What is proton beam therapy?

Cancers treated: Primarily cancers of the prostate, lung, head, neck, and 0

Why performed: Proton beam therapy, like traditional radiation therapy, is performed to kill cancer cells. It may be performed as a primary treatment for a tumor, to kill any cancer cells that remain after surgical cancer treatment, or in addition to other cancer treatment options. Proton beam therapy is only offered at a few locations in the United States, and for many cancers it is still considered an experimental treatment. In fact, studies have indicated that its effectiveness is debatable.

Proton beam therapy is a site-specific therapy, so it is designed to treat cancers that have not spread to large areas or throughout the body. It is mainly used to treat cancers occurring in the prostate, lung, head, neck, and brain, although it has been tested in clinical trials for use on cancers occurring in many other areas as well.

Patient preparation: The patient should discuss with the cancer care team any necessary preparation for the specific procedure that he or she is undergoing. Necessary preparation may vary depending on the type of cancer being treated and the patient’s previous response to any radiation therapy.

Steps of the procedure: Proton beam therapy uses high-speed protons to kill cancer cells, instead of the electrons used by most radiation therapy techniques. Atoms are made up of a nucleus of protons and neutrons surrounded by orbiting electrons. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. When free protons come very close to an atom, the electrons orbiting the nucleus are attracted to the positive charge of the protons. The electrons are then pulled out of their orbits. This is called ionization of the atom. Atoms that have been ionized are not as stable as normal atoms. This change to the atom means that changes also occur to the molecule of which the atom is part, and eventually to the cell of which the molecule is part. If the cell cannot repair the damage caused, then it eventually dies.

Proton beam therapy begins with protons traveling around a synchrotron, a machine that makes the protons go very fast and energizes them. The protons are then sent through vacuum tubes to the machine that actually aims them at the area of the patient that will receive the radiation. The patient is positioned and held still so that the proton beam can be aimed as accurately as possible. Complex computer technology helps the doctors and technicians aim the proton beam very accurately so that it hits as little healthy tissue as possible.

The protons are released in a directed stream toward the cancer cells. The protons are traveling very quickly at first, but they slow down as they get closer to the cancer. When they are traveling fast, they do not have a very strong effect on the atoms they are passing. When they are slower, however, they have an extremely strong effect. This is one reason that proton beam therapy causes less damage to healthy cells than does traditional radiation therapy. Traditional radiation therapy uses x-rays, which strongly affect all the cells with which they come into contact, which makes it hard to deliver enough radiation to the cancer cells without also killing healthy cells. With proton beam therapy, doctors aided by computers can determine the right way to release the protons so that they have the maximum impact just as they come into contact with the cancer cells. Proton beam therapy may need to be repeated one or more times depending on the size and type of the cancer and other factors.

After the procedure: After proton beam therapy, many individuals experience no negative side effects. Some individuals, however, may experience pain, fatigue, nausea, or diarrhea.

Risks: The risks associated with proton beam therapy are believed to be somewhat lower than those associated with traditional radiation therapy for most people. This is the case because proton beam therapy causes less damage to surrounding healthy cells, so healthy tissue is less likely to be significantly damaged. Some of the risks of proton beam therapy include nausea, diarrhea, and fatigue. Damage to healthy tissue is still a possible risk of proton beam therapy.

Results: The goal of proton beam therapy is generally to destroy a tumor, to reduce the size of a tumor, to reduce related symptoms, or to kill any residual cancer cells left after a tumor has been surgically removed. Success rates for proton beam therapy can vary drastically depending on the type of cancer, its size, and how far it has spread. It is generally found to be successful at reducing the side effects usually associated with traditional radiation therapy. If the procedure is done to completely destroy a tumor or residual cancer, then the procedure is generally considered to have been successful if the cancer does not return for five years or more. If the procedure was done to reduce tumor size, then it is considered successful if quality of life is improved. However, not enough studies have been done to determine the true effectiveness of the procedure.

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