Biphasic Synovial Sarcoma With Neuroendocrine Differentiation: A Case Report

ABSTRACT: Synovial sarcoma is a malignant soft tissue neoplasm that can closely mimic carcinoma by histology and immunohistochemistry, making accurate diagnosis challenging in small or superficial biopsies. In this study, we report a rare case of a superficial biphasic synovial sarcoma with striking neuroendocrine differentiation, a biopsy of which was initially interpreted as a poorly differentiated carcinoma due to diffuse cytokeratin and neuroendocrine marker expression. Resection subsequently revealed a biphasic morphology, and molecular testing confirmed the presence of an SS18::SSX fusion, confirming the diagnosis of synovial sarcoma. This case highlights a potential diagnostic pitfall in dermatopathology and stresses the importance of confirmatory molecular testing in cases where both sarcoma and carcinoma are included in the differential diagnosis.