Synovial sarcoma (SS) is a common mesenchymal malignancy which represents approximately 10% of all soft tissue sarcomas. ¹ It frequently affects the extremities, and only 6.8% of cases have been reported in the head and neck region. ² It typically occurs in young adults and is more common in males than females. The most common sites in the head and neck region are hypopharynx and parapharyngealspaces. ³ However, SS can also occur in tonsils, tongue, and orofacial soft tissues. ⁴ It is not difficult to diagnose SS microscopically with its classic biphasic appearance, but it is seriously problematic to diagnose the monophasic forms, and in this situation immunohistochemistry is very useful. ⁵ When monophasic variants of SS arise in unusual sites, such as the head and neck region, recognition and differential diagnosis become more difficult. It is important to diagnose this entity in such locations correctly because wide surgical excision without regional node dissection is still the main approach to treatment. ⁶ Because of the rarity of SS in the head and neck region, especially in the jaws, and the risk of misdiagnosis of monophasic variants in unusual sites, the diagnosis of SS requires an integrated multidisciplinary approach by means of powerful diagnostic tools. In this article, we report a rare case of primary monophasic SS of the mandible and its clinical, histopathological, and immunohistochemical features. Also, a thorough search in the literature was performed, and a comparative summary of all reported cases is presented here.

A 76-year-old man was referred to Oral and Maxillofacial Pathology Department of Tehran University of Medical Sciences, Iran, in February, 2011, with a progressive swelling in the left side of his face of 1-month duration. He also complained of dull pain in the same region and paresthesia of the left half of the lower lip. Clinical examination revealed bony hard, nontender swelling of the posterior region of the mandibular body. Also, submandibular lymph nodes at the same side were enlarged and palpable. No other intra-oral sign was observed and no significant event was noted in his past medical history. A conventional panoramic radiograph showed ill-defined destruction of the posterior mandibular body and ascending ramus Figure 1. A computed tomography (CT) scan revealed a destructive lesion at the same region. Figure 1

Panoramic radiography showed a destructive lesion in the left mandibular body and ramus

SS is a clinically and morphologically well-defined entity that despite its name is not common in joint cavities and is frequently diagnosed in areas having no relation with synovial structures. It occurs primarily in para-articular regions of extremities, usually in a close association with tendon sheaths and joints. ⁷

In the head and neck region, the hypopharynx is considered as a common site of involvement, but intraoral cases are extremely rare and based on our knowledge, only 39 cases have been reported until 2011. ⁶ , ⁸ , ⁹ Although the tongue is considered as a common intra-oral site, SS has been reported in other intra-oral areas such as buccal mucosa, floor of the mouth, and jaw bones. ¹⁰ To our knowledge, only six cases of reported intraoral SSs have occurred in the jaws and our case represents the seventh one. A comparative summary of these cases is shown in Table 1.

Microscopically, the classical form of SS is a biphasic tumor composed of sharply segregated epithelial and sarcomatous components. The epithelial areas usually appear in the form of glands or solid nests of large pale cells. The sarcomatous component is made up of spindle cells with a fibroblast-like appearance. It tends to be hypercellular but with a relatively monotonous appearance and plump nuclei. Monophasic synovial sarcoma is composed of only one of the two components. In most cases, the spindle cell sarcomatous component is observed, which is easily misdiagnosed as fibrosarcoma or other spindle cell neoplasms. ¹⁶ The monophasic form of SS in the jaws requires additional consideration and benign or malignant spindle cell neoplasms with odontogenic origin such as odontogenic fibroma (especially simple type), ameloblastic fibroma, and ameloblastic fibrosarcoma with scant epithelial nests should be included in the differential diagnosis. A careful and comprehensive histopathologic examination of the lesion should be performed to rule out such lesions. Regarding special histochemical staining, mucin stains reveal the presence of acid mucopolysaccharides (hyaluronic acid, chondroitin sulfate, and heparitin sulfate) in the spindle cell areas. ¹⁷ Immunohistochemical staining can help to differentiate monophasic SS from the other more common nonodontogenic spindle cell neoplasm of the jaws. Strong co-expressionof mesenchymal (vimentin) and epithelial (cytokeratin or/and EMA) markers in spindle cells is rarely identified in other neoplasms (neither epithelial nor mesenchymal origin), and combined with positivity for CD99 and Bcl2, the definitive diagnosis of SS can be made. However, it is also recommended to investigate the negative reactivity of spindle-shaped neoplastic cells with S100 and SMA to rule out neurogenic or smooth muscle origin of the cells (although immunoreactivity of some SSs with S100 has been reported). ¹⁷ , ¹⁸ {Table 1}

Various clinical and histopathologic prognostic factors have been proposed for SS including patient′s age, tumor size and location, necrosis, and mitotic index. Recent studies have suggested new prognostic indicators. A Ki67 index of 10% or more is indicative of highly proliferative behavior. ¹⁹ Our case showed a Ki67 index of more than 25%.

When histopathologic studies are ambiguous in confirming the diagnosis of SS, cytogenetic studies can be helpful. The detection of a specific translocation between chromosome X and 18, t(x; 18)(p11.2; q11.2), found in 95% of tumors is a more significant diagnostic tool. ²⁰ However, molecular analysis is not required if the diagnosis of SS is probable based on clinical, histologic, and/or immunohistochemical evaluation. ¹⁰

In addition to difficulties in the diagnosis of monophasic SS, a troubling pattern of locoregional recurrence as well as distant metastasis has been described. Because of the rarity of this tumor in the head and neck, the ideal treatment has not been established yet, but complete surgical resection followed by radiation therapy, without chemotherapy, is suggested ¹ Our patient was treated by local and regional surgery without radiation and chemotherapy. The poor prognosis was related to regional as well as distant metastasis.

Based on our knowledge, we reported the seventh case of primary SS of the jaws. Because of the rarity of SS in this location, a careful step by step analysis is recommended to prevent misdiagnosis and improve the treatment outcome. Also, we would like to emphasize the critical role of special techniques such as immunohistochemistry and cytogeneticsin the diagnosis of monophasic SS.