A type of cancer that usually forms in the soft tissue, but it may also form in bone. It can occur anywhere in the body, but it usually occurs in the legs (especially the thighs), arms, or back of the abdomen. It may also occur in a part of the body where a patient received radiation therapy in the past. Undifferentiated pleomorphic sarcomas often grow quickly and spread to other parts of the body, including the lungs. They usually occur in older adults, and they may sometimes occur as a second cancer in patients who had retinoblastoma. Also called malignant fibrous cytoma and malignant fibrous histiocytoma.