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The aim of this study is to improve the characterization and modeling of the radiation field surrounding the Leksell Gamma Knife®-PerfexionTM. The improved characterization of the radiation field enables more accurate shielding calculations to be performed for the areas adjacent to the treatment room. With the aid of a high-purity germanium detector and a satellite dose rate meter, γ -ray spectra and ambient dose equivalent H*(10) data were acquired at various locations in the field of a Leksell Gamma Knife unit in a treatment room at Karolinska University Hospital, Sweden. These measurements were used to validate the results of the PEGASOS Monte Carlo simulation system with a PENELOPE kernel. The levels of the radiation that passes through the shielding of the machine (leakage radiation) are shown to be much lower than what is suggested by various bodies, e.g. the National Council on Radiation Protection and Measurements, to be used when calculating radiation shielding barriers. The results clearly indicate that Monte Carlo simulations may be used in structural shielding design calculations for γ rays from the Leksell Gamma Knife. • A HPGe detector has been used to study the radiation field around the Gamma Knife; • A phase space has been used to study the radiation in the field of the Gamma Knife; • A novel pile-up modeling has been used to compare MC simulations with measurements. [ABSTRACT FROM AUTHOR]