The patient's inherent reaction to ionizing radiation is variable and influences the outcome. Because the likelihood of controlling a tumor by radiation is related directly to the dose given, the radiation oncologist prefers to give as high a dose as possible. However, because normal tissues usually are admixed with tumor tissues or are located close by, the radiation oncologist often cannot give as high a dose as she or he would like, for fear of causing both irreversible damage to those normal tissues and the late effects that the patient and the physician want to avoid. Often, then, the radiation oncologist prescribes a lower total dose, to give the normal tissues an opportunity to recover and repair the radiation damage they have suffered. Unfortunately, the lower dose may not control the tumor.
Protons are more massive than photons or electrons, and so scatter less as they pass through tissue.
This contrasts with x-rays or gamma rays, which pass through the patient's body, depositing energy all the while. That is what is being attempted every day at the Proton Treatment Center. The observation has long been made that radiation effects are related to dose and volume, but the underlying mechanism is only partially understood. In the last 10 years interest has increasingly been focussed on cytokines, small proteins secreted by cells. Some investigators have described the potentially harmful effects of increased cytokine expression following radiation therapy.