What Makes Protons Different?
Protons, which are positively charged subatomic particles, deposit energy differently than x-ray beams do. Compared to an x-ray beam, a proton beam that is delivered with sufficient energy (or "modulated") has a low "entrance dose" (the dose in front of the tumor). A proton beam also has a high-dose "Bragg peak" region, which is designed to cover the entire tumor. There is no "exit dose" beyond the tumor involved in proton radiation therapy. X-ray beams, by contrast, may deposit most of their dose in tissues in front of the tumor.
More than 41,000 patients around the world have been treated through proton cancer therapy. Because of the number of patients treated and the amount of follow-up data available, we can assess the effectiveness of proton beams for treating cancer. In almost every tumor site examined, the higher tumor doses and lower normal-tissue exposure have improved local control and reduced complications as compared with x-ray therapy. When Medicare and National Cancer Institute (NCI) officials reviewed the available data on proton therapy in the early 1990s, they classified proton radiation therapy as "accepted.” In other words, proton therapy is considered a non-experimental treatment for several localized tumors and for intracranial aneurysms.
Loma Linda University and Harvard University are currently performing studies sponsored by the NCI to determine the optimal proton dose for certain cancers, such as prostate cancer and a variety of brain tumors. These studies do not determine if the treatment is effective. Its effectiveness has already been established. The studies are being done to find out the best ways to use this tool against cancer. Similar studies are performed all the time with other standard forms of cancer therapy, such as chemotherapy and surgery.