MCNPX Simulation Study for Array-type Prompt Gamma Detection System for Real-time Distal Dose Edge Determination in Proton Therapy

Abstract

Real-time determination of distal dose edge is important for the safety of patients in proton therapy. In the present study, the MCNPX code was used to determine the optimal design of an array-type prompt gamma detection system that can determine the location of the distal dose edge in the patient during proton therapy. The array-type prompt gamma detection system will be constructed with an array of CsI(Tl) scintillation detectors and a multi-slit tungsten collimation system. Our simulation results show that 2-3 millimeters of CsI(Tl) scintillator thickness is enough to absorb more than 80-90% of the initial kinetic energies of the secondary electrons produced by prompt gammas. The optimal dimensions of the multi-slit collimation system was determined by observing the variation of the prompt and background gamma distributions at the CsI(Tl) scintillation detectors for different dimensions of the collimation system. Finally, the performance of the array-type detection system, with the optimal design, was predicted by Monte Carlo simulations for 80, 150, and 220 MeV proton beams. The results were encouraging; that is, a good agreement was found between the location of the distal dose edge and the distribution of the prompt gamma counts at the CsI(Tl) scintillation detectors for 80 and 150 MeV proton beams. For the 220 MeV proton beam, the distribution of the prompt gammas changes only slightly at the location of the distal dose edge, which requires additional advanced detection techniques during measurement for accurate determination of distal dose edge.