中文摘要：

采用BEAMnrc模拟医用加速器治疗头，研究均匀轫致辐射分裂、选择轫致辐射分裂和方向轫致辐射分裂三种方差减小技术并设置参数，计算相空间文件；将相空间文件输入至DOSXYZnrc中，选择光子分裂、粒子循环技术提高效率，计算水模体中的剂量分布。结果显示，在BEAMnrc中，使用无电子分裂的方向轫致辐射，当分裂数为2500时，计算效率最高；在DOSXYZnrc中，当光子分裂数为40时计算效率最高。表明合适的轫致辐射方式及参数，一定的光子分裂数和粒子循环数能提高计算效率。

英文摘要：

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 EGSrtrc modeling of clinical linear accelerator by selecting proper parameters. Methods： Several variance reduction techniques including tmiform 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 oarticle recvcling number.