Pediatric Synovial Sarcoma with SMARCB1 Loss and Aberrant ALK Expression: A Diagnostic Pitfall Necessitating Molecular Confirmation

Synovial sarcoma (SS) is a malignant soft tissue tumor characterized by biphasic mesenchymal and epithelial differentiation and is histologically classified into biphasic, monophasic spindle cell, and poorly differentiated subtypes.The monophasic spindle cell variant is particularly challenging to diagnose due to its overlap with other spindle cell tumors, including infantile fibrosarcoma (IFS), inflammatory myofibroblastic tumor (IMT), and malignant peripheral nerve sheath tumor (MPNST).Over 90% of SS harbor the characteristic t(X;18) (p11.2;q11.2) translocation, resulting in an SS18::SSX fusion. 1 The tumor typically occurs in the deep soft tissues near joints, especially around the knee; however, despite its name and frequent periarticular location, it does not originate from synovial tissue. 2 Clinically, it occurs most frequently in adolescents and young adults, whereas pediatric presentations are relatively uncommon.On magnetic resonance imaging (MRI), SS typically appears as a well-defined, multilobulated soft-tissue mass with heterogeneous T2 signal and variable contrast enhancement.