Chapter 20: Intensity Modulated Radiation Therapy

In the traditional external beam photon radiation therapy, most treatments are delivered with radiation beams that are of uniform intensity across the field (within the flatness specification limits). Occasionally, wedges or compensators are used to modify the intensity profile to offset contour irregularities and/or produce more uniform composite dose distributions such as in techniques using wedges. This process of changing beam intensity profiles to meet the goals of a composite plan is called intensity modulation.

The term "intensity modulated radiation therapy" refers to a radiation therapy technique in which nonuniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution.

Inverse planning.

The clinical implementation of IMRT requires at least two systems:

• 1.) a treatment planning computer system that can calculate nonuniform fluence maps for multiple beams directed from different directions to maximize dose to the target volume while minimizing dose to the critical normal structures 
• 2.) A system of delivering the nonuniform fluences as planned.

The principle of IMRT is to treat a patient from a number of different directions (or continuous arcs) with beams of nonuniform fluences, which have been optimized to deliver a high dose to the target volume and an acceptably low dose to the surrounding normal structures.

• 1.) Analytic Methods.
• 2.) Iterative Methods.