ABSTRACT
According to Chu et al. (1993) and later agreed upon by the International Commission on Radiation Units and Measurements (ICRU) (Wambersie et al. 2004) light ions are ion species with an atomic number less than or equal to 10 (neon). Most patients treated with light ions for stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT) to date have been irradiated with either hydrogen-1 ions (protons) or helium-4 ions. Recently, there have been several clinical trials to study reducing the number of treatment fractions to one to five for some large-field radiotherapy treatments using carbon-12 ions; these programs will not be specifically discussed in this chapter, but the techniques that are described here are applicable to all light ions. There are two primary dosimetric characteristics that suggest the use of light ion beams for SRS/SRT. The first is that the large mass of the ions minimizes scattering as they penetrate a patient, resulting in a narrow lateral penumbra width. The second is that, unlike x-ray beams with which the dose decreases with depth because the x-rays are attenuated exponentially as they pass through tissue, the dose deposited by light ions increases with depth because the ions lose energy and slow down, thereby transferring energy to tissue at an even greater rate. This energy deposition peaks just before the ions lose all their energy and stop, thereby creating a peak in the depth dose distribution.
