A leiomyosarcoma is a malignant tumor made up of smooth muscle tissue - the intestines, stomach and uterus. However, they can occur anywhere in the body where there is a blood vessel. They are very rare, occurring in about one of 100,000 people in a given year. However, it is one of the more common forms of sarcoma in soft tissues. They are very unpredictable and, although they can go years without growing, they are also capable of very fast growth. As such, the preferred treatment is to remove them surgically while they are still small.