Construction of Voxel-type Phantom Based on Computed Tomographic Data of RANDO Phantom for the Monte Carlo Simulations

The Monte Carlo simulations using the mathematical phantom of a human body model is useful as a method of getting to know the details of the radiation exposure in a human body. However, it is very difficult to evaluate experimentally the validity of the calculation result of the simulation using the human body model. Then, the mathematical phantom of voxel-type (voxel phantom) for the Monte Carlo simulations was constructed from Computed Tomographic (CT) data of this phantom using a human body phantom which can actually be measured. The Alderson RANDO phantom RAN-100 type (female model) made by The Phantom Laboratories, Inc. was selected as the target of this study. Electron Gamma Shower version 4 (EGS4) which is the Monte Carlo transport code was used for the simulation of the radiation dose distribution. Some kinds of voxel phantoms with different voxel size were created, and the radiation dose distribution obtained in the simulation was considered about capability expressed finely. Moreover, 4MV X-ray is directly irradiated from the outside at the RANDO phantom, the simulation result in the same conditions as an actual measurement is compared. The fluorescence glass dosimeter (Dose Ace made by Asahi Technoglass corp.) was used for dose evaluation by the measurement. The dose distribution of the voxel phantom by EGS4 simulation expressed more detailed dose distribution structure as voxel size became fine. In comparison of the measured value and the calculated value, it differed 12 % at the maximum. The meaning which builds a voxel phantom from a RANDO phantom with the structure more near a human body is very effective in order to bury the gap of an actual measurement and a simulation, and in order to evaluate the accuracy of the human body model building method, it offers many information.