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DEAR MAYO CLINIC: What's the difference between standard radiation therapy and proton beam therapy? How do doctors decide which one to use?

ANSWER: Both standard radiation therapy and proton beam therapy can be used to treat tumors. A key difference is that standard radiation passes through a person's body. In contrast, proton beam therapy delivers radiation to the tumor, where it stops. Because the proton beams stop, less healthy tissue is exposed to radiation, and the risk of side effects is lower.

Protons are a component of all matter. Atoms have protons and neutrons together in the central nucleus with electrons surrounding them. Radiation is the energy released from atoms as either a wave, such as an X-ray, or a tiny particle of matter, such as a proton. Radiation therapy can be used to kill cancerous and noncancerous tumors.

Standard radiation therapy uses X-rays. Because X-rays have no mass, they cannot be stopped from traveling all the way through the body. Proton beam therapy directs protons into a tumor. Because protons have mass, how far they travel and where they are placed can be controlled.

For example, a person who has a tumor near the back of one lung and is treated with standard radiation therapy may receive a dose through the back. As the X-ray leaves the body, the rest of the lung, the heart and other tissues in front of the tumor also receive radiation. With proton beam therapy, protons are directed inward from the back and stop in the tumor, delivering radiation to the tumor but none to organs or tissue in front of it.

Because much of the healthy tissue around the tumor does not receive radiation with proton beam therapy, side effects are less likely than with standard radiation therapy. In addition, because radiation can be more precisely controlled using proton beam therapy, the amount of radiation delivered to the tumor can often be safely raised, potentially increasing the treatment's effectiveness and possibly decreasing the number of treatments needed.

Proton beam therapy can be used for many kinds of tumors. It is particularly useful in sites near critical organs or for tumors located deep within the body, when concern about damage to healthy organs and tissue may require that the standard radiation treatment dose be decreased. In particular, proton beam therapy is often a reasonable option for brain, esophageal and lung cancers.

Proton beam therapy can also be a good choice for children. With radiation therapy, there is an increased risk that children will develop another tumor sometime during their lifetime due to the radiation. Proton beam therapy lowers that risk because the child's body is exposed to less radiation than that from standard radiation therapy. Proton beam therapy also lowers the risk of damage to children's brain cells, decreasing the potential for long-term memory and thinking problems.

Proton beam therapy, although applicable in a variety of situations, is unlikely to completely replace standard radiation therapy. Standard radiation is typically a better choice for skin cancers or other surface-level tumors. In addition, proton beam therapy is not a treatment option for cancer that has spread, or metastasized, to other parts of the body through the bloodstream. Standard radiation is useful and effective at decreasing symptoms related to metastatic cancer.

The expense of the large equipment required for proton beam therapy also makes it unlikely that this therapy will be widely available in the near future. Finally, although this therapy appears to hold promise for treating a wide array of tumors, additional research is ongoing to determine the most effective way to use proton beam therapy. - Steven Schild, M.D., Radiation Oncology, Mayo Clinic, Scottsdale, Ariz.

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