256 Monophasic Synovial Sarcoma Presenting as a Parapharyngeal Mass—Report of a Rare Case

Abstract

Synovial sarcoma (SS) is an aggressive soft tissue sarcoma, most commonly arising in the peri-articular tissue of extremities in young adults, harboring a specific chromosomal translocation t(X;18)(p11.2;q11.2). SS is a misnomer because it does not arise from the synovium; it appears to arise from pluripotent stem cells that are capable of differentiating into mesenchymal and/or epithelial structures and lacks synovial differentiation. SS accounts for ~5%-10% of soft tissue sarcomas. Occasionally, it arises in the head and neck region and carries a poor prognosis. Morphologically, SS can be biphasic (BSS), monophasic (MSS), or poorly differentiated. We report a rare case of MSS presenting as a parapharyngeal mass. The patient is a 27-year-old-man presenting with left-sided facial and mandibular pain. Imaging showed large parapharyngeal soft tissue mass in the region of the left pterygoid musculature measuring 3.6 x 2.9 x 2.5 cm with focal calcifications. He underwent resection of this mass, which showed spindle cell neoplasm with variably prominent hemangiopericytoma-like vascular pattern. Vague nuclear palisading was also noted. Immunohistochemical stains were focally positive for cytokeratins (AE1/AE3, CK7, CK8 (Cam5.2), and CK19), calponin, calretinin, EMA, GFAP, and synaptophysin with Ki67 labeling being 25%-30%. The tumor cells were negative for CD34, CD117, desmin, HMB45, myogenin, P63, S100, and SMA. RT-PCR showed SYT/SSX2 fusion transcript. Combining all, the diagnosis of MSS-spindle cell type was confirmed. Based on AFIP data, ~3% of SS arise in this location. SS is a rare entity to be encountered in the head and neck region and is often a challenge in terms of diagnosis, especially the MSS type. The differential of monophasic tumors is fibrosarcoma, malignant schwannoma, and hemangiopericytoma. We report this case of MSS because of its rare presentation as a parapharyngeal mass. Clinicians and pathologists should be aware of this unusual location and presentation of SS.