Immobilizing and Positioning Patients for Radiotherapy.
Rigid immobilization of patients and accurate positioning of their targets have long been recognized as critically important aspects of quality radiotherapy. Radiobiological studies have indicated that the slope of the dose-response curve for many tumors is sufficiently large that a dose reduction of 3% to 5% to a portion of the tumor could significantly reduce the probability of local control. On the other hand, accurate positioning combined with rigid immobilization might permit reduced margins around the target, resulting in a decrease in dose to normal tissue and a potential increase in dose to the target. The increasing use of computed tomography-based three-dimensional treatment planning programs has made highly conformal dose distributions possible, thus further emphasizing the need for accurate positioning. The development of new immobilization materials and methods has made it possible to immobilize almost any area of the body of a cooperative patient to 3 mm, allowing the use of margins of no more than 5 mm except when target motion within the immobilized patient is an issue. Special techniques for intracranial targets or for targets in the head and neck can yield positioning accuracies of 1 to 2 mm, or even less for invasive immobilization. Through the use of electronic portal imagers, stereo video cameras, or stereo radiography, anatomical reference points can be followed during treatment and the target position varied as needed. Quantitative positioning studies are required for each disease site and immobilization method so that the target position uncertainty, which is the basis for the choice of treatment-planning margins, can be minimized and evaluated, leading to an increased level of uncomplicated local tumor control.
Verhey
LJ
Department of Radiation Oncology, University of California, San Francisco, CA, USA