Intensity modulated radiotherapy (IMRT) | Cancer Research UK
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What IMRT is

Intensity modulated radiotherapy (IMRT) is a type of conformal radiotherapy. Conformal radiotherapy shapes the radiation beams to closely fit the area of the cancer.

You can have IMRT on a standard radiotherapy machine, called a linear accelerator (LINAC). 

The LINAC has a device called a multileaf collimator. The multileaf collimator is made up of thin leaves of lead which can move independently. They can form shapes that fit precisely around the treatment area. The lead leaves can move while the machine moves around the patient. This shapes the beam of radiation to the tumour as the machine rotates. This means that the tumour receives a very high dose and normal healthy cells nearby receive a much lower dose.

Each radiotherapy beam is divided into many small beamlets that can vary their intensity. This allows different doses of radiation to be given across the tumour.

IMRT can also create a U shaped (concave) area at the edge of the radiotherapy field. This avoids high radiation doses to structures that would otherwise be damaged by the radiotherapy. So IMRT can reduce the risk of long term side effects. It is very helpful in areas such as the head and neck, for example to avoid the spinal cord or salivary glands.


Planning intensity modulated radiotherapy

Planning IMRT treatment involves several steps.

You start with a session in the radiotherapy department. You have a CT scan. The 360° photo is of a CT scanner. Use the arrows to look around the room.

You may also have MRI scans or PET scans of the area of the body to be treated.

The information from these scans feeds directly into the radiotherapy planning computer.

The treatment team carefully plans the treatment using the scan images. They use advanced computerised dose calculations to find the dose strength pattern that best matches the tumour shape. The planning may take longer than for some other types of radiotherapy.

Skin markings

The radiographers may make marks on your skin to make sure the same area is treated at each session.

Moulds and masks

If you have intensity modulated radiotherapy to your head or neck, you may need to wear a plastic mould during your treatment. You may hear this called a shell or mask. Some types of mask are see through but others aren't. You can have a mould for other parts of the body, such as the breast, limbs and sometimes the whole body.

The mould or mask keeps the treatment area completely still. So your treatment will be as accurate as possible. This also means that you can have any markings you need made on the mask, instead of on your skin.


After the planning session

After the planning session you usually have to wait for 2 weeks or more for the physicist and your radiotherapy doctor to create your treatment plan. You then get an appointment for your first dose of radiotherapy.


Having intensity modulated radiotherapy

To have the treatment you lie on a radiotherapy couch. The radiographers will help you to get into the right position and put on any moulds that you may need.

You may have the treatment from a machine called a LINAC

A photo of a linear accelerator, which gives radiotherapy

Or you may have it from a machine called a Tomotherapy machine

TomoTherapy is a particular brand of radiotherapy machine that has a built in scanner (imaging unit). It combines image guided radiotherapy and IMRT. The scanner part of the machine takes a scan before each treatment session.

Once you are in the right position the staff leave you alone in the room. This is so they are not exposed to the radiation. They watch you carefully either through a window or on a closed circuit television screen. They may ask you to hold your breath or take shallow breaths during the treatment.

You can't feel radiotherapy when you actually have the treatment. It doesn’t hurt. It may take between 15 and 30 minutes or more.


Volumetric modulated arc radiotherapy (VMAT)

VMAT is a new type of IMRT technique. The radiotherapy machine rotates around the patient during treatment. The machine continuously reshapes and changes the intensity of the radiation beam as it moves around the body. 

Giving the radiotherapy in this way makes it very accurate, shortens the treatment time, and uses a lower overall dose of radiation. The treatment usually takes about 10 minutes.


Which cancers IMRT can treat

Clinical trials have tested IMRT in a number of cancer types, including breast cancer and head and neck cancer. It is a standard form of treatment for some cancer types. But research is always going on, looking into using new treatments for other types of cancer.

Some trials are looking at improving the way you have radiotherapy treatment. You can search for IMRT trials on our clinical trials database.


Possible side effects

With intensity modulated radiotherapy techniques there is very little normal tissue in the treatment area. So the risk of side effects is low. But unfortunately you can still have side effects. As with any external beam radiotherapy, the side effects only affect the part of the body that the radiotherapy treatment is aimed at.


More information about radiotherapy

Look at our general information about radiotherapy.

We have more detailed information about

Conformal radiotherapy

Image guided radiotherapy (IGRT)

Radiotherapy skin markings

Radiotherapy moulds and masks

Clinical trials and research

Radiotherapy side effects

For general information and support 

Contact the Cancer Research UK nurses on freephone 0808 800 4040 (Open 9am to 5pm, Monday to Friday).

Share experiences on our online forum – Cancer Chat.

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Updated: 29 February 2016