Sarcomas are cancers that develop from connective tissues in the body, such as muscles, fat, membranes that line the joints, or blood vessels. There are many types of sarcomas. Rhabdomyosarcoma is a cancer made up of cells that normally develop into skeletal muscles. The body contains 3 main types of muscles.

• Skeletal (voluntary) muscles are muscles that we control to move parts of our body.
• Cardiac muscle is the main muscle type in the heart.
• Smooth muscle is present in internal organs (except for the heart). For example, smooth muscles in the intestines push food along as it is digested. We do not control this movement.

About 7 weeks into the development of an embryo, rhabdomyoblasts (cells that will eventually form skeletal muscles) begin to form. These are the cells that can develop into the cancer called rhabdomyosarcoma. Because this is a cancer of embryonal cells, it is much more common in children, although it does occur in adults occasionally.

Although we think of our skeletal muscles as being in our arms and legs, these skeletal muscle cancers are often found elsewhere in the body. Common sites include:

• head and neck (near the eye, inside the nasal sinuses or throat, or near the spine in the neck)
• urinary and reproductive organs (bladder, prostate gland, or any of the female organs)
• arms and legs
• trunk (chest and abdomen)

But these cancers can occur anywhere in the body, including sites that don't normally have skeletal muscle.

Types of rhabdomyosarcoma

There are 2 main types of rhabdomyosarcomas.

Embryonal rhabdomyosarcoma

Embryonal rhabdomyosarcoma (ERMS) is the most common type of rhabdomyosarcoma. It usually affects infants and young children. The cells of ERMS look like the developing muscle cells of a 6- to 8-week-old fetus. ERMS tends to occur in the head and neck area, bladder, vagina, and in or around the prostate and testicles.

Two types of ERMS, botryoid and spindle cell rhabdomyosarcomas, tend to have a better prognosis (outlook) than the more common forms.

Alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma (ARMS) typically affects older children or teenagers and occurs more often in large muscles of the trunk, arms, and legs. ARMS cells look like the normal muscle cells seen in a 10-week-old fetus.

Anaplastic rhabdomyosarcoma and undifferentiated sarcoma

Anaplastic rhabdomyosarcoma (formerly called pleomorphic rhabdomyosarcoma) is a type that occurs in adults but is very rare in children.

Also, some doctors group undifferentiated sarcomas with the rhabdomyosarcomas. Although these uncommon cancers are sarcomas, the cells don't have any features that help classify them further.