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

Background: skyshine describes the radiation scattered by the atmosphere abovea linac facility to a point on the ground. the aim of this study was to measure theskyshine photon dose rates from two different (9 mv and 18 mv) medical linearaccelerators. materials and methods: the photon beam was directed upward(180° gantry position), with a maximum photon field size (40 × 40 cm2) at theisocenter. measurements were obtained around the external points selectedoutside the room facilities at a horizontal distance from the target by the calibratedrds-110 survey meter at four points around the isocenter. results: the measuredvalues of the skyshine photon exposure rates at four points for 9 mv and 18mv were 0.6, 0.5, 0.5, and 0.4 μsv/h, and 0.6, 0.4, 0.4, and 0.5 μsv/h,respectively. all the measured skyshine photon exposure rates were lowerthan the values recommended by ncrp 147. conclusion: there is a pooragreement between the measured and the calculated values; therefore it seemsthat caution is needed while using the equations available in ncrp 147 or 151.