Rectal cancer radiotherapy

Radiotherapy uses high-energy rays to destroy cancer cells. It destroys cancer cells in the area where treatment is given, while doing as little harm as possible to normal cells.

It is often given in combination with chemotherapy for rectal cancer. This is called chemoradiation. Chemotherapy can make cancer cells more sensitive to radiotherapy.

Radiotherapy does not make you radioactive. It is safe for you to be with other people, including children, throughout your treatment.

How radiotherapy is given

Radiotherapy can be given externally or internally.

External radiotherapy

External beam radiotherapy is given from a radiotherapy machine outside the body.

This treatment is normally given as a number of short, daily treatments in a radiotherapy department. These are called treatment sessions or fractions. It is given using a machine that looks like a large x-ray machine or CT scanner. Each treatment takes 10 to 15 minutes. The treatments are usually given Monday to Friday, with a rest at the weekend.

Your doctor will talk to you about the treatment and possible side effects.

Internal radiotherapy

Internal radiotherapy uses a radioactive material that is put inside the body to treat cancer.

High-dose-rate (HDR) brachytherapy is a newer treatment. We do not know all the possible risks and benefits yet. Before you decide to have this treatment, your doctor will explain what it involves. They will talk with you about possible benefits and risks. They may also give you written information to help you make your decision.

HDR brachytherapy uses radioactive material (the source) that is placed close to or inside the tumour. The source is left in place to give the correct dose of treatment. As with external radiotherapy, the source uses high-energy rays to treat the cancer.

Before the treatment, the rectum needs to be emptied. Your nurse will give you a liquid into your back passage. This is called an enema. This liquid helps you to empty your bowel. Brachytherapy is usually given under a general anaesthetic and the treatment takes about an hour.

Some people with early-stage rectal cancers may be treated with a type of brachytherapy called Papillon treatment. This can be given as an outpatient and you do not need a general anaesthetic. It may be given on its own or with external radiotherapy. It is only available in a few hospitals, so you may need to travel to have it. If you want to find out if this is an option for you, ask your cancer doctor. You can read more about it on Papillon's website.

When radiotherapy is given

Radiotherapy may be used instead of surgery if you are unable to have surgery. Your doctor will talk to you about the benefits and risks of surgery and radiotherapy.

Radiotherapy before surgery

Radiotherapy is sometimes given before surgery. This aims to:

  • shrink the cancer, so that it is easier to remove with surgery
  • reduce the risk of the cancer coming back.

You may have a short course of radiotherapy. It is given once a day, Monday to Friday, the week before surgery.

Or you might have a long course of radiotherapy that lasts up to 6 weeks. To help make the radiotherapy more effective, it is usually given with chemotherapy. This is called chemoradiation.

After a long course of radiotherapy, you wait for at least 6 weeks before having surgery. During this time, the radiotherapy or chemoradiation will continue to work, shrinking the cancer.

Radiotherapy after surgery

If radiotherapy was not given before surgery, you may have it afterwards. This is if cancer cells are found on the edge of healthy tissue (margin). You may have radiotherapy with chemotherapy (chemoradiation).

External radiotherapy is usually given Monday to Friday, for 4 to 5 weeks.

Radiotherapy for advanced cancer

External radiotherapy may be used to treat rectal cancer that has spread or come back after treatment. It is most likely to be used to treat cancer in the pelvis (the area between the hip bones). The aim is to shrink the cancer and help with symptoms such as bleeding or pain.

You can usually only have radiotherapy for advanced cancer if you have not had it to treat cancer in the same area before.

Planning your radiotherapy treatment

You will have a hospital appointment to plan your treatment. You will usually have a CT scan of the area to be treated. During the scan, you need to lie in the position that you will be in for your radiotherapy treatment.

Your radiotherapy team use information from this scan to plan:

  • the dose of radiotherapy
  • the area to be treated.

You may have some small, permanent markings made on your skin. The marks are about the size of a pinpoint. They help the radiographer make sure you are in the correct position for each session of radiotherapy.

