IMRT is a method of delivering high-precision radiotherapy which employs a computer-controlled linear accelerator to deliver precise radiation doses to a malignant tumour or to specific areas within the tumour.

The radiation dose is planned to conform to the shape of the tumor in three dimensions by 'modulating' the intensity of the radiation beam: this increases the radiation dose to the tumor while concurrently minimising radiation dose to surrounding normal tissues.

Treatment is carefully planned by employing computed tomography (CT) images of the patient in conjunction with inverse planning methods of dose calculation to determine the pattern of radiation dose that will best conform to the tumour shape.

Typically, combinations of several intensity-modulated fields coming from different beam directions produce a customised treatment plan which tailors the radiation dose, thus maximising the radiation dose to the tumour while also minimising the radiation dose to normal surrounding tissues.

Compared with conventional radiotherapy techniques, as the ratio of normal tissue dose to tumor dose is reduced to a minimum with the IMRT approach, higher and more effective radiation doses can safely be delivered to tumour with fewer side effects.

IMRT also has the potential to reduce treatment toxicity, even when doses are not increased.