Radiation therapy uses high-energy rays (such as x-rays) or particles to destroy cancer cells. It may be part of treatment for either colon or rectal cancer. Chemotherapy can make radiation therapy more effective against some colon and rectal cancers, and these 2 treatments are often used together.

Radiation therapy is mainly used in people with colon cancer when the cancer is found to have attached to an internal organ or the lining of the abdomen. When this occurs, the surgeon cannot be certain that all the cancer has been removed, and radiation therapy may be used to try to kill any cancer cells that may remain after surgery. Radiation therapy is seldom used to treat metastatic colon cancer because of side effects, which limit the dose that can be used.

For rectal cancer, radiation therapy is usually given along with chemotherapy to help prevent the cancer from coming back in the pelvis where the tumor started. It may be given either before or after surgery. Many doctors now favor giving it before surgery, as it may make it easier to remove the cancer. Giving radiation before surgery may also result in fewer complications such as scar formation that can interfere with bowel movements. It may also lower the risk that the tumor will come back (recur) in the pelvis. If a rectal cancer's size and/or position make surgery difficult, radiation may be used to try to shrink the tumor first to make surgery easier. Radiation therapy can also be given to help control rectal cancers in people who are not healthy enough for surgery or to ease (palliate) symptoms in people with advanced cancer causing intestinal blockage, bleeding, or pain.

Types of radiation therapy

Different types of radiation therapy can be used to treat colon and rectal cancers.

External-beam radiation therapy: The radiation is focused on the cancer from a machine outside the body called a linear accelerator. This is the type of radiation therapy most often used for people with colorectal cancer.

Before treatments start, the radiation team takes careful measurements to determine the correct angles for aiming the radiation beams and the proper dose of radiation. External radiation therapy is much like getting an x-ray, but the radiation is more intense. The procedure itself is painless. Each treatment lasts only a few minutes, although the setup time -- getting you into place for treatment -- usually takes longer. Most often, radiation treatments are given 5 days a week for several weeks, although the length of time may be shorter if it is given before surgery.

Endocavitary radiation therapy: A small device placed into the anus delivers the radiation. The device delivers high-intensity radiation over a few minutes. This is repeated about 3 more times at about 2-week intervals for the full dose. The advantage of this approach is that the radiation reaches the rectum without passing through the skin and other tissues of the abdomen, which means it is less likely to cause side effects. This can allow some patients, particularly elderly persons, to avoid major surgery and a colostomy. It is used only for small tumors. Sometimes external-beam radiation therapy is also given.

Brachytherapy (internal radiation therapy): Brachytherapy uses small pellets of radioactive material placed next to or directly into the cancer. The radiation travels only a short distance, limiting the effects on surrounding healthy tissues. Internal radiation is sometimes used in treating people with rectal cancer, particularly people who are not healthy enough to tolerate curative surgery. This is generally a one-time only procedure and doesn't require daily visits for several weeks.

Side effects of radiation therapy

If you are going to get radiation therapy, it's important to speak with your doctor beforehand about the possible side effects so that you know what to expect. Potential side effects of radiation therapy for colon and rectal cancer can include:

• skin irritation at the site where radiation beams were aimed
• nausea
• rectal irritation, which can cause diarrhea, painful bowel movements, or blood in the stool
• bowel incontinence
• bladder irritation, which can cause frequent urination, burning sensations while urinating, or blood in the urine
• fatigue
• sexual problems (impotence in men and vaginal irritation in women)

Most side effects should lessen after treatments are completed, but problems such as rectal and bladder irritation may remain. Some degree of rectal and/or bladder irritation may be a permanent side effect. If you begin to develop these or other side effects, talk to your doctor right away so steps can be taken to reduce or relieve them.

Last Medical Review: 05/18/2009
Last Revised: 05/18/2009