11.04 - Principles Of IMRT

Intensity modulated radiation therapy (IMRT) is the use of beams with non-uniform fluence to deliver radiation to a target volume. It typically involves the use of inverse planning techniques where the target volume is specified, organs at risk are specified, and a computer calculates the most appropriate field and fluence arrangement.

IMRT Planning

IMRT is typically planned using inverse planning. This may be done using analytical methods which attempt to construct a beam arrangement that would lead to the desired dose distribution (difficult with competing dose minimums and maximums). Another method is iterative, which adjusts beams gradually to try and maximise the dose to the target volume and minimise the dose to critical structures. A dose volume histogram is used by the iterative method to try and accomplish this. Many computer programs use both methods to achieve a beam arrangement.

IMRT Delivery

Currently the most useful device for delivery of radiotherapy beams with dynamic modulation of energy fluence is the multileaf collimator. This is computer controlled and may shift between the delivery of radiation (multisegmented static fields) or during delivery (dynamic delivery). The latter technique requires rapidly moving MLC leaves (over 2 cm per second) and requires additional resources to measure the position of leaves and ensure accuracy. The former technique is easier to implement as it requires fewer resources, but faces issues with beam ‘on’ and ‘off’ cycling and takes longer to deliver.
A third technique is the use of arc therapy combined with IMRT (intensity modulated arc therapy). In this technique the gantry rotates around the patient with the beam on, and the MLC leaves shift dynamically during treatment. This has the benefit of rapid treatment delivery but increases the volume of tissue being exposed to radiation.

IMRT Commissioning and Quality Assurance

IMRT requires additional commissioning and quality assurance checks to maintain patient safety. This includes:

  • Testing of the dynamic MLC to ensure stable leaf speed, dose profile between leaves, the acceleration of individual leaves and the accuracy of leaf position readings. Regular mechanical checks are required.
  • Testing of dosimetry, including the interleaf transmission and increased head scatter.
  • Treatment verification using phantoms and ionisation chambers must be carried out to ensure that the computer designed plan accurately represents the dose distribution with the patient.
  • Regular quality assurance should be performed, including verification of dose prior to the start of a course of treatment, daily checks of dose to a test point in each field, weekly checks of dose distribution based on gantry and collimator positions and annual checks of all machine features.