Synovial sarcoma is a malignant tumor of the soft tissues, usually around joints. Under the microscope the tumor resembles synovial tissue (the lining tissue of joints). Synovial tissue is found around the tendons (bands of fiber that connect muscle to bone), and can form bursa (fluid filled cushioning pouches or sacs found in spaces between tendons, ligaments and bones) found in the area of joints.
The most common location of sarcoma origin is the thigh near the knee, but synovial sarcoma can also occur near other joints, mainly in the arms and legs.