What is radiotherapy?

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

Radiotherapy helps to reduce the risk of DCIS coming back in the area it is given to. You have it after surgery to destroy any remaining DCIS cells. It also helps reduce the risk of invasive cancer developing.

After breast-conserving surgery, your cancer doctor will usually recommend you have radiotherapy to the breast if the DCIS is high grade. It is sometimes given for larger intermediate-grade DCIS. You will not usually be recommended radiotherapy if the DCIS is low grade.

You usually start radiotherapy about 4 to 6 weeks after surgery.

Your cancer doctor and breast care nurse will explain why radiotherapy is recommended for you. It is important to talk to them about any concerns you have.

Getting support

Macmillan is also here to support you. If you would like to talk, you can:

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 are made in the same way as a tattoo. The marks 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. Tell the radiographer if you are worried about them or already have a tattoo in the area to be treated.

Having radiotherapy for DCIS

You have the radiotherapy as an outpatient. It is usually given using equipment that looks like a large x-ray machine. This is called external beam radiotherapy (EBRT) or external radiotherapy. The person who operates the machine is called a therapy radiographer. They will give you information and support during your treatment.

You usually have radiotherapy as a series of short daily treatments. Each treatment is a called or a session. You usually have sessions from Monday to Friday, over 1 to 3 weeks.

Some people may be offered more treatment to the area where the DCIS was. This is called a radiotherapy boost. Your cancer doctor will tell you how many treatments you will need.

If you have radiotherapy to your left breast, you will usually be asked to take a deep breath and hold it briefly. This is called deep inspiration breath hold (DIBH). You do this at each of your planning and treatment sessions. DIBH helps protect your heart during radiotherapy treatment to your left side.

Your heart is behind the left side of your chest. DIBH moves the heart away from the area being treated. It also keeps you still and reduces the risk of late effects. A website called respire.org.uk explains more about DIBH.

You may have intensity-modulated radiotherapy (IMRT). This is another type of external radiotherapy. It shapes the radiotherapy beams and allows the radiographer to give different doses of radiotherapy to different areas. This means you have lower doses of radiotherapy to healthy tissue surrounding the tumour.

External radiotherapy does not make you radioactive. After your treatment, it is safe for you to be with other people, including children.

Treatment sessions

Your radiographer will explain what happens during treatment. At the beginning of each session, they make sure you are in the correct position. Usually, you lie with your arms above your head. If your muscles and shoulder feel stiff or painful, a physiotherapist can show you exercises that may help.

When you are in the correct position, the radiographer leaves the room and the treatment starts. The treatment is not painful and only takes a few minutes.

The radiographers can see and hear you from outside the room. You can usually talk to them through an intercom, if you need to.

During treatment, the radiotherapy machine may stop and move into a new position. This is so you can have radiotherapy from different directions.

If your muscles and shoulder feel stiff or painful, a physiotherapist can show you exercises that may help.

Side effects of radiotherapy for DCIS

Radiotherapy can cause side effects in the area of your body that is being treated. You may also have some general side effects, such as feeling tired. Sometimes side effects get worse for a time during and after you have finished radiotherapy before they get better.

If you are having the radiotherapy over 1 week, sometimes the side effects may not start for 2 to 3 weeks after treatment.

Your cancer doctor, breast care nurse or radiographer will tell you what to expect. They will give you advice on what you can do to manage side effects. If you have any new side effects or if side effects get worse, tell them straight away.

Skin irritation

If you have white or pale skin, the treated area may get red, dry and itchy. If you have black or brown skin, the treated area may get darker, dry and itchy. Your nurse or radiographer will give you advice on looking after your skin. If it becomes sore and flaky, your doctor can prescribe creams or dressings to help this.

Skin reactions usually get worse after treatment for a few weeks. But they slowly start to improve 2 weeks after radiotherapy ends.

Here are some tips for skin reactions:

  • Do not put anything on your skin in the treated area without checking with your cancer doctor, nurse or radiographer.
  • Have cool or warm showers or baths. Turn away from shower spray to protect the treated area.
  • Avoid shaving, waxing or using epilators on your underarm on the affected side.
  • Gently pat the area dry with a soft towel – do not rub.
  • Wear loose clothing. This is less likely to irritate your skin.
  • Avoiding swimming if your skin is irritated.

You need to avoid exposing the treated area to the sun during radiotherapy and after treatment finishes. Use suncream with a sun protection factor (SPF) of at least 30.


This is a common side effect that may last for a few weeks or months after treatment. Studies show that exercise can help to manage tiredness caused by treatment. Try to get enough rest and pace yourself. But it is important to balance this with some physical activity, such as going for short walks. This can give you more energy.

Aches and swelling

You may have a dull ache in the treated area. Or you may get shooting pains that last for a few seconds or minutes. You may also notice that the area becomes swollen. These effects usually improve after treatment. But you may still have some aches and pains in the area after treatment ends. The area can sometimes stay a little swollen after treatment.

Late effects of radiotherapy for DCIS

Radiotherapy to the breast may cause side effects that happen months or years after radiotherapy. These are called late effects. Newer ways of having radiotherapy are helping reduce the risk of late effects. If you are worried about late effects, talk to your cancer doctor, breast care nurse or radiographer.

The most common late effect is a change in how the breast or chest area looks and feels.

Radiotherapy can damage small blood vessels in the skin. This can cause red, spidery marks to show. These are called telangiectasia. They may be more common if you had boost doses of radiotherapy.

After radiotherapy, your breast may feel firmer and shrink slightly in size. If your breast is noticeably smaller, you can have surgery to reduce the size of your other breast. If you had breast reconstruction using an implant before radiotherapy, you may need to have the implant replaced.

You may find the treated area sore or uncomfortable for some time. This usually improves over years. It is not uncommon to get pain in the muscle or ribs at the edge of the breast if you overdo things. Very rarely, radiotherapy may cause lung problems or problems with the ribs.

If you have radiotherapy to the left breast, very rarely it can cause heart problems. Tell your cancer doctor, nurse or radiographer if you notice any problems with your breathing, or have any pain in the chest area. We have more information about the late effects of breast cancer treatment.

About our information

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Dr Rebecca Roylance, Consultant Medical Oncologist and Professor Mike Dixon, Professor of Surgery and Consultant Breast Surgeon.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

The language we use

We want everyone affected by cancer to feel our information is written for them.

We want our information to be as clear as possible. To do this, we try to:

  • use plain English
  • explain medical words
  • use short sentences
  • use illustrations to explain text
  • structure the information clearly
  • make sure important points are clear.

We use gender-inclusive language and talk to our readers as ‘you’ so that everyone feels included. Where clinically necessary we use the terms ‘men’ and ‘women’ or ‘male’ and ‘female’. For example, we do so when talking about parts of the body or mentioning statistics or research about who is affected.

You can read more about how we produce our information here.

Date reviewed

Reviewed: 01 October 2023
Next review: 01 October 2026
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.