Study of dose enhancement effects in the vicinity of high Z atoms with Geant4 Monte Carlo simulation

Dose enhancements in the vicinity of different high Z (HZ) materials in radiation therapy (RT) are studied by several groups. In this work, extensive and systematic Monte Carlo (MC) studies were performed with Geant4 toolkit focusing on factors that influence the dose enhancements, including the atomic number of HZ materials, the concentration of the HZ material, and the X-ray energies. Gold (Au), Platinum (Pt), Gadolinium (Gd), and Iodine (I) are used in the study. A new method has been implemented to determine the individual contributions from pair production, Compton scattering and the photoelectric effect to the total dose. The dose enhancement ratios (DERs) and their contributions to the overall dose deposition are calculated for these processes. It is also shown in the study that with fixed X-ray energy and molar concentration, the DERs for these HZ materials are approximately linear proportional to their Z number. The DER increases linearly with HZ concentration; however, the slope of the increase depends on the X-ray energies. Analysis of the DERs from different processes reveals an interesting phenomenon: although the contribution from the photoelectric effect to the overall dose enhancement for X-rays in the MeV range is low, being normally less than 3%, the DER for the photoelectric effect from MeV X-rays are higher than that from X-rays in keV range.