Etiology and demography Synovial sarcoma is a misnomer which represents a tumor that only resembles synovial tissue under an optical microscope, and which originates from multipotent stem cells capable of differentiating into mesenchymal and/or epithelial structures, which explains the different extra-articular locations [14-19].

Synovial sarcoma occurs most commonly in young people, accounting for approximately 5-10% of all soft tissue sarcomas, but approximately 15-20% of cases occur in adolescents and young adults with an estimated incidence of 2.5 per 100,000 [15,20,21].

Although the most common location of synovial sarcoma is the particular region (close to large joints), particularly the joint capsules, tendon sheaths, and bursae, other unusual extra-articular locations have been reported in the literature such as: the abdominal wall, abdominal cavity, retro peritoneum, head, cervical region, lungs, mediastinum, pleura, heart, kidney, prostate, vessels, nerves... [5-10].

Clinical, radiological, and pathological study Primary abdominal synovial sarcoma most often presents in the form of a painless, palpable, slowly growing mass, whose clinical symptoms (especially signs of compression) depend on the site and the size of the tumor. Metastases occur in the first 2 to 5 years, they are present in 16-25% of cases, particularly in the lungs, and less frequently in the lymph nodes and bones [19].

Even if imaging does not confirm the diagnosis, it can be suspected in the presence of a heterogeneous mass with necrotico-hemorrhagic areas and eccentric or peripheral calcifications. These calcifications are present in 30% of cases [22,23]. CT scanning is more sensitive than abdominal ultrasound because it can detect calcifications as well as necrosis and haemorrhagic areas.

Regarding magnetic resonance (|MR) imaging, synovial sarcoma appears isointense to slightly hyperintense on T1-weighted images, and hyperintense to muscles on T2-weighted images due to the presence of a mixture of solid, cystic, fibrous and myxoid stromal tissues.

Marked heterogeneity and enhancement are strongly suggestive of synovial sarcoma, as are CT findings [22,23].

The main differential diagnoses of intra-abdominal synovial sarcoma are fibro sarcomas, malignant schwannomas, malignant fibrous histiocytomas. Histological study after a biopsy or surgical resection represents the only way to confirm the diagnosis of synovial sarcoma. The most sensitive immunohistochemical stains for synovial sarcoma are EMA, cytokeratin AE1/AE3 and E-cadherin, in association with CD 34 negative [17].

Keratin positivity (around 90% of cases), measured by immunostaining associated with the histological appearance, confirms the positive diagnosis of a synovial sarcoma [1]. Synovial sarcoma has three histological subtypes: monphasic, biphasic and the poorly differentiated type [1].

The monophasic subtype accounts for 50-60% (the most common subtype) of all lesions, it consists mainly of spindle cells resembling fibrosarcoma, while the biphasic subtype is composed of a bimorphic form, with mixed spindle and epithelioid cell components [1,18]. Concerning the poorly differentiated subtype, it is generally of epithelioid morphology associated with geographic necrosis with high mitotic activity (15-20/10 field). This subtype represents 15 to 25% of all synovialsarcomas [1].

Genetic background: The specific chromosomal aberrations are the translocation (X;18) (p11.2:q11.2) found in 90% of SS and the fusion of the SYT gene on chromosome 18 with either SSX1 (67% of cases) or SSX2 (33% of cases) on chromosome X [19 ].

Tumors of monophasic subtypes mainly express the SYT-SSX2 fusion transcript while biphasic tumors mainly carry the SYT-SSX1 transcript [17].

Treatment and prognosis: Wide surgical excision remains the only curative treatment. For non-metastatic tumor of small size < 5cm, wide surgical excision is indicated, whereas for large, locally advanced, metastatic tumor, neoadjuvant chemotherapy seems indicated to reduce the tumor volume and thus allows in 61 to 80% of cases a surgical procedure that was initially impossible [24-26].

Adjuvant chemotherapy aims to reduce the rate of recurrence and the incidence of metastases [24, 25].

However, recidivism still varies between 28% and 36% [18].

Close follow-up of at least every 3 months for the first 2 years then semi-annually for an additional 5 years is recommended due to the high local recurrence [19].

Survival and prognosis are correlated with tumor size, histological subtype, mitotic rate, glandularity percentage, tumor necrosis and vascular invasion [26]. The poorly differentiated subtype includes epithelioid cells with high mitotic activity which explains the high grade (3/3). Monophasic and biphasic synovial sarcomas are generally intermediate grade (2/3) [18].