Assessment of skyshine photon dose rates from 9 and 18 MV medical linear accelerators

Background: Skyshine describes the radiation scattered by the atmosphere above a LINAC facility to a point on the ground. The aim of this study was to measure the skyshine photon dose rates from two different (9 MV and 18 MV) medical linear accelerators. Materials and Methods: The photon beam was directed upward (180 degrees gantry position), with a maximum photon field size (40 x 40 cm2) at the isocenter. Measurements were obtained around the external points selected outside the room facilities at a horizontal distance from the target by the calibrated RDS-110 survey meter at four points around the isocenter. Results: The measured values of the skyshine photon exposure rates at four points for 9 MV and 18 MV were 0.6, 0.5, 0.5, and 0.4 mu Sv/h, and 0.6, 0.4, 0.4, and 0.5 mu Sv/h, respectively. All the measured skyshine photon exposure rates were lower than the values recommended by NCRP 147. Conclusion: There is a poor agreement between the measured and the calculated values; therefore it seems that caution is needed while using the equations available in NCRP 147 or 151.