Estimation of influence of material assignment in CT data for hadron therapy using GEANT4

The influence of material assignment in a patient geometry for hadron therapy had been studied using GEANT4 simulation. For the purpose of evaluating the influence of material differences, the dose distributions by proton and carbon beams were compared in a homogeneous medium for water and tissue materials, where all mass densities were normalized to 1 g/cm³. The differences were observed in the depth of Bragg peak positions. For clinical treatments, the dose distributions in CT data of sample patients were studied with actual treatment parameters. The tissue materials were assigned by dividing CT space into material groups. Within the group, the mass density was varied with the CT value using a look up table in a treatment planning system. The dose distributions in the sample patients were consistent each other between material assignments using inhomogeneous water and heterogeneous medium of tissue materials.