Particles/radioactivity: Particle accelerators for proton beam therapy

Proton beam therapy (PBT) is used to treat cancers in sensitive areas such as the eyes or brain. High energy protons are produced by a particle accelerator and steered toward the patient’s tumor by a series of magnets. Once in the patient’s body, 80% of the protons only interact with electrons, not nuclei. Protons lose energy by ionizing atoms in those interactions. Those protons will slow slightly in the skin and other “shallow” tissues without significantly deviating from a straight path. Once slowed, protons rapidly lose their remaining energy over a short distance as they capture an electron and decelerate to thermal speeds. This is sometimes called a “depth charge” effect. How deep the proton penetrates before releasing this burst of energy depends on its initial speed and the composition of the tissue. By adjusting how much energy protons have when they enter the patient, physicians can target where the proton will most rapidly ionize tissue atoms. This gives doctors some control over how much energy gets deposited in the shallower tissue (the “skin dose”), and all but eliminates irradiation of deeper tissues (the “shadow dose”). This pattern of energy loss is illustrated by the proton’s narrow “Bragg peak” (Figure 1). Combining beams with different energies allows the new beam to deposit energy with a plateau-like profile that can damage a tumor across its full depth. This combined dose forms a Spread Out Bragg Peak (SOBP; see Figure 1’s combined dose curve).

Figure 1. Percent therapeutic energy dose (dose necessary to kill tumor cells) deposited in tissue by proton beams by depth. Curve for a single proton energy is a normal Bragg Peak. The Combined dose is (offset from the 250MeV dose at the right end for illustration only) is an SOBP.

Adapted from Aamiller, Creative Commons. CC by SA 3.0