Abnormal tissue growth in or originating from muscle tissue.
There are three major types. Leiomyomas are tumours of smooth muscles, seen most often in the uterus but also in the digestive, urinary, and female genital systems. Part of the tumour may become malignant, but it usually does not spread or recur. Rhabdomyomas occur most often in cardiac muscle. Some forms spread, and it may remain contained in tissue or become diffuse and hard to remove. Rhabdomyomas involving both smooth and striated muscle are often malignant and may grow very large. The several types of rhabdomyosarcoma are rare; they arise in skeletal muscle, usually in the leg or arm, and are extremely malignant.