Gamma Knife Stereotactic Radiosurgery (GKSRS) treatment requires the high accuracy of delivering single high dose without negative impact on the surrounding tissue. During radiosurgery treatments using GKSRS, the surrounding tissue will receive high scatter radiation because the single dose treatment will employed long irradiation time. Therefore, radiation doses in critical organs of patients during GKSRS treatment is important to be evaluated. In this study, we investigated the radiation dose to the lens, thyroid, breast, uterine fundus, ovaries and testes during treatment using Leksell Gamma Knife Perfexion (LGK; Elekta AB, Stockholm, Sweden). The dose was simulated using the variation of tumour target from 5 to 20 cc. The measurement were performed using variation of collimator size of 4 mm, 8 mm, 16 mm and employed thermoluminescent dosimeters (TLDs) to be placed at the surface of Rando anthropomorphic phantom. TLDs reader Harshaw model 3500 were employed to evaluate the measurement result. The result of this study explains that the dose of organs at risk decrease linearly depend on the distance of the target, target volume and the size of collimator. © Published under licence by IOP Publishing Ltd.