Propagation of Swift Protons in Liquid Water and Generation of Secondary Electrons in Biomaterials

Abstract

A proper description of the propagation of a swift proton beam through biomaterials, accounting for the energy deposited as well as the geometrical evolution of the beam as a function of the target depth and nature, is a crucial issue in proton therapy. For this purpose, simulation is a very adequate tool, since the most relevant interactions that take place between the projectile and the target constituents (electrons and nuclei) can be conveniently accounted for in a controlled manner. In this chapter an overview and relevant results for hadron therapy are presented of the simulations we have developed using the code SEICS (Simulation of Energetic Ions and Clusters through Solids), which combines Monte Carlo and Molecular Dynamics, to follow in detail the motion and energy deposition of swift protons through targets of hadron therapeutic interest, mainly liquid water. The main interactions considered in our study are of elastic nature (affecting mainly the projectile’s direction) and inelastic processes (leading to either nuclear reactions or electronic energy loss). The performance of the code, as well as the quality of its main input, namely the stopping force for proton beams in liquid water (which is the main tissue constituent), are benchmarked by comparing the results of the simulations with available experimental proton energy spectra as a function of the detection angle after traversing a micrometric liquid water jet. The excellent agreement with experiments validates the SEICS code, which we can use then to study several problems of interest for proton therapy, including the calculation of depth-dose curves and lateral dose profiles, the energy evolution of the proton beam along the target, as well as the production of secondary electrons at the Bragg peak in relevant biomaterials.