Synovial sarcoma is a distinct aggressive neoplasm that occurs most often in teenagers and young adults (mean age:35) (1). The etiology of this tumor is unknown, but some authors believe that it arises from multi-potential stem cells with differentiation into the epithelial or mesenchymal structures (2, 3). The most common clinical findings are palpable and deep-seated mass associated with the pain or tenderness (4). There are no known risk factors for this tumor (7), but some patients had a history of trauma (4-7). Mostly lower extremities are affected by the tumor (8), and involvement of the articular space is rare (4). Most cases occur in juxta-articular areas (8). This tumor is classified histologically into biphasic, monophasic and poorly differentiated types (4, 5). The pure epithelial pattern is rare (2, 4) and the two major differential diagnosis for the epithelial predominant synovial sarcoma is metastatic adenocarcinoma and malignant adnexal tumor (2, 4). Therefore, an exact diagnosis of this subtype of synovial sarcoma, identifying minute foci of spindle cells and cytogenetic or molecular findings [t (X,18)], are necessary (4).
A 35-year-old male referred to our center with a painful thigh mass. He had first noticed the mass 8 years ago after minor trauma. The tumor was growing slowly during these years, until about 6 months ago that started to grow rapidly and became painful. On physical examination, a 10 cm, well-defined, firm movable and mildly tender mass was palpated in the medial aspect of the right thigh. Magnetic resonance imaging (MRI) revealed a large lobulated hypervascular mass lesion in the deep portion of the right rectus femoris muscle (Fig. 1). Computed tomography scan of the abdomen, pelvis and whole body bone revealed no pathologic findings. The patient was planned for the incisional biopsy. A wedge-shaped portion of the tumor was removed and sent for the pathologic examination. Macroscopically, the specimen consisted of multiple fragments of creamy-tan soft tissue totally measuring 5x3x1 cm and was embedded entirely. Microscopically, the tumor showed variable-sized well-differentiated gland-like structures lined by the cuboidal cells with clear to pinkish cytoplasm. Some of these glands contained intraluminal eosinophilic material (Fig. 2A). Small foci of spindle cells arranged in the fascicles were also identified between glandular structures (Fig. 2B). Immunohistochemically, the glandular components showed strong reactivity for pan CK, CK7, CK19 and EMA (Fig. 3A&B). CD99 was positive in both spindle and epithelial components (Fig. 3C). Bcl2 was only positive in spindle cells, and CD34 was negative in both components. The TLE1 was positive in both epithelial and spindle cell components (Fig. 3D). The diagnosis was done for the epithelial predominant synovial sarcoma.
Figure 1. A) AP, B) transverse and C) lateral view of right femur MRI showing large lobulated hyper-vascular mass lesion with the diameter of 16.5X7cm in deep portion of the right rectus femoris muscle