Synovial sarcoma: A malignant tumor of soft tissue that arises near, but not in, a joint. Synovial sarcomas most often occur in adolescents or young adults, are typically slow-growing, and may escape notice until they become painful. They occur mainly in the arms and legs, near large joints, especially the knees. Although the tumor is called synovial sarcoma, it has never been shown to arise from synovial cells (the cells that line a joint), and the cell of origin is not known. The diagnosis of synovial sarcoma can be suspected via X-ray or imaging, made via biopsy, and confirmed via cytogenetic studies that show a translocation (an exchange of material) between the X chromosome and chromosome 18 in the tumor cells. The key treatment is surgery to remove the entire tumor, nearby muscle, and lymph nodes. Radiation, chemotherapy, or a combination of treatment methods may follow surgery. The tumor tends to recur locally and to involve local lymph nodes. Distant spread (metastasis) occurs in half of cases, sometimes many years after the initial diagnosis. Also known as synoviosarcoma.