A rare type of cancer that begins in fat cells. It usually forms in the layer of fat just under the skin or in the deep soft tissues of the legs (especially in the thigh or back of the knee) or in the abdomen. It may also form in other parts of the body. Most liposarcomas are painless and tend to grow slowly, but some may grow quickly and spread to nearby tissue or to other parts of the body. Liposarcomas usually occur in adults, and are rare in children and adolescents. They are a type of soft tissue sarcoma.