Dose impact of a brass aperture on low dose tails for a proton wobbling nozzle with a multileaf collimator — A Monte Carlo study

At the Chang Gung Memorial Hospital, a wobbled beam together with a set of single scatterers produces a uniform proton field at the first and second gantries for proton radiotherapy. Conformal field shaping was accomplished by using only a multileaf collimator (MLC) or a combination of an MLC and a brass aperture (downstream of the MLC). The MLC-only technique typically delivered extra low dose tails in penumbra compared to the dose delivered using both the MLC and brass aperture/block. This study attempted to provide a physical explanation of this phenomenon by Monte Carlo simulation and phase space analysis. The Particle Therapy System Simulation Framework was used as the Monte Carlo tool for beam propagation simulation. This research team investigated four possible causes for low dose tails including, (a) same-side MLC scattering; (b) opposite-side MLC scattering; (c) MLC transmission; and (d) proton angular distribution broadened by the wobbling magnets. The results indicated that the proton angular distribution broadened by the wobbling magnets is the single cause of low dose tails and can be stopped by inserting a downstream brass aperture.