Hybrid dose calculation with a pencil beam algorithm and a GEANT4 simulation for hadron therapy application

Monte Carlo method has been widely approved to the way of precise dose calculation. The GEANT4 Monte Carlo has been also demonstrated its wide applicability in medical physics. In our previous work, we had developed a GEANT4 based simulation framework for particle therapy system, PTSSim, for composing an irradiation system in various radiation therapy facilities. The performance study had been also carried out for voxel geometry of a patient volume which uses numerous number of voxles. However, calculation time for obtaining statistically reliable dose distribution was critical for the use of treatment planning that requires completing dose calculation within five minutes. In order to improve the calculation efficiency, a hybrid dose calculation method using a Monte Carlo method adapting a pencil beam algorithm has been proposed and introduced in PTSSim for proton therapy application. The calculation method is switched from Monte Carlo method to the pencil beam algorithm at surface of patient volume depending on particle’s phase-space. The hybrid dose calculation successfully reduced the statistical fluctuations in dose distribution while reproducing the dose distribution in pure Monte Carlo method.