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. Unclassified sarcomas tend to grow quickly and spread to other parts of the body. Some types of unclassified sarcomas may form in a part of the body where a patient had radiation therapy in the past. An unclassified sarcoma is a type of soft tissue sarcoma. Also called undifferentiated sarcoma.