Radiation therapy uses high-energy beams of X-rays or other particles to kill cancer cells or stop them from growing.

Radiation therapy is a common part of the treatment for rectal cancers. It also may be used to treat colon cancer. But surgery is the main treatment for both colon and rectal cancer.

Sometimes advanced colorectal cancer can't be removed with surgery. And some people are not healthy enough to have surgery. Then radiation can be very helpful. In this case, it's often used along with chemotherapy.

Here are some reasons your doctor may suggest radiation therapy:

• To try to shrink a tumor before surgery. This may make it easier to take out the tumor and may allow a smaller operation that removes less normal tissues. Often chemotherapy is given with radiation to shrink rectal cancers before surgery. In some cases, if the cancer shrinks a lot, this preoperative treatment may allow the surgeon to stay away from performing a permanent colostomy.
• To try to kill any cancer cells left after surgery. For colon cancer, it may be used if a cancer growth was attached to an organ or to the lining of your belly (abdomen). In these cases, the surgeon may not be sure all the cancer cells were removed. Radiation can kill any cancer cells that are left behind. This helps reduce the chance that the cancer will come back.
• To treat the exact area where the tumor was. Radiation can be used during surgery. After the tumor is removed, it can be aimed right where the tumor was to kill any cancer cells left there. This is called intraoperative radiation therapy.
• To ease symptoms. Radiation can ease problems caused by tumors that can't be taken out or that have spread to other organs. For instance, it can help shrink tumors that are causing blockages, bleeding, or pain.
• To treat other places where the cancer has spread. Radiation might be used to treat colorectal tumors that have spread to the lungs, brain, or bone.

To plan your treatment, you will talk with a team of cancer specialists. This will include a radiation oncologist, the doctor who creates your treatment plan. The plan shows what kind of radiation therapy you'll have and how long the treatment will last. If you need radiation along with chemotherapy, you'll also see a doctor called a medical oncologist.

For colorectal cancer, radiation most commonly comes from a large machine that sends X-ray beams into the tumor from outside the body. This is called external beam radiation therapy . It's a lot like getting an X-ray, but more radiation is used.

In some cases of rectal cancer, a radioactive source is put right into or next to the tumor. It gives off radiation for a short time over a short distance. This is called internal radiation or brachytherapy. This may be done a few times or in just one treatment. The implants are usually placed in a hospital operating room. You will receive medicine so you don't feel pain during the procedure. Your doctor can tell you more about what to expect during and after this procedure. They can also tell you if you need to take any special precautions after this treatment.

You may get external radiation therapy in a hospital or a clinic. You will likely be an outpatient. This means you go home the same day. External radiation is often given 5 days a week for several weeks. If you also need chemotherapy, you'll get it in a different outpatient area or be given an oral chemotherapy pill. Sometimes chemotherapy is given for several weeks; then chemotherapy and radiation are given together; then surgery follows. This may allow the best chances of shrinking the tumor.

Before your first treatment, you'll have an appointment to plan for the treatment. This is called a simulation. During the simulation:

• You may be asked to drink a contrast agent, like barium. This is a liquid that coats the inside of your stomach and intestines so they can be seen on a CT scan or X-ray image. Barium may also be put in your rectum with a soft, small rubber tube. In some cases, you may need to drink a liquid contrast agent the day before.
• You'll lie on a table. A radiation therapist uses a machine and the scans to find exactly where the radiation will be aimed.
• The therapist may mark your skin with tiny, long-lasting ink dots or tattoos. These are used to aim the radiation at the exact same place each time.
• Body molds might be made to help put you in the same position and stay still during each treatment.

Radiation treatment is like getting an X-ray. The radiation is stronger so it can kill cancer cells. You can't feel radiation, so the process will be painless. Also, you will not be radioactive afterward.

On the days you have treatment, you'll lie on a table. A radiation therapist may place blocks or special shields to protect healthy parts of your body. The therapist then lines up the machine. You may see lights and laser lines projected from the machine lined up with the marks on your skin. These help the therapist know you are in the right position. The radiation machine may be used to take X-rays or CT-scans before treatment. This is done to help with alignment.

The therapist will leave the room while the machine sends radiation to your tumor. The machine may whirr and buzz and will move, but it won't touch you. During this time, the therapist can see you, hear you, and talk to you. The treatment itself often takes only a few minutes.

Radiation therapy affects normal cells as well as cancer cells. This can cause side effects. The side effects depend on the amount and type of radiation. Some people have few or no side effects. But if you have them, your doctor may change the dose of your radiation or how often you have treatments. Or your doctor may stop treatment until your side effects go away. Talk with your health care team about any side effects you have.

These are some common short-term side effects:

• Diarrhea
• Gas and bloating
• Nausea
• Feeling the need to urinate often
• Pain or burning feeling when you urinate or have a bowel movement
• Blood in your stool
• Trouble controlling your bowel movements
• Feeling very tired
• Low blood counts (seen on blood tests)
• Skin irritation or skin changes in areas that get radiation

Some of these side effects can be controlled and even prevented with medicine. Most get better over time after treatment ends. Some may be helped with diet. Talk with your health care team about what side effects you should watch for, how to deal with them, and how to know when they become serious. Make sure you know what number to call with questions or problems. Is there a different number to call after office hours or on holidays and weekends?

It may be helpful to keep a diary of your side effects. A written list will make it easier to remember your questions when you go to appointments. It will also make it easier for you to work with your health care team to make a plan to manage side effects.

Radiation therapy can cause some long-term side effects that may not show up until many years after treatment. It depends on where the radiation was aimed and the dose used. These can be more serious, so you should watch for them and talk with your health care team about what to expect.

Long-term side effects can include:

• 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 your urine or stool. Be sure your doctor is aware of these problems so that they can be treated, if needed.
• Early menopause (hormone changes) or infertility (inability to become pregnant). If you are planning on having children, discuss your choices with your health care team before starting radiation treatments. If you haven't gone through menopause, you may have menopausal symptoms. Talk with your doctor about what you can do to ease the symptoms. Please let your doctor know if there is any chance of you being pregnant. Radiation can cause damage or death to a fetus.
• Vaginal dryness. You may have vaginal dryness and narrowing. This can lead to pain or bleeding during sex. Lubricants can help with vaginal dryness. Vaginal narrowing may be helped by using a vaginal dilator or by having sex. Both can help stretch the vagina.
• Lower sperm count. You may make less semen. Semen may have a lower sperm count. This could lower a male's fertility. Still, it might be possible to get someone pregnant, so radiation does not count as birth control. If you're planning on having children, you may want to discuss saving sperm before starting radiation treatments.
• Erectile dysfunction. You may have trouble getting or keeping erections. Certain medicines or a penile pump can help.