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Proton therapy aims for more effect, fewer risks

HOUSTON — When a person picks proton therapy at MD Anderson Cancer Center, he or she is seen by a team of health providers from multiple specialties.

They analyze the approaches for treating a cancerous tumor and reach a consensus.

A CT scan simulates the position a patient will need to lie in for treatment. The image is sent to a dosimetrist, a person who determines how to deliver the prescribed dose. The proton beam information is entered into a computer and used to determine from which direction protons should shoot toward the tumor, said Dr. James Cox.

The tumor's shape is delineated by the physician. The treatment plan is reviewed repeatedly by specialists.

Proton beam radiation therapy uses protons, which are different from the photons used with traditional X-ray radiation. Photons deliver radiation to the tumor, but also can damage nearby organs as they enter and leave the body.


Proton particles can be likened to a baseball, said Dr. Andrew Lee, director of the Proton Therapy Center at MD Anderson Cancer Center. A baseball at rest won't have much of an effect on your body. But pitcher Randy Johnson's 95-mph throw killed a bird that got in the way. Similarly, protons can deliver significant energy when accelerated like a pitched baseball.

With proton therapy, Lee said, you can control how far the "baseball" goes into the body before releasing the bulk of its accelerated energy into the tumor target. That's why proton therapy is so attractive to patients who want to destroy cancerous tissues but minimize harm to surrounding organs.

One option with traditional radiation is to use a "radiation band" by shooting X-rays from, say, eight angles that intersect at a point in the tumor. The goal is to affect healthy tissue with only small doses while compounding the effect when all the doses converge at the site of the tumor.

But that means more parts of the body are exposed to X-rays, and the side effects that come with it. It's important to keep the dose low, or as close to zero as possible, to surrounding tissues.

"The primary goal," Lee said, "is to treat the tumor and usually some area around the tumor." Targeted pencil beam therapy, in particular, can be useful for treating children, he said.

Parents must consider side effects from traditional radiation treatments, such as risks to a child's future fertility, the possibility of treatment-induced secondary tumors triggered by therapy, heart concerns, radiation-induced nausea and thyroid problems.

Pencil beam therapy means the heart, for example, gets no stray radiation that normally occurs with X-ray radiation.

Protons are accelerated the way you might remember getting spun by another child on a merry-go-round as a kid. They might travel the equivalent of three times around the surface of the planet, Lee said. That accelerates the protons so that they release the greatest energy upon impact.


"It's only going to be effective if you can get it going fast, and the deeper the tumor the faster you have to get this going," Lee said.

A patient's treatment might last six to eight weeks, he said. At MD Anderson, patients get to ring a gong afterward — symbolically celebrating the completion of that part of their cancer journey.

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