These marks will only be made with your permission. If you are worried about them, talk to your radiographer.

Support from Macmillan

Macmillan is here to support you. If you would like to talk, you can do the following:

  • Call the Macmillan Support Line on 0808 808 00 00.
  • Chat to our specialists online.
  • Visit our radiotherapy and side effects forum to talk with people who have been affected by radiotherapy, share your experience, and ask an expert your questions.

Treatment sessions

At the beginning of each session of radiotherapy, the radiographer will position you carefully on the couch and make sure you are comfortable. During your treatment you will be alone in the room, but you can talk to the radiographer who will watch you from the next room.

Radiotherapy is not painful, but you will have to lie still for a few minutes during the treatment.

Side effects of radiotherapy

Side effects depend on:

Side effects usually begin a week or two after starting treatment. They may continue to get worse for a few weeks after treatment, before beginning to get better. Side effects usually improve gradually over the next few weeks or more.

Smoking can make side effects worse. If you smoke, try to give up or cut down. Drink at least two to three litres of fluid a day. Water is best. Drinks containing caffeine and alcohol can make bowel and bladder symptoms worse.

It is important to tell your radiographer, cancer specialist or specialist nurse if you have side effects. They can give you advice on how to manage them and prescribe treatments that can help. It may take some time to recover, particularly after longer courses of radiotherapy or chemoradiation. Look after yourself by getting enough rest and gradually increasing your physical activity. This will help with your recovery.

Sometimes some side effects do not completely go away. Sometimes side effects develop months or years later after treatment. These are called long-term or late effects. Tell your specialist nurse or cancer doctor if side effects do not get better, or you notice new side effects. There are many things that can be done to help.

We have more general information about the side effects during pelvic radiotherapy treatment.


Tiredness (fatigue) can continue for weeks or a few months after your treatment has finished. You might be more tired if you have to travel to hospital each day. If you are also having other treatment such as chemotherapy or surgery, this can make you even more tired.

Try to get as much rest as you can, especially if you have to travel a long way for treatment. Balance this with some physical activity, such as short walks, which will give your more energy.

Skin reactions

The skin in the area that is treated may:

  • redden
  • darken
  • feel sore or itchy.

Your radiographer or specialist nurse will give you advice about looking after your skin. If your skin becomes sore or itchy or changes colour, tell them straight away. They can give you advice and treatments if needed.

Skin reactions should get better within 4 weeks of treatment finishing.

During your treatment, you are usually advised to:

  • wear loose-fitting clothes made from natural fibres, such as cotton
  • wash your skin gently with mild, unperfumed soap and water and gently pat it dry
  • avoid rubbing your skin
  • avoid wet shaving
  • avoid hair-removing creams or products, including wax
  • follow your radiotherapy team’s advice about using moisturisers
  • protect the treated area from the sun.

Bowel changes

You may experience problems with your bowel such as:

  • loose stools (poo)
  • constipation
  • needing to poo urgently and more frequently
  • cramping pains in your tummy or back passage
  • passing a lot of wind.

If you have diarrhoea, your specialist will prescribe anti-diarrhoea tablets to help.

If you have tummy cramps or constipation, tell your cancer doctor or specialist nurse. They can prescribe things to help.

Tell your specialist nurse or radiographer if you have any incontinence. They will give you advice on coping with this. They will also explain how to look after the skin in that area. 

You may be advised to make changes to your diet during radiotherapy. Bowel side effects usually start to improve about 2 weeks after radiotherapy finishes. Sometimes they may take a few months to get better.

Bladder changes

Radiotherapy can irritate the bladder. You may feel like you need to pass urine (pee) more often. You may also have a burning feeling when you pass urine.

Always tell your radiotherapy team if you have any side effects during, or after your course of treatment. There are drugs that can improve bladder side effects and help you feel more comfortable.

Try drinking at least 2 to 3 litres (3½ to 5½ pints) of fluids a day. Try to drink more water and less drinks that may irritate the bladder. These include drinks containing caffeine, such as tea, coffee, drinking chocolate and cola. You should also try to drink less alcohol, fizzy drinks, acidic drinks (orange and grapefruit juice) and drinks with artificial sweeteners (diet or light drinks).

