What Is Proton Beam Therapy?

The characteristics of proton beam therapy enable physicians to deliver higher, more conformed doses to tumor volume while almost completely sparing normal healthy tissue.
Protons are hydrogen atoms whose electrons have been removed. Proton beam therapy uses a special machine called a cyclotron or synchrotron to energize protons. Protons are extracted from the cyclotron or synchrotron and directed with magnetic fields to the tumor. The depth of penetration of the protons is related to their energy and can be precisely controlled to match the location of the tumor.

Protons deliver the majority of their energy at a very narrow area within the body.  This unique dose delivery property of protons is known as the Bragg Peak. We can manipulate the Bragg Peak area to deliver the desired radiation dose to the tumor itself without any exit dose beyond the tumor.  Conventional external beam radiation therapy uses photons or x-rays that enter and exit through the body. The special properties of protons generally reduce the radiation dose to the uninvolved normal tissues surrounding the tumor.

Principles of Proton Beam Therapy

Irregularly shaped lesions located near critical structures, tumors in children, and large tumors near any critical organ are well suited for proton beam therapy. Protons have a physical advantage over gamma rays and x-rays when it comes to sparing normal tissues. Protons deposit most of their radiation energy in what is known as the Bragg Peak, which occurs at the point of greatest penetration of the protons in tissue. The exact depth to which protons penetrate, and at which the Bragg Peak occurs, is dependent on the energy or modulation of the proton beam. This energy can be very precisely controlled to place the Bragg Peak within a tumor or other tissues that are targeted to receive the radiation dose. Because the protons are absorbed at this point, normal tissues beyond the target receive very little or no radiation. Proton energy can be adjusted to match the depth of the target with a sharp drop in dose beyond the Bragg Peak.

Tumors can have very irregular shapes and can be located close to critical organs. Every patient’s tumor shape, size and location are unique. Patient specific hardware, which helps sculpt the proton beam, is customized to maximize the dose to the tumor while minimizing the dose to normal structures. The shaping of the proton beam can also be controlled by magnetically scanning across the tumor volume. Aiming proton beams, each with customized field shaping, from various directions further ensures that the dose to normal tissues is reduced as much as possible, therefore reducing the risk of treatment related complications.

Customized dose-shaping devices used for proton beam therapy

Pictured Above: Customized dose-shaping devices used for proton beam therapy. A brass aperture shaped to the outline of a target blocks the proton beam outside a specified safety margin. The penetration depth of the protons that pass through the aperture opening is adjusted to match the shape of the target with a Lucite range compensator. A target is depicted in red in the rightmost figure with the proton radiation dose conforming to its shape and avoiding a critical structure shown in green.