A rare, fast-growing type of cancer that is made up of bone and cartilage cells, and forms in soft tissue near bones. It usually occurs in the thigh, buttock, shoulder, or trunk (chest and abdomen). It often recurs (comes back) after treatment and spreads to other parts of the body, including the lungs. Extraskeletal osteosarcoma usually occurs in middle-aged or older adults, and is rare in children and adolescents. It is a type of soft tissue sarcoma. Also called extraosseous osteosarcoma.