Proton Therapy FAQs

James Metz, MD, associate professor and vice chair of the clinical division of Penn Radiation Oncology, answers some frequently asked questions about proton therapy.

Q: What is proton therapy?

A: Proton therapy is a kind of external beam radiation in which protons (hydrogen atoms from which the electrons have been

removed) are directed at a tumor. Proton therapy is effective because of its ability to accurately target and kill tumors, both near the surface and deep within the body, while minimizing damage to the surrounding tissues.

Q: How is proton radiation different from that produced by conventional radiation?

A: Protons exist in the nuclei of atoms and have electrons orbiting them, and thus have both a positive charge and a relatively large mass. In contrast, conventional or photon radiation lacks both charge and mass. These differences account for the distinct physical properties of the proton beam. X-rays are produced by linear accelerators that excite electrons; gamma rays originate with the decay of radioactive elements such as cobalt-60. Proton beam radiotherapy is generated by a hydrogen ion source and accelerated in a large machine called a cyclotron. (The cyclotron at the Roberts Proton Therapy Center weighs more than 200 tons.)

Q: How is proton therapy different from other forms of radiation therapy?

A: As a result of the differences in its electromagnetic characteristics, proton therapy offers potential advantages over conventional radiation therapies. Many of these advantages are derived from the ability of radiation oncologists to limit exposure to normal tissue with proton radiation.

X-rays and gamma rays gradually lose energy as they move through the body and the energy deposited tends to be greater at shallow depths and exponentially lower as the beams move through the body. Charged protons can be delivered at specific velocities with the greatest energy loss occurring just before the particle stops. Little or no energy extends beyond the target point. The concentrated energy is released at the tumor site so more energy reaches the cancerous cells, and more damage occurs with each burst of radiation. Side effects caused by the irradiation of normal tissue in front of and behind the tumor are not totally eliminated, but dramatically reduced.

Q: How does proton therapy work?

A: Proton therapy, like all forms of radiation therapy, works by aiming energized particles at a tumor. These particles damage the DNA of the targeted cells, ultimately causing their death. Because cancer cells have a high rate of division, and a reduced ability to repair damaged DNA, they are particularly vulnerable to radiation.

Q: What cancers can proton therapy be used to treat?

A: Proton beam radiation therapy is useful in treating a variety of cancers including:

• Brain and cranial base tumors.
• Tumors of the eye.
• Sarcomas.
• Prostate cancer.
• Spine tumors.
• Thoracic cancers.
• Gastrointestinal cancers.
• Pediatric cancers.
• Head and neck cancers.

Q: Why use protons?

A: The advantages of proton therapy include:

• Same tumor-killing properties as X-rays.
• Decreased dose to normal tissues by 50 to 70 percent.
• Decreased side-effects and complications.
• Decreased toxicity.
• Ability to treat tumors close to critical organs.
• Potential to increase radiation dose administered to the targeted tumor.
• Potential reduction in the number of daily patient treatments.
• Possibility of increased cure rates.
• Potential to re-treat tumors after recurrences.
• Added capacity to treat benign conditions.

Q: Who performs proton therapy?

A: Proton therapy is administered by a team of highly trained specialists including radiation oncologists, radiation physicists, dosimetrists and radiation therapists. A diagnostic radiologist may perform imaging studies to help design the treatment. In addition, radiation therapy nurses help patients in their daily treatments, including management of the side effects.

Q: What are the side effects of proton therapy?

A: It depends on the area of the body being treated. Typically, because much less normal tissue is affected, the side effects of proton therapy are less frequent and less intense than with conventional photon radiation.

Q: How is proton therapy integrated with other therapies (combination therapy)?

A: Proton therapy may be used in conjunction with other treatment modalities, including conventional radiation and chemotherapy. The combination of proton therapy and conventional radiation therapy permits an escalation of dose to the tumor, while minimizing radiation dose to normal tissues.

The combination of radiation therapy with chemotherapy can be difficult due to the side effects seen with some combinations, but protons may allow for the development of more effective and less toxic combination therapies. To improve patient outcomes, Penn hopes to combine chemotherapies with protons in ways that are not possible with conventional radiation.