Radiation Therapy for Anal Cancer

Radiation therapy uses a beam of high-energy rays (or particles) to kill cancer cells or slow their growth. Radiation therapy can be used:

  • As part of the main treatment (along with chemotherapy) for most anal cancers (This is called chemoradiation.)
  • After surgery if the doctor is concerned that all of the cancer might not have been removed
  • To help treat cancer that has come back in the lymph nodes after initial treatment
  • To help control cancer that has spread or to relieve symptoms it causes (This may be called supportive or palliative care.)

There are 2 main forms of radiation therapy: external beam and internal radiation.

External-beam radiation therapy (EBRT)

The most common way for anal cancer to be treated with radiation is by using a focused beam of radiation that comes from a machine outside the body. This is known as external-beam radiation therapy.

Radiation can harm nearby healthy tissues along with the cancer cells. This causes side effects. To reduce the risk of side effects, doctors carefully figure out the exact dose you need and aim the beams as accurately as they can. Before treatment starts, the radiation team will get PET/CT or MRI scans of the area to be treated to help figure this out. Radiation therapy is much like getting an x-ray, but the radiation is stronger. The procedure itself doesn't hurt. Each treatment lasts only a few minutes, but the setup time – getting you into place for treatment – usually takes longer. Treatments are usually given 5 days a week for a period of 5 weeks or so.

Newer techniques allow doctors to give higher doses of radiation to the cancer while reducing the radiation to nearby healthy tissues:

Three-dimensional conformal radiation therapy (3D-CRT) uses special computers to precisely map the location of the cancer. Radiation beams are then shaped and aimed at the tumor from several directions. This makes them less likely to damage normal tissues. You will most likely be fitted with a plastic mold like a body cast to keep you in the exact same position each time so that the radiation can be aimed more accurately.

Intensity-modulated radiation therapy (IMRT) is an advanced form of 3-D therapy and the preferred type of EBRT for anal cancer. It uses a computer-driven machine that actually moves around you as it delivers radiation. Along with shaping the beams and aiming them from several angles, the intensity (strength) of the beams can be adjusted. This helps limit the dose reaching normal tissues. IMRT lets doctors deliver an even higher dose to the cancer.

Side effects of external radiation therapy

Side effects vary based on the part of the body treated and the dose of radiation given. Some common short-term side effects include:

  • Diarrhea
  • Skin changes (like a sunburn) in areas being treated
  • Short-term anal irritation and pain (called radiation proctitis)
  • Discomfort during bowel movements
  • Tiredness
  • Nausea
  • Low blood cell counts

In women, radiation may irritate the vagina. This can lead to discomfort and discharge.

Most of these side effects get better over time after radiation stops.

Long-term side effects can also occur:

  • Damage to anal tissue by radiation may cause scar tissue to form. This can sometimes keep the anal sphincter muscle from working as it should, which could lead to problems with bowel movements.
  • Radiation to the pelvis can weaken the bones, increasing the risk of fractures of the pelvis or hip.
  • Radiation can damage blood vessels that nourish the lining of the rectum and lead to chronic radiation proctitis (inflammation of the lining of the rectum). This can cause rectal bleeding and pain.
  • Radiation can affect fertility (the ability to have children) in both women and men. (For more on this, see Fertility and Men With Cancer and Fertility and Women With Cancer.)
  • Radiation can lead to vaginal dryness and even a narrowing or shortening of the vagina (called vaginal stenosis), which can make sex painful. A woman can help prevent this problem by stretching the walls of her vagina several times a week. This can be done using a vaginal dilator (a plastic or rubber tube used to stretch out the vagina). (To learn more, see Sex and the Woman With Cancer.)
  • If radiation is given to the lymph nodes in the groin, it can lead to swelling problems in the genitals and legs, called lymphedema. (See For People at Risk of Lymphedema.)

Internal radiation (brachytherapy)

Internal radiation is not commonly used to treat anal cancer. When it is used, it's usually given as a radiation boost along with external radiation when a tumor isn't responding to regular chemoradiation (chemo plus external radiation).

Internal radiation involves putting small sources of radioactive materials in or near the tumor. It may also be called brachytherapy, interstitial radiation, or intracavitary radiation. It's used to focus the radiation in the area of the cancer.

The possible side effects are a lot like those seen with external radiation.

More information about radiation therapy

To learn more about how radiation is used to treat cancer, see Radiation Therapy.

To learn about some of the side effects listed here and how to manage them, see Managing Cancer-related Side Effects.

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.

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Last Medical Review: November 13, 2017 Last Revised: November 13, 2017

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