abnormal tissue growth located in or originating from muscle tissue. Tumours (tumour) may either arise in muscle tissue or spread to it. Three major types of muscle tumours are leiomyomas, rhabdomyomas, and rhabdomyosarcomas.

A leiomyoma is a benign tumour of smooth muscles (such as those in the walls of the intestines (intestine) and of blood vessels (blood vessel)). It is most frequently located in the uterus. Leiomyomas also may occur in the ovaries (ovary), the fallopian tubes (fallopian tube), the alimentary canal, the bladder, and the ureters (ureter). The tumour is firm or rubbery and can be easily removed. Although part of the tumour may become malignant, it usually does not spread, nor does it recur once it is removed.

A rhabdomyoma is a rare, usually benign tumour of striated (striped) muscles. It most commonly occurs in the heart. Some forms of this tumour do spread; metastases (secondary tumours at distant sites) may occur in the uterus, the bladder, the prostate (prostate gland), the esophagus, the digestive tract, and the kidneys (kidney). The tumour is soft and may occur in nodes, flat masses, round clusters, or polyps. Rhabdomyomas of the heart grow in the wall and may project into the heart cavities. Rhabdomyomas affecting other parts of the body commonly involve both the smooth and the striated muscles. Many of these mixed tumours are likely to be malignant and may grow to great proportions. Tumours of this type occurring in the uterus, the vagina, or the prostate are large and polyp-shaped masses that protrude from these structures. In the prostate they may obstruct the bladder and invade the adjacent pelvic tissue.

A rhabdomyosarcoma is a malignant tumour that arises in the skeletal muscles. Most tumours of this type are located in the leg or arm muscles. A rhabdomyosarcoma may recur even after amputation of the involved extremity. The only symptom may be a slowly growing mass; it appears most often in the fifth to sixth decade of life and has usually been growing for 10 or more years before it is discovered.