Radiation Therapy for Colorectal Cancer

Radiation therapy uses high-energy rays (such as x-rays) or particles to destroy cancer cells. Chemotherapy can make radiation therapy more effective against some colon and rectal cancers. Using these 2 treatments together is called chemoradiation or chemoradiotherapy.

When is radiation therapy used for colorectal cancer?

It is not common to use radiation therapy to treat colon cancer, but it may be used in certain instances:

  • After surgery, if the cancer has attached to an internal organ or the lining of the abdomen. If this occurs, the surgeon can’t be certain that all the cancer has been removed. Radiation therapy may be used to try to kill any cancer cells that may have been left behind.
  • To help control 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.
  • To help treat cancer that has spread to other areas, such as the bones or brain.

For rectal cancer, radiation therapy may be used:

  • Either before or after surgery to help prevent the cancer from coming back. In this case, it is often given along with chemotherapy. Many doctors now favor giving radiation therapy before surgery, as it may make it easier to remove the cancer, especially if the cancer's size and/or location might make surgery difficult.
  • 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.
  • To help treat cancer that has spread to other areas, such as the bones or brain.

Types of radiation therapy

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

External-beam radiation therapy

This is the type of radiation therapy used most often for people with colorectal cancer. The radiation is focused on the cancer from a machine outside the body. It is much like getting an x-ray, but the radiation is more intense. How often and how long the radiation treatments are delivered depends on the reason the radiation is being given and other factors. It can last anywhere from a few days to weeks. 

Internal radiation therapy (brachytherapy)

This type of radiation therapy can be used to treat some rectal cancers. For this treatment, a radioactive source is put inside your rectum next to or into the tumor. 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.

Endocavitary radiation therapy: For this treatment, a small device is placed through the anus and into the rectum to deliver high-intensity radiation for a few minutes. This is typically done in 4 treatments (or less), with about 2 weeks between each treatment. This can let some patients, particularly elderly patients, avoid major surgery and a colostomy. This type of treatment is used for some small rectal cancers. Sometimes external-beam radiation therapy is also given.

Interstitial brachytherapy: For this treatment, a tube is placed into the rectum and directly into the cancer. Small pellets of radioactive material are then put into the tube for several minutes. The radiation travels only a short distance, limiting the harmful effects on surrounding healthy tissues. It is sometimes used to treat people with rectal cancer, particularly people who are not healthy enough for surgery. This can be done a few times a week for a couple of weeks, but it can also be just a one-time procedure.


Radiation can also be given during an embolization procedure. This is covered in more detail in Ablation and Embolization to Treat Colorectal Cancer.

Side effects of radiation therapy

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

  • Skin irritation at the site where radiation beams were aimed, which can range from redness to blistering and peeling
  • Nausea
  • Rectal irritation, which can cause diarrhea, painful bowel movements, or blood in the stool
  • Bowel incontinence (stool leakage)
  • Bladder irritation, which can cause problems like feeling like you have to go often (called frequency), burning or pain while urinating, or blood in the urine
  • Fatigue/tiredness
  • Sexual problems (erection issues in men and vaginal irritation in women)

Most side effects should lessen after treatments are completed, but some problems may not go away completely. If you notice any side effects, talk to your doctor right away so steps can be taken to reduce or relieve them.

You can learn more about radiation treatments in the Radiation Therapy section of our website, or in A Guide to Radiation Therapy.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: January 15, 2017 Last Revised: March 2, 2017

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