Intensity Modulated Radiation Therapy, IMRT

Abstract

Radiation therapy has been the important modality in treatment of cancer patients. Initially, radiation oncologists used conventional radiograph or fluoroscopy to set up the radiation treatment field. Radiation field aperture and angle were defined based on the correlation of tumors and bony structures seen on 2D radiograph. Subsequent advances in imaging technology had resulted in improvement in radiotherapy planning. In mid 1980, computer tomography was generally used in diagnostic department and then widely adopted in radiation oncology community. The treatment

planning systems allowed CT image data to be incorporated into radiotherapy treatment plans, heralding 3-D conformal radiation therapy (3D CRT). Based on this 3D volume, radiation oncologists decided the radiation beam configuration (beam energy, beam aperture, gantry angle, wedge and weighting factors) in the computer. This process is called "virtual simulation". The aims of the treatment are to maximize target coverage as well as minimize normal tissue dose.

Although CT and MRI are used in 3D treatment planning system to define gross tumor volume and subclinical volume, there are some limitations of 3D CRT. 3D CRT needs experienced planner. The planners manually adjust beam directions, beam shapes and beam intensities on the basis of their planning experience and "trial and error" to meet the optimal dose distribution. This process is called "forward planning" which works well for tumors with simple shapes. For complex tumor geometies, such as concave tumors and tumors surrounded by sensitive structures, the forward planning method may be limited by the experience of the individual planner and the restricted beam intensity variation inside each beam.

During mid 1990, the advanced in radiological imaging and computer technology have significantly enhanced the development of intensity modulated radiation therapy. IMRT is an advanced method of 3D CRT that utilizes sophisticated computer-controlled radiation beam delivery to improve the conformality of the dose distribution to the shape of the tumor. IMRT also incorporates computerized inverse treatment plan optimization as opposed to the manual optimization techniques of conventional 3D CRT. The planner designs beam angle, beam energy as well as dose volume constraint; the computer optimization determines the intensities of each beam.

This article will introduce the IMRT to the reader. Discussions of the advantage, limitation and clinical use are also provided.