, desmin, SMMS, H‐Caldesmon, myogenin, and S‐100. Expression of INI1 stain was retained. The unusual histomorphology and inconclusive immunophenotypic profile lead to next‐generation sequencing identifying an SS18::NEDD4 gene fusion with genomic coordinates 5′‐SS18 (ex1‐9 NM_005637)‐NEDD4 (ex14‐29 NM_006154). Fluorescence in situ hybridization confirmed SS18 gene rearrangement. Within 2 years, the patient developed widespread metastatic disease. Despite aggressive multimodality treatment, the patient succumbed to disease. In summary, we report a unique case of previously unclassified cytokeratin positive malignant cutaneous spindled and epithelioid sarcoma with aggressive behavior, harboring an SS18::NEDD4 fusion.