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Side effects of radiotherapy

Find out more about the side effects of radiotherapy.

1. Skin reactions

Radiotherapy can cause skin reactions. 

Most people have some redness around the area being treated. 

The skin may also: 

  • Become pinker or darker over time 
  • Feel tender, dry, itchy and sore 
  • Peel or flake as treatment goes on
  • Blister or become moist and weepy

You may hear the skin reaction being called radiation dermatitis.

Skin reactions may start during or after treatment. The skin reaction is normally at its worst between 7 and 14 days after treatment. After this it usually starts to get better.

Let your treatment team know if you develop a skin reaction. Most skin reactions are mild and should heal within 3 to 4 weeks of your last treatment, but some may need treating or monitoring more closely. For example, skin that has blistered or is peeling will take longer to heal.

The Society and College of Radiographers has information you may find useful on skin reactions and how to care for your skin.

Caring for your skin during radiotherapy

It’s important to look after your skin during treatment. This will help prevent infection, reduce pain and help keep the area being treated comfortable.

You will be given skincare instructions by your radiotherapy team. The following tips may also help.

Washing the skin

Wash the treated area gently with warm water and pat the skin dry with a soft towel. 

Skincare products

If you want to use anything on the skin in the treatment area, discuss this with your therapeutic radiographer first. 

You can continue to use moisturiser that is free of sodium lauryl sulphate, which can cause irritation, but it’s not recommended to apply this immediately before your treatment. If your skin blisters or peels let the radiotherapy staff know. You can continue to use soap and deodorant that suits your skin unless your treatment team has told you not to. 

Heat and cold

Avoid exposing the treated area to very hot or cold temperatures such as hot water bottles, heat pads, saunas or ice packs during treatment. 

Taking care in the sun 

Avoid exposing the treated area to the sun while having radiotherapy and until any skin reaction has settled down. 

The skin in the treated area will remain sensitive to the sun for some time after treatment. Use a sunscreen with a high SPF. Apply the cream under clothes too as you can get sunburnt through clothing.

Read our tips for taking care of your skin in the sun after radiotherapy.

Clothing, bras and prostheses

Friction or rubbing from clothing can cause or worsen skin reactions. 

You’ll usually be advised not to wear an underwired bra until your skin heals. Wearing clothing made from natural fibres, such as a soft cotton bra or vest, may help. You may prefer to go without a bra.

If you’ve had a mastectomy and wear a silicone , you may find it more comfortable to wear the soft, lightweight prosthesis (softie or cumfie) you used after surgery.


Skin changes from radiotherapy can be irritated by chlorine or chemicals in the pool. Speak to your treatment team if you want to go swimming during or shortly after treatment.

2. Swelling (oedema) of the breast

Your breast or chest area may appear swollen and feel uncomfortable. This usually settles within a few weeks after treatment. 

If it continues after this time, talk to your treatment team as you may need to be seen and assessed by a specialist. 

3. Pain in the breast or chest area

You may have aches, twinges or sharp pains in the breast or chest area. These are usually mild. They may continue for months or years, but they usually become milder and less frequent over time.

You may also have stiffness and discomfort around the shoulder and breast or chest area during and after treatment. 

Continuing to do arm and shoulder exercises during radiotherapy and for several months afterwards may help minimise or prevent stiffness or discomfort. 

4. Hair loss in the armpit

Radiotherapy to the armpit will make the underarm hair fall out on that side. You will also lose any hair on the area of the chest that’s being treated.

Hair in the treatment area usually starts to fall out 2 to 3 weeks after treatment has started and it may take several months to grow back. For some people, hair lost from radiotherapy may never grow back.

5. Sore throat

If you have radiotherapy to the area around your collarbone or near your breastbone, you may develop a sore throat or discomfort when swallowing. If this happens, talk to your treatment team.

It may help to take pain relief in liquid form, particularly before eating, until the discomfort improves.

