What Is Rhabdomyosarcoma?   Sarcomas are cancers that develop from connective tissues in the body, such as muscles, fat, membranes that line the joints, or blood vessels. Rhabdomyosarcoma is a cancer made up of cells that normally develop into skeletal muscles of the body. The body contains 3 main types of muscles. Skeletal (voluntary) muscles are muscles that we control to move parts of our body. Cardiac muscle powers the heart's pumping action. Smooth muscle is present in internal organs. For example, smooth muscle in the intestines pushes food along as it is digested. This movement is not under our control. Skeletal muscles first begin to form in embryos about 7 weeks after the egg cell is fertilized by a sperm cell. At that time, rhabdomyoblasts (cells that will eventually form muscles) begin to form. It is these cells that can become malignant and develop into the cancer called rhabdomyosarcoma. Because this is a cancer of embryonal cells, it is more common in younger people. Rhabdomyosarcoma does occur in adults, but it is uncommon. Over 85% of rhabdomyosarcomas occur in infants, children, and teenagers. Although most of our skeletal muscles are in our limbs and trunk, these cancers are most often found elsewhere in the body. The most common site is in the head and neck (30%-40%) where it can grow near the eye, inside the mouth or even near the spine in the neck. The next most common sites are the urinary and reproductive organs (20%-25%) where it can start in the bladder, prostate gland, or any of the female organs. The least common sites are the arms and legs (18%-20%), and trunk (7%). But these cancers can occur anywhere else in the body and not necessarily in a site that normally contains skeletal muscle. There are 2 main types of rhabdomyosarcomas that occur in children. Embryonal Rhabdomyosarcomas The most common type, embryonal rhabdomyosarcoma (ERMS) tends to occur in the head and neck area, bladder, vagina, and in or around the prostate and testes. These usually affect infants and young children. Cells of embryonal (embryo-like) rhabdomyosarcomas resemble the developing muscle cells of a 6- to 8-week-old fetus. Two variants of ERMS, botryoid and spindle cell rhabdomyosarcomas, tend to have a better prognosis (outlook) than the more common forms. Alveolar Rhabdomyosarcomas The second main type, alveolar rhabdomyosarcoma (ARMS), occurs more often in large muscles of the trunk, arms, and legs and typically affects older children or teenagers. This type is called alveolar because the malignant cells form little hollow spaces, or alveoli. Alveolar rhabdomyosarcoma cells resemble the normal muscle cells seen in a 10-week-old fetus. Revised: 02/23/2005 From: American Cancer Society