Monte Carlo Modeling of Skyshine Dose Distribution in Vicinity of Medical Accelerator Facilities

In the vicinity of some of radiation therapy centers, measured exposure may not agree with calculated values due to the skyshine phenomenon. The aim of this study was to determine a computational model to predict the dose of radiation skyshine via Monte Carlo simulations.
The simulation of linear accelerator head, skyshine geometry, and skyshine dose calculation at several heights and for different roof thicknesses was done using Monte Carlo N-Particle eXtended (MCNPX) code. The simulation results were validated via measuring and then were compared with the Council on Radiation Protection-151 (NCRP151) model results.
Findings: The results of dose distribution in skyshine simulated model for all thicknesses of roof and every height from ground level showed an increase in the dose up to about 10 meters distance from the isocenter and then a decrease by distance from the isocenter; while the dose in the NCRP151 model, continuously decreases by distance. In addition, the amount of dose in each point was around 50 times larger than the simulated values. Dose reduction in both methods was proportional to the inverse-square.
The model developed in this study can be used to predict the skyshine dose when designing a radiotherapy facility. The developed model is superior to the NCRP151 model due to its ability to calculate the dose in every point, inside and outside of treatment room, to investigate the influence parameters such as ceiling thickness and height from ground level and to determine the point with maximum dose.