Abstract

Ion beam therapy is a rapidly emerging treatment modality that makes use of the favorable properties of ion interaction with tissue. When ions are slowed down in matter, they transfer most of their energy in Coulomb interactions with the atomic electrons, and the energy transfer rate is relatively low at the initial therapeutic beam energies of 50–230 MeV for protons and 90–430 MeV/u for ¹²C ions, but rapidly increases toward the end of the beam penetration depth (Schardt et al., 2010; Newhauser and Zhang, 2015). This energy loss process is responsible for the main feature of the ion depth–dose deposition, exhibiting a pronounced sharp maximum, the Bragg peak, at an energy-dependent depth (Wilson, 1946). Additional processes that influence the exact shape of the three-dimensional (3D) dose distribution of an ion beam are (1) energy loss straggling, affecting the longitudinal Bragg peak width; (2) multiple Coulomb scattering, affecting the lateral beam spread; and (3) nuclear interactions, which modify the particle energy fluence spectrum. Energy straggling and multiple Coulomb scattering are especially responsible for the broader Bragg peak and marked lateral spread of proton beams compared to carbon ions at the same penetration depth (Figure 23.1). Nuclear reactions prevent primary particles from reaching the Bragg peak and are especially responsible for the complex mixed radiation field in the case of carbon ion beams, where secondary projectile fragments of reduced charge give rise to a characteristic dose tail distal to the Bragg peak (Figure 23.2). More information about the interaction of light ions with matter can be found in Chapter 12.