目的：模擬加馬刀放射手術單一射束深度與徑向劑量分佈，並與加馬刀治療計畫系統一加馬計畫（GammaPlan?飽^進行比較。材料與方法：利用EGS4蒙地卡羅計算機程式，模擬加馬刀放射手術治療機18mm孔徑單一封束通過聚苯乙烯均質假體。結果：模擬過程中，體射源模擬除射源自我屏蔽降低絕對深度劑量外，其他相對劑量分佈計算結果與點射源模擬大致相同，故利用點射源射束模擬實際情形，可大幅減短程式計算時間。加馬計畫除表面劑量增建區無法計算外，相對深度劑量計算可謂相當準確；徑向劑量方面，加馬計畫利用焦點平面相對徑向劑量分佈取代其他平面劑量分佈，亦無不妥。結論：均質假體中，加馬計畫劑量評估除深度劑量分佈之表面增建現象無法計算外，其餘深度與徑向劑量分佈可謂相當準確。

　　　　　Purpose: To simulate the single beam depth and radial dose distribution forthe 18 mm collimator of a Gamma Knife radiosurgery unit.Materials and Methods: EGS4 Monte Carlo code was used to simulate the single beampassing a poly-styrene phantom.Results: Simulation results show no difference in the relative dose distributionsfor point and body sources, it is desirable to use a point source to replace the realbody source for all the simulations in this study to reduce computation time. Thesimulated relative depth dose distribution show good agreement with that calculatedby the treatment planning system (GammaPlan??) provided by the manufacturerexcept at the region adjacent to phantom surface. The simulated relative radial dosedistributions at various depths also indicate that the assumption made by theGammaPlan to use the radial dose distribution at the focus to replace those at otherdepths generates only minimum errors.Conclusions: Excluding the build up effect on depth dose distribution at the surfaceregion, GammaPlan @ show a good agreement with EGS4 simulation both on depth and radialdose distribution in a homogeneous phantom for single beam of gamma knife.