Radiation therapy for colorectal cancer

Radiation therapy uses high-energy rays or particles to destroy cancer cells. Radiation therapy is used more often in treating rectal cancer than colon cancer. Your healthcare team will consider your personal needs to plan the type and amount of radiation, and when and how it is given. You may also receive other treatments.

Radiation therapy is often combined with chemotherapy to treat rectal cancer. This is called chemoradiation. The 2 treatments are given during the same time period.

Radiation therapy is given for different reasons. You may have radiation therapy or chemoradiation to:

  • shrink a tumour before other treatments such as surgery or chemotherapy (called neoadjuvant therapy)
  • destroy cancer cells left behind after surgery to reduce the risk that the cancer will come back (recur) (called adjuvant therapy)
  • treat colorectal cancer if you aren’t healthy enough to have surgery
  • relieve pain or control the symptoms of advanced colorectal cancer (called palliative therapy)
  • treat colorectal cancer that has spread to other areas such as the bones or brain

The following types of radiation therapy are most commonly used to treat colorectal cancer.

External radiation therapy

During external radiation therapy (also called external beam radiation therapy), a machine directs radiation through the skin to the tumour and some of the tissue around it.

Radiation therapy is commonly given before surgery for rectal cancer. It can shrink the tumour and help make the tumour easier to remove. Depending on where the tumour is in the rectum, radiation therapy may be given alone or as part of chemoradiation. Doctors will also decide how long radiation therapy is given based on the stage and location of the tumour.

Radiation therapy may be given after surgery for cancer in the colon to help prevent a local recurrence. This is not a common treatment because colon cancer usually spreads to distant organs, such as the liver.

External radiation therapy to the abdomen or pelvis may be used as palliative therapy for advanced colorectal cancer or colorectal cancer that can’t be removed with surgery. It may also be used for colorectal cancer that has spread to the bone or brain. Find out more about bone metastases and brain metastases.


Brachytherapy is a type of internal radiation therapy. It uses a radioactive material called a radioactive isotope. It is placed right into the tumour or very close to it. Radioactive materials can also be placed in the area where the tumour was removed. The radiation kills the cancer cells over time.

In some cases, brachytherapy may be used instead of external radiation therapy before surgery to treat rectal cancer.

Side effects

Side effects can happen with any type of treatment for colorectal cancer, but everyone’s experience is different. Some people have many side effects. Other people have only a few side effects.

During radiation therapy, the healthcare team protects healthy cells in the treatment area as much as possible. But damage to healthy cells can happen and may cause side effects. If you develop side effects, they can happen any time during, immediately after or a few days or weeks after radiation therapy. Sometimes late side effects develop months or years after radiation therapy. Most side effects go away on their own or can be treated, but some side effects may last a long time or become permanent.

Side effects of radiation therapy will depend mainly on the size of the area being treated, the specific area or organs being treated, the total dose of radiation, the type of radiation, if chemotherapy is given at the same time (chemoradiation) and the treatment schedule. Side effects of chemoradiation can be more severe than those of radiation therapy alone.

Some common side effects of radiation therapy used for colorectal cancer are:

Tell your healthcare team if you have these side effects or others you think might be from radiation therapy. The sooner you tell them of any problems, the sooner they can suggest ways to help you deal with them.

Questions to ask about radiation therapy

Find out more about radiation therapy and side effects of radiation therapy. To make the decisions that are right for you, ask your healthcare team questions about radiation therapy.

Expert review and references