Hair loss

Most people lose their pubic hair. It should grow back after your treatment finishes, although the hair loss may be permanent.

Additional side effects in women

Changes to the vagina

Radiotherapy can make the lining of the vagina sore and inflamed. You may be advised not to have sex during treatment and for a few weeks after. This is to allow any inflammation or side effects to settle. Ask your doctor or nurse for advice.

If you do have sex during treatment, it is very important to use effective contraception to prevent a pregnancy. Radiation may cause damage to a baby conceived during or shortly after radiotherapy.

After radiotherapy, the vagina may be narrower, less stretchy and drier than before. This may make sex uncomfortable. Your specialist nurse may recommend you use vaginal dilators to try to prevent the vagina from narrowing. Dilators are tampon-shaped, plastic tubes of different sizes that you use with a lubricant.

Vaginal dryness can be relieved with vaginal lubricants or creams. Hormone creams can also help with dryness and vaginal narrowing. These are available on prescription from your doctor.

Early menopause

If you are still having menstrual periods, radiotherapy to the pelvic area will cause the menopause. The menopause means your ovaries are no longer producing eggs. This means you will not be able to get pregnant. Your team will give you more information about this.

Hormone replacement therapy (HRT) replaces the hormones your ovaries can no longer produce. This can improve the symptoms of menopause.

Effects on fertility

Radiotherapy for rectal cancer can affect the ovaries and the lining of the womb. This often means you will not be able to get pregnant or carry a pregnancy after treatment. Your team will give you information about this.

If you would like to have children in the future, talk to your doctor or specialist nurse before you start treatment. There may be options for preserving your fertility.

Additional side effects in men


It is important that you do not make someone pregnant during radiotherapy, and for a few months after it has finished. Sperm produced during treatment and for some time after may be damaged. This can affect the developing baby if a pregnancy is started at this time. To prevent a pregnancy, your doctors may recommend that you use contraception during treatment and for a time after.

Radiotherapy can cause problems getting or keeping an erection. This is called erectile dysfunction or ED. You may find your erections are not as strong as they were before the treatment. You may get an erection, but then lose it. Or you may be unable to get an erection at all. Your radiotherapy team can explain what is likely to happen.

You may have a sharp pain when you ejaculate. This is because radiotherapy can irritate the tube that runs through the penis from the bladder (the urethra). The pain should get better a few weeks after treatment finishes.

Effects on fertility

Radiotherapy may make you unable to make someone pregnant (infertile). Your doctor or specialist nurse can talk to you about this.

For some men, it may be possible to have sperm stored before the treatment starts. This is called sperm banking. The sperm can then be used in the future. It is important to talk to your cancer doctor or specialist nurse before your treatment starts.

About our information

  • References

    Below is a sample of the sources used in our rectal cancer information. If you would like more information about the sources we use, please contact us at

    R Glynne-Jones, PJ Nilson, C Aschele et al. ESMO-ESSO-ESTRO Clinical practice guidelines for diagnosis, treatment and follow up for anal cancer. July 2014. European Society of Medical Oncology. Available from (accessed October 2019).

    National Institute for Health and Excellence (NICE). Colorectal cancer: diagnosis and management clinical guidelines. Updated December 2014. Available from (accessed October 2019).

    Association of Coloproctology of Great Britain & Ireland (ACPGBI). Volume 19. Issue S1. Guidelines for the management of cancer of the colon, rectum and anus. 2017. Available from (accessed October 2019).

    National Institute for Health and Care Excellence. Preoperative high dose rate brachytherapy for rectal cancer. 2015. Available from (accessed October 2019).

    BMJ. Best practice colorectal cancer. Updated 2018. Available from (accessed October 2019).

Date reviewed

Reviewed: 30 April 2020
Next review: 30 April 2023

This content is currently being reviewed. New information will be coming soon.

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