粒子線治療装置と技術開発の現状：陽子線の線量分布

Current status of proton therapy：physical properties of proton therapy beams focusing on depth dose and radiation quality

現在治療に用いられている粒子線には陽子線と炭素線とがあり，いずれの粒子線も体内のある深さで線量が急激に高くなり，その部分でピタッと止まるという特徴を有している。

This chapter focuses on the physical properties of proton beams. Proton beams are physically superior to X-ray beams in terms of their “range” and “Bragg-peak,” which are unique to charged particle beams. Spread-out Bragg peak（SOBP）is an application of these superior physical characteristics to improve dose concentration and uniformity to the tumor. The significance of SOBP is explained in the first half of this chapter. In the latter part, we present an example of the application of Monte Carlo calculations to proton beams. The Monte Carlo calculations enable not only the evaluation of absorbed doses, but also various other evaluations, such as the beam quality of proton beams and the secondary radiation produced by the proton beams, which are difficult to evaluate with general-purpose treatment planning systems. These evaluations are expected to create new value in the future, as they may be applied to the adaptation of spatial distributions optimized for dose and beam quality as well as to the detailed analysis of patient exposure doses.