Quantitative and qualitative evaluation of density resolution of pCT images using Cirs062m phantom simulation in Geant4

Abstract

Accuracy in the treatment planning (TP) of proton therapy (PT) depends on the precision of the information employed to calculate the relative stopping power (RSP) of the tissues in the patient's body. This information is obtained from the x-ray computed tomography (xCT) images and the calibration curve needed to convert the Hounsfield unit to RSP values. Using xCT bears errors in proton range estimation and dose calculation of TP. Proton computed tomography (pCT) can decrease the inherent uncertainty in PT by directly measuring RSP. Though by using pCT in the integral mode of energy loss, this error is removed, and the RSP map of tissues is calculated directly. This study simulated a modern pCT imaging system with particle-by-particle tracking capability using Geant4 Monte Carlo code. This simulation sought to improve the density resolution of tissue images without increasing the dose. The CIRS062M standard phantom was irradiated with 300 MeV protons, and the energy, position, and direction of the particle were recorded before and after the phantom. The matrix of the RSP image was modified using the normalization algorithm of the amount of integral energy loss and the correction of the angles of the projections.