Validation and clinical implementation of a full Monte Carlo code for scanned proton pencil beams

Abstract

We present a universal method to model a proton PBS dedicated nozzle by using acceptance and commissioning measurements, together with a full Monte Carlo (MC) code (Topas/Geant4), to address both the halo inherent from the nozzle as well as a simplified implementation of a range shifter . A double Gaussian source spot profile model was implemented to better address the halo due to interaction of protons with components in the nozzle. The phase space parameters (including beam size, angular divergence and energy spread) and protons per MU were extracted and tuned without simulating any components of the nozzle by comparing Topas simulation with a series of commissioning measurements using scintillation screen/CCD camera detector and ionization chambers. The range shifter was simulated as an independent object. The beam model was validated by comprehensive measurements of the size of single spots, field size factors (FSF) and three dimensional dose distributions of Spread Out Bragg Peaks (SOBPs) both without and with the range shifter. Figure 1 shows FSF along beam path in air and in water after the range shifter for energies of 115 and 225 MeV. The excellent agreement between a TOPAS and measurement reflects high accuracy of Topas in halo modeling. To faciltate assessment of clinical treatment plans, the source model was directly implemented into a second fast, PBS dedicated MC code, MCsquare. The difference between FSF of 200x200mm and FSF of 40x40mm can be as large as 15% at air gap 195mm after the range shifter, which indicates that comprehensive modeling of the spot profile is mandatory for TPS commissioning of range shifter. Figure 2 shows a representative head-and-neck case calculated using TOPAS, MCsquare and a commercial treatment planning system (Eclipse 13.7). In conclusion, two different MC codes have been implemented with universal commissioning method for treatment quality assurance