Proton beam therapy physics

Contrary to popular opinion, the concept of treating cancer using high energy beams of protons is not new. First conceived in 1946, following a period in which patients were treated only in laboratory facilities, the first hospital-based facility opened in 1990 at Loma Linda University Medical Center (United States). Currently there are 67 centres in operation, with a further 32 in the construction or planning stages. This rapid expansion has been driven by the potential clinical advantages of proton beam therapy (PBT) over conventional photon radiotherapy (RT) due to the interaction characteristics of protons. Being positively charged particles with mass, protons lose energy as they traverse patient anatomy, slowing down and becoming more densely ionising as they approach their endof-range, at which point they stop. This results in a distribution with a low entrance dose increasing to a maximum, the Bragg peak, beyond which no further dose is deposited. By comparison, photons continue depositing dose at depths beyond that of the target (figure 1). The depth of the Bragg peak is determined by the initial energy of the proton beam, which is selected to deposit the maximum dose at the position of the target. A therapeutic dose can be realised with reduced dose to surrounding healthy tissue compared to RT (figure 2) resulting in the potential for reduced acute and late toxicities, reduced secondary cancer risk and an improvement in patients’ quality of life.