Colorectal cancer

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Side effects of radiation therapy for colorectal cancer

Side effects can happen with any type of treatment for colorectal cancer, but everyone’s experience is different. Some people have many side effects. Other people have few or none at all.

During radiation therapy, the healthcare team protects healthy cells in the treatment area as much as possible. Damage to these healthy cells may cause side effects. Side effects can happen any time during, immediately after or a few days or weeks after radiation therapy. Sometimes late side effects develop months or years after radiation therapy. Most side effects go away on their own or can be treated, but some side effects may last a long time or become permanent.

Side effects of radiation therapy will depend mainly on the:

  • size of the area being treated
  • specific area or organs being treated
  • total dose
  • treatment schedule
  • type of radiation therapy given (external beam radiation therapy or brachytherapy)

Tell your healthcare team if you have these side effects or others you think may be from radiation therapy. The sooner you tell them of any problems, the sooner they can suggest ways to help you deal with them.

Bowel problems

The intestine is very sensitive to radiation, so bowel problems are common with radiation therapy for colorectal cancer. Most problems usually go away after treatment is finished, but some can last several months or years after radiation therapy.

Diarrhea and incontinence often occur after radiation therapy for colorectal cancer. Diarrhea is an increase in the number and looseness of stools. Incontinence is the inability to control bowel movements. Find out more about diarrhea.

Bowel obstruction is when part of the intestine is blocked so stool, or feces, can’t move through it normally. It can happen after radiation therapy if this treatment causes adhesions in the intestine. Adhesions are pieces of scar, or fibrous, tissue that make organs and tissues stick together. This can lead to a blockage in the intestine. Find out more about bowel obstruction.

Radiation enteritis is irritation and inflammation of the intestine caused by radiation to the abdomen, pelvis or rectum. It can occur when radiation therapy is first given and up to 8 weeks after radiation therapy begins (called acute radiation enteritis). Radiation enteritis can cause pain or discomfort in the abdomen and rectum, bleeding from the rectum and diarrhea. Find out more about radiation enteritis.

The radiation therapy team can suggest ways to help manage bowel problems. They may prescribe medicines for diarrhea, pain or inflammation. Surgery may be needed to treat a bowel obstruction, adhesions or radiation enteritis.

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Fatigue is one of the most common side effects of radiation therapy. Fatigue may be caused by anemia, poor appetite or depression. It may also be related to toxic substances that are made when cancer cells break down and die.

Radiation therapy to any area of the body can make a person feel more tired than usual, but fatigue is more common when larger areas of the body are treated. Anemia is more likely to occur when the treatment area includes where blood cells are formed in the bone marrow, such as the pelvic bones. During radiation therapy, the body uses more energy to heal itself, so fatigue will not always be relieved by rest. Making frequent, daily trips for radiation treatments can also be tiring.

Fatigue usually starts during or after the second week of radiation therapy. Symptoms of fatigue may get worse over the course of treatment. Fatigue usually goes away gradually after treatment has ended, but some people continue to feel tired for several weeks or months after radiation therapy.

Find out more about fatigue and anemia.

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Skin reactions

Skin reactions happen because external beam radiation has to travel through the skin to reach the area being targeted for treatment. The skin in the treatment area may become red or dry. It can also become darker or tanned looking. The skin around the anus is very sensitive to radiation, so if it is in the treatment area it can become red and sore.

Most skin reactions occur within the first 2 weeks of receiving radiation treatment. They usually go away a few weeks after treatment ends. Some skin changes, like skin darkening or scarring, can last for a long time.

Find out more about skin problems.


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Nausea and vomiting

Nausea and vomiting happen more often when radiation is used as part of chemoradiation than when radiation is given alone. You may start to feel nauseated about 1–3 hours after your daily treatment about 1–2 weeks into therapy.

Some people find that eating a small snack before treatment helps prevent nausea and vomiting. Others find it better not to eat for a few hours before treatment or to wait a few hours after treatment before eating.

Tell your radiation therapy team if you have nausea and vomiting. They can usually be managed with medicines. Nausea and vomiting usually go away after treatment is finished.

Find out more about nausea and vomiting.

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Bladder problems

Radiation therapy for colorectal cancer can irritate the bladder and make it inflamed (called radiation cystitis). This can lead to bladder problems, including:

  • the need to urinate often (called frequency)
  • burning or pain during urination
  • blood in the urine (called hematuria)

Bladder problems usually occur 3–5 weeks after radiation therapy starts and go away 2–8 weeks after radiation therapy is finished. Some problems, such as cystitis, may continue or develop after treatment is over.

Try to drink plenty of fluids to ensure a regular flow of urine and empty the bladder often. Report burning, pain and frequent urination to the radiation therapy team. These can also be symptoms of infection.

The healthcare team may recommend medicines to help treat some bladder problems.

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Sexual problems

Radiation to the pelvis can damage nerves, blood vessels and organs in the pelvis. This can lead to sexual problems for men and women.

In men, radiation to the pelvis can cause erectile dysfunction (inability to have or keep an erection) or ejaculation problems. In women, it can make the vagina irritated and dry.

Talk to your healthcare team if you have these problems. They may prescribe medicines to manage sexual problems.

Find out more about sexual problems for men and sexual problems for women.


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Pelvic fractures

Radiation therapy can damage bones and the blood supply to the bones. This can make the bones weak, and they can crack. In rare cases, radiation therapy for rectal cancer can lead to cracked or broken bones in the pelvis.

Pelvic fractures can cause pain. They are usually treated with pain medicines.


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Second cancers

A very small number of people develop a second cancer after they have radiation therapy. The benefit of treating your cancer usually far outweighs the risk of developing a second cancer.

People who receive both chemotherapy and radiation therapy have the highest risk of developing a second cancer. A second cancer can develop a few years after radiation therapy, but most do not occur for 10–20 years or more after treatment.

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