A group of rare cancers that do not look like other types of sarcomas under a microscope and may be hard to diagnose. They usually form in the muscles that are attached to bones and that help the body move. Undifferentiated sarcomas tend to grow quickly and spread to other parts of the body. Some types of undifferentiated sarcomas may form in a part of the body where a patient had radiation therapy in the past. An undifferentiated sarcoma is a type of soft tissue sarcoma. Also called unclassified sarcoma.