Leiomyosarcoma is a type of sarcoma which is a neoplasm of smooth muscle. (When benign, it is called a leiomyoma.) Smooth muscle cells make up the involuntary muscles, which are found in most parts of the body: in uterus, stomach and intestines, walls of all blood vessels, and skin. Leiomyosarcoma is a very rare cancer. It makes up 7% of soft tissue sarcomas.