Developing a Novel Method for Photon Skyshine Dose Estimation in Radiation Therapy Facilities Compared to the NCRP151 Approach

M. Robatjazi (Department of Medical Physics and Radiological Sciences, Sabzevar University of Medical Sciences, Sabzevar, Iran)
F. Koosha (Department of Radiology Technology, School of Allied Medical Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran)
M. Molazadeh (1. Medical Radiation Sciences Research Center, Tabriz University of Medical Sciences, Tabriz, Iran 2. Department of Medical Physics, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran)

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This study aimed to enhance the accuracy of estimating photon beam skyshine dose rates by refining the National Committee on Radiation Protection (NCRP) 151 method. Simulations were conducted using the MCNPX Monte Carlo (MC) code for 6, 10, and 18 MV photon beams of Varian 2300 C/D linear accelerators to estimate the skyshine dose rates of photon beams. The analytical methods of NCRP 151 and our proposed formulation were used to estimate the skyshine dose around the vault. The results of the three methods were compared and analyzed. Our modified method demonstrated improved accuracy compared to the NCRP method and agreed with the MC method under similar conditions and geometries for the linac photon beams. The ratio of skyshine dose rates calculated by NCRP 151 to MC simulation results ranged from 5.39 to 16.59 for 18 MV, 2.25 to 19.20 for 10 MV, and 3.33 to 22.50 for 6 MV linacs. However, employing our derived formula reduced these ratios to 0.91 to 1.22 for 18 MV, 0.92 to 1.10 for 10 MV, and 0.12 to 1.40 for 6 MV machines. Our modified method exhibited good agreement with MC modeling across various conditions. Application of the novel method in different situations and comparison with measurements are recommended for further use in radiation therapy and radiation protection calculations.