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What Happens During Radiation Therapy for Colorectal Cancer

For colorectal cancer, the most common way to receive radiation is from a machine outside your body that emits an invisible X-ray beam. This is called external radiation.

A doctor who specializes in cancer and radiation is called a radiation oncologist. This doctor works with you to determine the kind of radiation you need. This doctor also determines the dose and how long you need the therapy. If you need radiation combined with chemotherapy, you’ll also see another doctor called a medical oncologist.

You can receive external radiation therapy on an outpatient basis in a hospital or a clinic. The standard treatment is given five days a week for several weeks. This type of radiation may come from a machine called a linear accelerator. If you also have chemotherapy, you will receive it in a different outpatient area.

Preparing for radiation

Before your first radiation treatment, you will have a session to determine exactly where on your body the radiation beam needs to be directed. The process is called simulation. This session may take up to two hours. During this session, you may be given barium, which is a liquid that allows your doctor to see where your colon is on an X-ray picture. You’ll drink some of the barium, and some will be put in your rectum. Then, you’ll lie still on a table while a radiation therapist uses a machine to define your treatment field. The field is the exact area on your body where the radiation will be aimed. Sometimes it’s called your port. You may have more than one treatment field if you have cancer in more than one place. The therapist marks your skin with tiny dots of colored permanent ink so that the radiation will be aimed at the exact same place each time. You may also have other imaging scans, such as CT scans, MRI scans, PET scans, or ultrasound to help doctors know the exact location of your tumor to better aim the radiation. Also at this session, you may have body molds made to help keep you from moving during the treatment.

On the days you get radiation

On the days you receive radiation treatment, you’ll lie on a table while the machine is placed over you. You may have to wear a hospital gown. The experience is much like that of getting an X-ray, only longer. It takes about 15 to 30 minutes to complete. You should, though, plan on being there for about an hour.

At the start of the treatment session, a radiation therapist may place blocks or special shields to protect parts of your body that don’t need to be exposed to radiation. The therapist then lines up the machine so that radiation is directed to the spot that was marked during the simulation. When you are ready, the therapist leaves the room and turns the machine on. You may hear whirring or clicking noises, similar to the sounds of a vacuum cleaner, while the radiation is being given. During the session, you will be able to talk to the therapist over an intercom. You can’t feel radiation, so the process will be painless. Also, you will not be radioactive afterward.

What to expect after radiation therapy

Because radiation affects normal cells as well as cancer cells, you may have some side effects from this treatment. Some people have few or no side effects, but if you do have them, your doctor may change the dose of your radiation or the frequency of treatments. The doctor may stop treatment until the side effects are cleared up. So be sure you keep your doctor informed about the side effects you have.

Potential short-term side effects

These are some of the common short-term side effects:

  • Diarrhea

  • Nausea

  • Bladder irritation

  • Fatigue

  • Low blood counts, as noted from a blood test

  • Skin irritation or changes in areas that got radiation

These side effects can be unpleasant. Some of them can be controlled with medicine, and some may be helped with diet. Talk with your doctor or nurse about how to deal with them and how to know when they become serious. Usually these side effects go away a few weeks after you stop getting treatment.

Potential long-term side effects

Radiation therapy can cause some long-term side effects, depending on where the radiation was aimed. These can be more serious, so you should monitor them closely with your doctor:

  • Bowel or bladder irritation. You may feel the need to urinate or have bowel movements more often. You may also have some pain with urination or bowel movements, or see blood in the urine or stool. Be sure your doctor is aware of these problems so that they can be treated, if needed.

  • Early menopause. If you're a woman who has not gone through menopause, you may experience menopausal symptoms. Your period may stop, and you may become infertile. Talk with your doctor about what you can do to relieve the symptoms.

  • Vaginal dryness. Women may experience vaginal dryness and narrowing. Both conditions may be permanent. Lubricants can help with vaginal dryness. Vaginal narrowing may be helped by using a vaginal dilator or by having sexual intercourse.

  • Decreased sperm count. Men may produce less ejaculate, and their semen may have a lower sperm count. If you're planning on having children, you may want to visit a sperm bank before having your radiation treatments.


Online Medical Reviewer: Alteri, Rick, MD
Online Medical Reviewer: Sather, Rita, RN
Last Review Date: 10/22/2012
© 2013 The StayWell Company, LLC. 780 Township Line Road, Yardley, PA 19067. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare provider's instructions.