Decorative image

Intensity modulated radiotherapy (IMRT)

Intensity modulated radiotherapy (IMRT) is a treatment for cancer. It is given in a hospital or specialist centre.

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 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 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 are having conformal 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, others aren't. 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. 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 a few days or up to 2 weeks for the physicist and your radiotherapy doctor to create your treatment plan.

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

Having IMRT

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.

Photo of a linear accelerator
A photograph of a LINAC machine

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. 

Possible side effects of IMRT

With intensity modulated radiotherapy techniques there is very little normal tissue in the treatment area. So the risk of side effects is low.

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.

Last reviewed: 
29 Feb 2016
  • External Beam Therapy
    Peter Hoskin
    OUP Oxford, 30 Aug 2012

  • Advances in radiotherapy
    S Ahmad and others
    British Medical Journal, 2012; 345.

  • The Evidence Base for Multiple Volumetric Modulated Arc Therapy (VMAT) “A Quality Perspective”. 
    Faculty of Radiation Oncology, The Royal Australian and New Zealand College of Radiologists, 2011.

  • Evidence behind use of intensity modulated radiotherapy: a systematic review of comparative clinical studies. 
    L Veldeman, I Madani, F Hulstaert, G De Meerleer, M Mareel, W De Neve 
    Lancet Oncolog, 2008, Apr;9(4):367-75.

Information and help

Dangoor sponsorship

About Cancer generously supported by Dangoor Education since 2010.