Track Structure and Microdosimetry of Proton Beams

Abstract

Energy is deposited in matter by ionizing radiations through atomic interactions such as ionization and excitation. Over the past several decades, studies have investigated the microscopic distribution of such energy deposition events on the trajectory of ionizing radiations, known as track structure, and the influence on the relative biological effectiveness (RBE). Specifically, these are microdosimetry studies, which are generally distinguished from conventional dosimetry due to the need to consider the stochastic nature of energy depositions. Thus, two stochastic quantities specially used in microdosimetry (i.e., specific and lineal energies) were defined and measured. These were used instead of the corresponding nonstochastic quantities generally utilized in conventional dosimetry (i.e., absorbed dose and linear energy transfer (LET)). The use of such microdosimetric quantities as index for expressing the radiation fields has brought to the development of numerous models for RBE estimation of charged particle therapy. Of note, a subset of these has already been implemented in carbon-ion therapy’s treatment planning. In this chapter, we review the definitions of microdosimetric quantities. For discussion, we present examples of calculated track structures and microdosimetric quantities of several monoenergetic radiations. Additionally, we briefly review and summarize past studies on the applications of microdosimetry in proton therapy.