The histologic picture of synovial cell carcinoma can be confused with that of many other small, round blue cell tumors. The gross appearance is grayish-white and the tumor often has a greasy feel, with focal calcifications on radiography. It usually presents as a slow-growing, painful, deep-seated soft-tissue mass in close proximity to large joints (most in knee and ankle), and most commonly affects young adults. The histologic features are identical in children and adults, and this histologic picture may be confused with that of other round blue cell tumors.
Synovial cell sarcomas typically have a biphasic histology; however, this disease is often monophasic (lacking glandular differentiation), which produces the picture of a small, round blue cell tumor. Three types of synovial sarcoma have been described, as follows:
In the monophasic type, a predominance of monomorphic spindle cells resembling synoviocytes are observed; the tumor cells are arranged in fascicles, with poorly defined cytoplasm; no glandular elements are present.
The biphasic type contains layers of columnar epithelium admixed with the spindle cells; it consists of plump, round epithelial cells and spindle-shaped cells alternating with glandular-like areas that are lined by cuboidal-to-columnar cells containing mucin.
A third type, called "poorly differentiated,' has numerous mitoses, high cellularity, and tumor necrosis.
Identification of synovial cell sarcoma is obtained by RT-PCR in tumor tissue (and possibly in blood), with a sensitivity of 96% and specificity of 100%; SYT-SSX1 or SYT-SSX2 gene fusions are seen in > 90% of cases. FISH is less expensive than RT-PCR. This method has a lower sensitivity than RT-PCR does, but the results of the two methods are concordant in 76% of cases. Tissue sections stained with the immunohistochemical antibody TLE1 correctly identify 97% of synovial sarcomas.
CT is used to confirm the presence of a mass, its size, and its location, but it is nondiagnostic. It also may detect secondary bony involvement, which is not uncommon with larger synovial sarcomas. Synovial sarcoma is a malignant disease; therefore, CT of the chest is mandatory to exclude metastatic disease.
For more on the workup of synovial cell sarcomas, read here.
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