The Bragg Peak

The Bragg Peak serves as the foundation for proton therapy.

Proton beam therapy delivers a high dose of radiation to a very localized site. Protons, unlike x-rays, decelerate faster than photons. They deposit more energy as they slow down, culminating in a peak – known as the Bragg peak.

EXIT DOSE MINIMIZATION

The Bragg peak phenomenon is exploited in the particle therapy treatment of cancer, to concentrate the effect of light ion beams on the tumor being treated, while also minimizing the effect on the surrounding healthy tissue. Traditional radiation treatment uses photons to target tumors. Photons deposit energy along the path of the X-ray beam, delivering radiation to the tumor as well as adjacent normal tissue. The exposure of radiation beyond the target is called the exit dose. The exit dose can result in damage to nearby organs and normal tissues and can cause future health issues.

Figure 1: Traditional radiation treatment has a relatively high entrance dose and exit dose.

Proton therapy is different because of the Bragg peak phenomenon. Unlike photons, protons deposit the majority of their energy at a single point, the Bragg peak. A proton beam can be conformed to the shape and depth of the tumor, eliminating the exit dose and sparing surrounding organs and normal tissue from radiation exposure. Because of this, patients can receive higher doses, increasing the effectiveness of treatment. This allows radiation oncologists to treat patients with higher doses, increasing the effectiveness of treatment.

Figure 2: Because of the Bragg peak phenomenon, the exit dose is eliminated, sparing normal tissue from radiation exposure.

With traditional treatment by x-ray radiation, energy is greatest where it first encounters tissue. It then dissipates as it penetrates the tissue. The skin and any tissue in between the source of radiation and the tumor will get higher doses than the tumor itself. This only increases collateral damage and the chances of dangerous mutations in healthy cells.

PROTON PRECISION

In proton radiation, however, the amount of energy exerted from the ionizing radiation starts relatively low as it enters the tissue of the patient. The amount of energy imparted to surrounding tissue by a proton increases along an exponential curve right up until it reaches its target, where the energy imparted peaks before plummeting to zero.

Because proton beams are much narrower, they allow treatment of tumors in a much more precise manner. This allows doctors to control the location of the Bragg peak and, therefore, the deposition of radiation. As a proton enters the tissue, it is moving extremely fast, so fast that it barely interacts with the atoms of the surrounding tissue. As it interacts, it loses energy and slows down. As it slows down, it interacts with more atoms, and imparts more energy until the proton runs out of energy and comes to a stop.

Figure 3: The dose produced by a native proton beam, and by a modified proton beam passing through tissue, compared to the absorption of a photon or x-ray beam.

It may seem counterintuitive that protons, unlike x-rays, do less damage when they’re entering tissues at high speed and then do their greatest when they’ve slowed at their target. This, however, is the nature of quantum mechanics. Everything is different at the quantum scale. In view of this, we can see the physics that enables an option for an improved form of radiation therapy; one that kills cancerous cells but with reduced collateral damage to the healthy surrounding tissue.

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