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Treatment Comparison: What Makes Proton Therapy Superior?

Unlike other types of radiation therapy that use x-rays to destroy cancer cells, proton therapy uses a beam of special particles called protons. Doctors can better aim proton beams onto a tumor, so there is less damage to the surrounding healthy tissue. This allows doctors to use a higher dose of radiation with proton therapy than they can use with x-rays.

See the chart below for a side-by-side comparison of traditional cancer treatments versus proton treatment.

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Both conventional x-ray therapy and proton beams work on the principle of selective cell destruction. The major advantage of proton treatment over conventional radiation, however, is that the characteristic energy distribution of protons can be deposited in three-dimensional tissue volumes designated by the physician. This capability provides greater control and, therefore, superior management of treatment.

Conventional radiation therapy requires that photons--X rays--be delivered into the body in total doses sufficient to ensure that enough ionization events occur to damage all the cancer cells. Became photons lack charge and mass, most of the energy from a single photon beam is deposited in normal tissue near the body's surface, and some undesirable energy is deposited beyond the cancer volume. This undesirable pattern of energy placement can result in unnecessary damage to healthy tissues, often preventing physicians from using sufficient radiation to control the cancer.

Protons, on the other hand, are energized to specific velocities. These energies determine how deeply in the body protons will deposit their maximum energy. As the protons move through the body, they slow down, causing increased interaction with orbiting electrons. Maximum interaction with electrons occurs as the protons approach their targeted stopping point. Maximum energy is released within the designated cancer volume; surrounding healthy cells receive significantly less injury than the cells in the designated volume. This point, where the high-dose region of energy release occurs, is called the Bragg peak.

Protons' favorable absorption characteristics result from their charge and heavy mass, which is 1,835 times that of an electron. These factors allow the physician to predict and control their depth of travel within the patient. The protons' energy upon entering the patient and the tissue density along their track determine the depth of penetration of the beam, and thus, the placement of the Bragg peak. This peak can be enlarged to conform to the thickness of the designated volume along the beam axis. The heavy mass also results in minimal deviation and, therefore, minimal side-scatter--a significant factor in reducing unwanted side effects and maximizing treatment benefit.

Photons (standard X rays) and electrons lose most of their energy near the body's surface and exponentially deposit energy as they travel through tissue. Electrons, because of their low mass, are deflected easily from their initial direction and produce significant secondary lateral scatter. When photons or electrons are used for treating patients, healthy tissues surrounding the tumor target frequently receive a dose as high as that delivered to the designated tumor volume. Attempting to circumvent these problems, radiation oncologists often employ multi-field arrangements to build up the tumor dose and spare as much of the normal tissue as possible by restricting the dose in those tissues to a level they can tolerate.

Multi-field arrangements can be used with protons. When they are, the dose to normal tissues is reduced even further, thereby minimizing normal-tissue effects. When several proton fields are used, the dose in the overlapped beams is further increased relative to normal tissue, permitting more effective doses to be delivered to the designated volume than can be achieved with X rays.

As a result of protons' dose-distribution characteristics, the radiation oncologist can increase the dose to the tumor while reducing the dose to surrounding normal tissues. This allows the dose to be increased beyond that which less-conformal radiation will allow, while sparing healthy tissue and organs, permitting more direct impact on the tumor, and increasing the likelihood of tumor control. The patient feels nothing during treatment. Following treatment, the minimized normal-tissue injury usually results in fewer and milder side effects, such as nausea, vomiting, or diarrhea. This results in a better quality of life during and after proton treatment.