Unusual mimicker of odontogenic carcinoma: Primary synovial sarcoma of the temporomandibular joint- a critical role for molecular diagnosis

Background: Synovial sarcomas (SS) are malignant tumors originating from pluripotent mesenchymal cells, with predilection for periarticular areas, as deep-seated soft tissue tumors. Treatment of SS of the head and neck (HN) is usually radical local excision and additional radiation or (neo)adjuvant chemotherapy or both. SS are characterized by a specific SS18-SSX1/2/4 fusion gene. SS have several morphological variants: monophasic, biphasic, or poorly differentiated. Methods and results: We describe a SS of mandibular condyle that showed an overwhelming (>95%) epithelial cell component mimicking odontogenic carcinoma. One year prior to presentation, a woman developed a 2.5 cm destructive bone lesion in the left mandibular condyle. The initial diagnosis of ameloblastoma on biopsy was changed to odontogenic carcinoma on the surgical specimen. Four months later a computed tomography (CT) revealed local recurrence; another month later, magnetic resonance imaging (MRI) depicted a new left temporal lobe brain lesion. The patient started on a carboplatin and paclitaxel therapy, with no clinical or radiologic benefit. Subsequently she was presented for another opinion. The pathology material was re-reviewed. Fluorescence in situ hybridization (FISH) resulted a positive result for SS18 (SYT) rearrangement; next generation sequencing (NGS sarcoma fusion panel) reported an SS18-SSX2 fusion transcript. Based on molecular testing the tumor was reclassified as synovial sarcoma. Her systemic treatment was changed to anthracycline based systemic therapy. Conclusions: This case emphasizes the importance of molecular approaches in diagnostic pathology. Accurate diagnosis is imperative to avoid misclassification as carcinoma (metastasis or primary e.g., odontogenic carcinoma), carcinosarcoma or a different sarcoma type with epithelioid or epithelial differentiation and to determine proper treatment.