Intensity modulated radiotherapy (IMRT) is a type of conformal treatment where the radiation beam is shaped to closely fit the area of cancer. 

IMRT is delivered using a standard radiotherapy machine, called a linear accelerator (LINAC). 

The LINAC has a device called a multileaf collimator which is made up of thin leaves of lead that move independently. This leaf movement, occurring while the machine rotates around the patient, can shape the radiation beam to fit precisely around the tumor. This means that the tumor receives a very high dose of radiation and normal healthy cells nearby receive a much lower dose.

Additionally, 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 tumor.

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.