Monte Carlo studies on dose conversion factors from graphite to water for high energy X-ray beams

Abstract

An evaluation of the dose conversion factor from graphite to water is essential to measure the absorbed dose to water using a graphite calorimeter. However, it is difficult to obtain the conversion factors from measurements. In the present study, Monte Carlo (MC) studies were performed to evaluate the absorbed dose conversion factor from graphite to water. Based on the Electron Gamma Shower MC codes from National Research Council Canada, the modeling of the medical linear accelerator gantry at the Korea Research Institute of Standards and Science (KRISS) was optimized to obtain phase space files for evaluation of the absorbed dose conversion factor from graphite to water. The evaluated dose conversion factors for the KRISS graphite calorimeter were in the range of 1.040–1.085 for beams of 6–25 MV. A linear correlation between the dose conversion factors and tissue phantom ratio in water at depths of 20 cm and 10 cm (TPR20,10) of each X-ray beam was observed. To understand sensitivity of the obtained dose conversion factors, case studies were performed under different conditions in the simulation, such as different electron stopping power, assumed graphite bulk density, and different MC code. The uncertainties of the dose conversion factors will be finalized considering the results of sensitivity studies in the present study and additional studies in the future.