Molecularly Variant Synovial Sarcoma of the Lower Lip: MN1::SSX1 Fusion as a Diagnostic Pitfall in the Head and Neck.

Synovial sarcoma is a malignant mesenchymal neoplasm typically characterized by SS18::SSX gene fusions. Although most commonly arising in the extremities, it rarely occurs in the head and neck region. We report a case of monophasic synovial sarcoma of the lower lip harboring a rare MN1::SSX1 fusion that presented a diagnostic challenge due to negative standard molecular testing. A 23-year-old woman presented with a slowly enlarging right lower lip mass. Imaging revealed a well-defined subcutaneous lesion without mandibular invasion and mildly prominent bilateral cervical lymph nodes. Core biopsy demonstrated a cellular spindle cell neoplasm with uniform cells arranged in intersecting fascicles. Immunohistochemistry showed diffuse nuclear SSX and TLE1 expression with focal cytokeratin positivity. The SS18-SSX fusion-specific antibody was negative, and fluorescence in situ hybridization showed no SS18 rearrangement. Given the morphologic and immunophenotypic features suggestive of synovial sarcoma, next-generation sequencing was performed and identified an MN1::SSX1 fusion. The patient underwent complete surgical excision with negative margins. She didn’t receive additional therapy, and no recurrence or metastasis has been identified at 16 months follow-up. This case represents the second reported synovial sarcoma with MN1::SSX1 fusion and highlights an important diagnostic pitfall: rare variant fusions may yield negative SS18-based immunohistochemistry and molecular assays despite classic morphologic features. This case emphasizes the importance of comprehensive molecular testing when clinical suspicion for synovial sarcoma remains high despite negative standard SS18-targeted assays.