Collection efficiency of a monitor parallel plate ionization chamber for pencil beam scanning proton therapy

The collection efficiency of monitor parallel plate ionization chambers is the main uncertainty in the beam control of pencil beam scanning systems. Existing calculation methods for collection efficiency in photon or passive scattering proton systems have not considered the characteristics of non-uniform charge density in pencil beam scanning systems. In this study, Boag’s theory was applied to a proton pencil beam scanning system. The transverse distribution of charge density in the ionization chamber was considered to be a Gaussian function and an analytical solution was derived to calculate collection efficiency in the beam spot area. This calculation method is called the integral method and it was used to investigate the effects of beam parameters on collection efficiency. It was determined that collection efficiency is positively correlated with applied voltage, beam size, and beam energy, but negatively correlated with beam current intensity. Additionally, it was confirmed that collection efficiency is improved when the air filling the monitor parallel plate ionization chamber is replaced with nitrogen.