6. Extreme tiredness

Fatigue is extreme tiredness that doesn’t go away with rest or sleep. It’s a very common side effect of breast cancer treatment and may affect you physically and emotionally.

If you think you have fatigue, tell your GP or treatment team. They can assess you and offer advice on how to manage your energy levels.

7. Lymphoedema

Lymphoedema is swelling of the arm, hand, breast or chest area caused by a build-up of fluid in the surface tissues of the body. It can occur as a result of damage to the lymphatic system, for example because of surgery or radiotherapy to the lymph nodes under the arm and surrounding area.

Lymphoedema can occur at any time after treatment, sometimes years later.

If the arm, hand, breast or chest area on the side where you had the radiotherapy or surgery swells or feels uncomfortable and heavy, contact your treatment team or GP.

8. Change in breast shape, size and colour

If you’ve had radiotherapy after breast-conserving surgery, the breast tissue and nipple on the treated side may feel firmer than before, change colour or the breast may be smaller and look different.

Although this is normal, you may be concerned about differences in the size of your breasts, or worry that the difference is noticeable.

You can discuss this with your breast surgeon to see if anything can be done to make the difference less noticeable. These side effects may affect how you feel about your body, including how you feel about intimacy and sex.

9. Tenderness over the ribs

Tenderness can occur over the ribs during treatment. In some people, this discomfort may continue but it usually improves gradually over time.

10. Late side effects

Some side effects can develop months or years after radiotherapy treatment ends. But these side effects are much less common.  

Serious side effects are very rare and experts agree that the benefits of the treatment in reducing the chances of breast cancer returning outweigh the risk of possible side effects.

Hardening of tissue

Hardening of the tissue (fibrosis) is rare but may happen several months or years after radiotherapy has finished. If the fibrosis is severe, the breast can become noticeably smaller and firmer.

Changes to the reconstructed breast

If you have a breast reconstruction using an implant, radiotherapy can cause the reconstructed breast to become firmer, change shape or become uncomfortable. You may hear this called capsular contracture.

If you have a breast reconstruction using your own tissue (tissue flap), radiotherapy can cause the tissue of the reconstruction to change shape or shrink.

If you notice changes to your reconstructed breast, talk to your breast surgeon or breast care nurse.

Broken blood vessels 

Under the skin you may see tiny broken blood vessels. This is known as telangiectasia. Although it’s not harmful to you, it’s permanent and there’s no treatment for it. It may affect you emotionally and the way you see your body. If you're worried about this, talk to your breast care nurse.

Changes to the lungs

Sometimes after treatment to the breast or chest wall area, part of the lung behind the treatment area can become inflamed. This may cause a dry cough or shortness of breath. It usually heals by itself over time. More rarely, hardening of the upper lung tissue can occur which can cause similar side effects.

Heart problems

There is a small risk of heart problems in the future from radiotherapy given on the left side. The risk is very low as care is taken to reduce the dose of radiotherapy to the tissues of the heart.

Risk of another cancer developing

There is a small risk of developing another cancer in the future from having radiotherapy. This is very rare, and much less of a risk than your breast cancer returning if you do not have radiotherapy. Your specialist will discuss this risk with you.

Other effects

Other side effects include:

  • Weakening of the bones in the treated area – can lead to rib and collarbone fractures
  • Damage to the nerves in the arm on the treated side – may cause tingling, numbness, pain, weakness and possibly some loss of movement
  • Long-term skin reactions (chronic radiation dermatitis) – see the section on skin reactions at the top of this page

If you’re concerned about late side effects, speak to your treatment team.

11. Further support

If you’re worried about any side effects, regardless of whether they are listed here, talk to your treatment team. You can also talk things through with one of our nurses on our free helpline - please see below.

You never have to face breast cancer alone. Find somebody who understands what you’re going through with Someone Like Me, below.

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Quality assurance

This information was published in October 2022. We will revise it in October 2024.

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