A rare tumor that usually forms in bone, but may also form in cartilage, muscle, fat, blood vessels, or other supportive tissue in the body. Most giant cell tumors occur at the ends of the long bones of the arms and legs, near a joint (such as the knee, wrist, hip, or shoulder). Most are benign (not cancer) but some are malignant (cancer). Giant cell tumors usually occur in young and middle-aged adults. Also called GCT.