Background: Monte Carlo simulation techniques are presently considered to be the most reliable method for radiation therapy treatment planning.However

long simulation times involved when using the general-purpose Monte Carlo code systems have led to the development of special-purpose Monte Carlo programs.Purpose: This paper attempts to improve computing efficiency for dose calculation in the EGSnrc modeling of clinical linear accelerator by selecting proper parameters.Methods: Several variance reduction techniques including uniform bremsstrahlung splitting

selective bremsstrahlung splitting

directional bremsstrahlung splitting are applied in BEAMnrc simulating medical accelerator treatment head to generate phase-space file which is selected as a source for DOSXYZnrc simulation

both photon splitting and particle recycling are used to improve the efficiency in the calculation of dose profile in water phantom.Results: The splitting number for maximum efficiency in directional bremsstrahlung splitting（no electron splitting） is 2500 in the BEAMnrc simulation.The highest efficiency of DOSXYZnrc simulation is given when photon splitting number is set to 40.Conclusions: Efficiency can be significantly improved by setting appropriate bremsstrahlung splitting and optimized photon splitting number and particle recycling number.