What Is Rhabdomyosarcoma?

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

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

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

We might think of our skeletal muscles as being mainly in our arms and legs, but these skeletal muscle cancers can start nearly anywhere in the body, even in some parts of the body that don’t normally have skeletal muscle.

Common sites of RMS include:

• Head and neck (such as 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)

There are 2 main types of RMS, along with some less common types.

Embryonal rhabdomyosarcoma (ERMS) usually affects children in their first 5 years of life, but it is the most common type of RMS at all ages.

The cells of ERMS look like the developing muscle cells of a 6- to 8-week-old embryo. ERMS tends to occur in the head and neck area, bladder, vagina, or in or around the prostate and testicles.

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

Alveolar rhabdomyosarcoma (ARMS) typically affects all age groups equally. It makes up a larger portion of RMS in older children and teens than in younger children (because ERMS is less common at older ages).

ARMS most often occurs in large muscles of the trunk, arms, and legs. The cells of ARMS look like the normal muscle cells seen in a 10-week-old fetus.

ARMS tends to grow faster than ERMS and usually requires more intense treatment.

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

Some doctors also group undifferentiated sarcomas with the rhabdomyosarcomas. Using lab tests, doctors can tell that these cancers are sarcomas, but the cells don’t have any features that help classify them further.

Both of these uncommon cancers tend to grow quickly and usually require intensive treatment.

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