PRECISION RADIATION THERAPY
Because tumors are located within the body, they are often near or located within critical structures or organs. In order to treat these tumors without causing harm to the normal body structures nearby, precision radiation therapy techniques must be employed. One of the primary way to visualize a tumor and the rest of the body is with the use of a CT scan. This advanced imaging technique gives excellent location of the tumor and normal body structures as well as provides information regarding the density of the body structures which is a critical part of calculating how much radiation to administer.
Rigid immobilization of patients and accurate positioning of their targets have long been recognized as critically important aspects of high quality precision radiation therapy. 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.
Radiobiological studies have indicated that the slope of the dose-response curve for many tumors is sufficiently large that even a dose reduction of 3% to 5% to a portion of the tumor could significantly reduce the probability of local control (that means the tumor may not be completely destroyed and could come back eventually). The computed tomography-based three-dimensional treatment planning programs has made highly conformal dose distributions possible. The extremely precise treatments require 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 within 3 mm, allowing the use of margins of no more than 5 mm except when target motion within the immobilized patient is an issue, such as a moving lung tumor as the patient breathes. Special techniques for cranial targets or for targets in the head and neck result in positioning accuracies of 1 to 2 mm, or even less if rigid fixation is utilized. 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.