The single source channel of Leksell Gamma Knife was simulated using Monte Carlo N-Particle Transport Code System(MCNPX version).The accurate dose distribution in a water-equivalent sphere phantom of 16 cm diameter,such as Central-axis(CAX)dose curves and dose profiles in focus plane,were obtained with high resolution(0.1 mm in radial direction)and compared with those of previous studies.The influence of electron cutoff energy used in Monte Carlo simulation on the accuracy of the results was also studied through a series of simulation with different electron cutoff energies.The dose distributions generally showed good agreement with those of previous studies except that there was small but significant discrepancy(the maximum relative error is about 50%)in penumbra.This discrepancy resulted from the dose contribution from the photons penetrated secondary collimator.If the electron cutoff energy was greater than 0.050 MeV,the CAX dose for 4 mm secondary collimator significantly increased(about-60%at most)than those calculated with lower electron cutoff energy while the dose in penumbra decreased.This effect faded out with the increase of secondary collimator size and the radial distance from beam axis.So we concluded that the dose contribution to the penumbra from the photons penetrated secondary collimator is not negligible,especially when the secondary collimator is small.In addition,too high(greater than 0.050 MeV)electron cutoff energy should not be used in Monte Carlo simulation in order to avoid incorrect neglect of the scatter effect of high energy electrons.