A type of radiation therapy that uses streams of protons (tiny particles with a positive charge) that come from a special machine. This type of radiation kills tumor cells but does not damage nearby tissues. It is used to treat cancers in the head and neck and in organs such as the brain, eye, lung, spine, and prostate. Proton beam radiation is different from x-ray radiation.

There is a significant difference between standard (x-ray) radiation treatment and proton therapy. If given in sufficient doses, x-ray radiation techniques will control many cancers. But, because of the physician’s inability to adequately conform the irradiation pattern to the cancer, healthy tissues may receive a similar dose and can be damaged. Consequently, a less- than-desired dose is frequently used to reduce damage to healthy tissues and avoid unwanted side effects. The power of protons is that higher doses of radiation can be used to control and manage cancer while significantly reducing damage to healthy tissue and vital organs.

Understanding how protons work provides patients and physicians with an insight into this mainstream treatment modality. Essentially, protons are a superior form of radiation therapy. Fundamentally, all tissues are made up of molecules with atoms as their building blocks. In the center of every atom is the nucleus. Orbiting the nucleus of the atom are negatively charged electrons.

When energized charged particles, such as protons or other forms of radiation, pass near orbiting electrons, the positive charge of the protons attracts the negatively charged electrons, pulling them out of their orbits. This is called ionization; it changes the characteristics of the atom and consequentially the character of the molecule within which the atom resides. This crucial change is the basis for the beneficial aspects of all forms of radiation therapy. Because of ionization, the radiation damages molecules within the cells, especially the DNA or genetic material. Damaging the DNA destroys specific cell functions, particularly the ability to divide or proliferate. Enzymes develop with the cells and attempt to rebuild the injured areas of the DNA; however, if damage from the radiation is too extensive, the enzymes fail to adequately repair the injury. While both normal and cancerous cells go through this repair process, a cancer cell’s ability to repair molecular injury is frequently inferior. As a result, cancer cells sustain more permanent damage and subsequent cell death than occurs in the normal cell population. This permits selective destruction of bad cells growing among good cells.

Both standard 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 energy distribution of protons can be directed and deposited in tissue volumes designated by the physicians-in a three-dimensional pattern from each beam used. This capability provides greater control and precision and, therefore, superior management of treatment. Radiation therapy requires that conventional x-rays be delivered into the body in total doses sufficient to assure that enough ionization events occur to damage all the cancer cells. The conventional x-rays lack of charge and mass, however, results in most of their energy from a single conventional x-ray beam being deposited in normal tissues near the body’s surface, as well as undesirable energy deposition beyond the cancer site. 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. Thus, maximum energy is released within the designated cancer volume. The surrounding healthy cells receive significantly less injury than the cells in the designated volume.

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. The overall affects lead to the potential for fewer harmful side effects, more direct impact on the tumor, and increased tumor control.”

The patient feels nothing during treatment. The minimized normal-tissue injury results in the potential for fewer effects following treatment, such as nausea, vomiting, or diarrhea. The patients experiences a better quality of life during and after proton treatment.

*Source: National Cancer Institute

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