What Happens During Radiation Therapy for Colorectal Cancer

You can't see or feel radiation, but it kills cancer cells.

For colorectal cancer, the most common way to receive radiation is from a machine outside your body that emits an invisible 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 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 lasts 5 weeks, 5 days a week. 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 2 hours. During this session, you’ll 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 imaging scans, such as CT scans, 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 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 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.