Colorectal cancer

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Radiation therapy for colorectal cancer

Radiation therapy uses high-energy rays or particles to destroy cancer cells. It is sometimes used to treat colorectal 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 sometimes used as part of chemoradiation to treat rectal cancer. Chemoradiation is treatment that combines chemotherapy and radiation therapy. Chemotherapy is given during the same time period as radiation therapy.

You may have radiation therapy, either alone or as part of chemoradiation, to:

  • shrink a tumour in the rectum before other treatments such as surgery (called neoadjuvant therapy)
  • destroy cancer cells in the colon or rectum left behind after surgery or chemotherapy and reduce the risk of the cancer coming back, or recurring (called adjuvant therapy)
  • relieve pain or control the symptoms of advanced colorectal cancer (called palliative therapy)

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

External beam radiation therapy

During external beam radiation therapy, a machine directs radiation through the skin to the tumour and some of the tissue around it. It is most often used for rectal cancer.

External beam radiation therapy is commonly given before surgery (called neoadjuvant radiation therapy) for rectal cancer. It can help make the tumour easier to remove and lower the risk that cancer will come back in the pelvis. Depending on where the tumour is in the rectum, radiation therapy may be given alone or as chemoradiation. Doctors will also decide how long radiation therapy is given based on the stage and location of the tumour.

External beam radiation therapy may also be given as part of chemoradiation after surgery for rectal cancer. This is not very common because neoadjuvant radiation therapy or chemoradiation has become standard.

In some cases, external beam radiation therapy may be given after surgery for cancer in the colon. It is used to help prevent a local recurrence, which is when cancer comes back in the same area as the primary tumour. This is not a common treatment because colon cancer usually spreads to distant organs, such as the liver.

External beam radiation therapy to the abdomen or pelvis may be used as palliative therapy for advanced colorectal cancer or colorectal cancer that can’t be surgically removed (called unresectable). 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 internal radiation therapy. It uses a radioactive material called a radioactive isotope. It is placed right into, or very close to, the tumour. The radiation kills the cancer cells over time.

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

The radioactive material is in a special applicator or a catheter (tube). The doctor places the radioactive material in the rectum close to the tumour (called endorectal brachytherapy). Brachytherapy for rectal cancer is usually given as high-dose rate (HDR) therapy. This means that it is given as a single, high dose of radiation in a very short period of time. After the radiation is given, the applicator or catheter is removed. This process may be repeated for a few days.

Endorectal brachytherapy usually causes fewer side effects and long-term complications than external beam radiation therapy. More research is needed to fully compare these treatments.

Radiation oncologists need special equipment to give endorectal brachytherapy. As a result, it isn’t available in all treatment centres, and it isn’t a standard treatment for rectal cancer.

Questions to ask about radiation therapy

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


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