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Treatment Options > Proton Therapy > Resources

Proton Therapy: The Basics

Carolyn Vachani, MSN, RN, AOCN
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: August 5, 2009

What is radiation therapy?

Radiation therapy uses high energy x-rays to damage the DNA of cells, thereby killing the cancer cells, or at least stopping them from reproducing. Radiation also damages normal cells, but because normal cells are growing more slowly, they are better able to repair this radiation damage than are cancer cells. In order to give normal cells time to heal and to reduce a patient's side effects, radiation treatments are typically given in small daily doses, five days a week, over a 5-7 week period. It is estimated that more than 50% of cancer patients will receive radiation at some point during their treatment.

How is radiation therapy given?

Radiation therapy is considered to be a "local" therapy, meaning it treats a specific localized area of the body. This is in contrast to systemic therapy, such as chemotherapy, which travels throughout the body. There are two main types of radiation therapy: external radiation therapy, where a beam of radiation is directed from outside the body, and internal radiation therapy, also called brachytherapy or implant therapy, where a source of radioactivity is surgically placed inside the body near the tumor.

External radiation may also be called x-ray therapy, cobalt therapy, proton therapy, or intensity modulated radiation therapy (IMRT). This type of radiation is administered using a machine. (Learn more about the steps involved in radiation therapy). Treatment can be given once or twice a day, depending on the treatment protocol being used. Treatments are given 5 days a week for several weeks, depending on the total final dose of radiation that is planned. Patients are given a break from treatment on weekend days to give normal cells some time to heal, thus reducing side effects. A person receiving external radiation therapy is not radioactive or dangerous to the people around him or her.

Internal radiation therapy places the source of the high-energy rays inside the body, as close as possible to the cancer cells. This may be done by implanting "seeds" (small pieces of the radioactive substance) or by using an implanted reservoir, into which a liquid radioactive substance is injected. This delivers very intense radiation to a small area of the body and limits the dose to normal tissue. Internal radiation therapy allows the doctor to give a higher total dose of radiation in a shorter time than is possible with external treatments. The radioactive substances used (also called the "radiation source") typically include radium, cesium, iodine, and phosphorus. Depending on the substance, the implant may be temporary or permanent, although the effect wears off over time in all cases. Depending on the type of radiation source, patients with radiation implants may need to be isolated from visitors for a period of time, so as not to expose others to radioactivity.

What is proton therapy and how is it different?

Radiation therapy given by a linear accelerator, including 3D conformal radiation and IMRT (intensity modulated radiation therapy), uses x-rays as the form of radiation. X-rays are a form of photon radiation and use high-energy rays composed of photons (or "packets of energy") to disrupt the cancer cells. Particle radiation is another type of radiation, which uses subatomic particles, including electrons, neutrons and protons to generate a "particle beam" that kills the cancer cells. Proton therapy is one type of particle therapy.

The main difference between protons and X-rays is the physical properties of the proton beam itself. Protons are large particles with a positive charge that penetrate matter (in this case, tissue) to a limited depth, based on the energy of the beam, and deposit most of their energy at the end of the beam. X-rays are electromagnetic waves that have no mass or charge and are able to penetrate completely through tissue while losing some energy along the way. In turn, x-rays enter the patient on one side of the body and travel straight through, exiting out the other side, with the radiation dose gradually decreasing as it travels through the tissues. In order to decrease the amount of radiation healthy tissues receive, the beam is given from several different angles, allowing the dose to accumulate in the intended target, but be far less to surrounding healthy tissues. This colorful picture shows an IMRT treatment plan for a prostate tumor, which utilizes 6 beams to get the maximum dose (in red) to the prostate, while giving smaller doses (green and blue, which is the lowest dose) to surrounding tissues.

The proton beam is able to enter the body at a fairly low dose of radiation and increase in the last 3mm of the beam to the dose required for treatment. In addition, the beam then stops, resulting in virtually no radiation to the tissue beyond the target- or no "exit dose" as it is called. The ability of the proton dose to increase at a specified area is called the Bragg Peak. The depth of the Bragg Peak in tissue is dependent on the energy of the beam; the higher the energy, the deeper the Bragg Peak and therefore, the deeper the dose. This allows the radiation team to calculate the energy required to position the dose at the depth of the cancer and spare the healthy tissues surrounding it.

This ability to spare healthy tissue is the main difference between x-rays and protons. Research has shown that the biologic effect, or the damage to exposed tissues, is essentially the same for both therapies. This means the therapies will destroy tumor cells in the same manner, but protons should result in less toxicity to healthy tissues. This picture shows a prostate tumor treatment plan with proton therapy, using beams from 2 angles. The red area being the highest treatment dose (around the prostate), followed by yellow, green and blue being the lowest dose.

Example of modulator wheel

The Bragg Peak is just that, a peak, or spike, of energy that may only cover a few millimeters of tissue. Most tumors would not be confined to this peak, so a special wheel, called a modulator, is used to spread out the peak to the width of the target. This diagram shows the doses achieved by various radiation methods. You can see the protons (orange line) depicting the Bragg Peak, with the dose dropping off after the peak. The blue line represents the spread out proton beam (SOBP). You can see how the dose is "molded" around the tumor and how the dose drops off right after the target, sparing the tissues beyond it.

Examples of various different relative doses with respect to depth for a variety of photon energies, protons and carbon icons

The pictures below are an example of using proton therapy to treat a pediatric spinal cord tumor. The first image below is a treatment plan using x-rays, the second image is using proton therapy and the last is a side-to-side comparison, looking through the body from head to feet. The colors represent doses of radiation to that tissue, with purple being the most and green being the least.