Intensity modulated radiotherapy (IMRT)

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

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

How does IMRT work?

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.

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. This can vary the intensity of radiation. It can give different doses of radiation across the tumour.

IMRT can be very helpful in areas such as the head and neck, for example to avoid the spinal cord or salivary glands.

Planning IMRT

There are several steps involved in planning IMRT.

You begin with a CT scan at the radiotherapy department. You may also have MRI scans or PET scans. 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 calculations. This finds the dose strength pattern that best matches the tumour shape.

The planning may take longer for IMRT compared to other types of radiotherapy.

Skin markings

The radiographers might make marks on your skin that act as reference points. This is to make sure every treatment session is accurate.

Radiotherapy masks and moulds

If you have radiotherapy to your head or neck, you may need to wear a radiotherapy mask during your treatment. This is also called a shell or mould. 

Most types of masks are made of a mesh material with lots of small holes. You can have a mould for other parts of the body, such as the breast or limbs.

The mould or mask keeps the treatment area completely still. This is so your treatment will be as accurate as possible.

After the planning session

You usually have to wait a few days or up to 2 weeks for your treatment plan to be ready.

You then get an appointment for your first session of radiotherapy.

Having IMRT

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

You might have the treatment from a machine called a LINAC. Or another type of radiotherapy machine, for example a TomoTherapy machine. TomoTherapy is a CT scanner combined with IMRT. 

Photo of a linear accelerator

Once you are in the right position your radiographers leave you alone in the room. This is so they are not exposed to the radiation.

They will often take x-rays before you have treatment. This is to ensure that you're in the correct position and is called image guided radiotherapy (IGRT). 

Your radiographers watch you carefully either through a window or on a closed circuit television screen (CCTV). They can talk to you through an intercom. 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) or Rapid Arc

VMAT is a type of IMRT technique. VMAT stands for Volumetric Arc Therapy and is also called Rapid Arc. 

VMAT is different to normal IMRT. The radiotherapy machine rotates around the patient during a radiotherapy beam. This is in an arc shape.

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
  • uses a lower overall dose of radiation

The treatment usually takes about 10 minutes.


TomoTherapy is a type of IMRT.

For this treatment you lie on the radiotherapy couch which moves through a donut-shaped machine. The radiation source in the machine rotates around you in a spiral pattern. It is also sometimes called helical tomotherapy. 

Which cancers IMRT can treat

IMRT is a standard form of treatment for some types of cancer, including breast and prostate.

Some trials are looking at improving the way you have radiotherapy treatment. 

Possible side effects of IMRT

With IMRT there is less normal tissue in the treatment area. So the risk of side effects is lower, but you might still experience some 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.

  • External Beam Therapy (Radiotherapy in Practice) Third Edition
    Peter Hoskin
    Oxford University Press, 2019

  • Advances in radiotherapy
    S Ahmad and others
    British Medical Journal, 2012.

  • Devita, Hellman and Rosenberg's Cancer: Principles and Practice of Oncology (12th edition)
    VT Devita, TS Lawrence and SA Rosenberg
    Wolters Kluwer Health, 2023

Last reviewed: 
06 Nov 2023
Next review due: 
06 Nov 2026

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