本研究的目的在找出適用於瓦里安21EX的6MV光子射束之直線加速器機頭模組的初始電子參數，以改善模擬時的準確度。過去許多研究證明蒙地卡羅模擬是一種精確的劑量模擬方法，但要達到精確模擬的目的，必須要擁有詳盡的直線加速器機頭幾何構造以及初始電子的參數。為了找出最佳的初始電子參數，本研究參考相關文獻對於初始電子的能量及空間分佈作了一些假設，把它分成初始電子平均能量、初始電子高斯分佈能譜分佈之半高全寬、初始電子徑向分佈之半高全寬等三個參數，並分別討論個別參數對劑量分佈的影響，利用嘗試錯誤的方法找出最佳的參數。且訂定一套流程，可供之後的研究作為參考。

Determination of initial electron parameters for Monte Carlo simulation of Varian 21EX LINACs 6MV photon beams

PURPOSES
Previous studies have demonstrated that the Monte Carlo simulation is an accurate method for dose simulation in radiotherapy. Accurate dose calculation requires precise characterization of the accelerator geometry and parameterization of the initial electron beam incident on the target. The objective of the current study was to determine optimal initial electron parameters for Monte Carlo simulation of the 6 MV photon beam (Varian 21EX) at CGMH—Linkou.

MATERIALS AND METHODS
The EGSnrc user code BEAM (BEAM06) was used for dose simulation in this work. The geometry was input into BEAM from proprietary diagrams supplied by Varian for the 21EX Linac. The optimal initial electron parameters were determined by evaluating dose difference between simulated and measured percentage depth doses (PDD) and lateral profiles at 1.5, 10 and 20 cm depths for 10 10 and 40 40 cm2 fields through trial-and-error processes. It started with an initial guess of the mean energy of electrons with fixed energy and radial spread. Once the optimal mean energy was determined, optimized energy spread was sought followed by determination of radial intensity distribution. Dose difference evaluation was performed using the κα factor and off-axis ratio (OAR). The κα factor is defined as the fraction of the voxels with absolute dose difference less than α% of the maximum measured central axis dose. Data evaluation (κα test) for PDD was done for depth range between 1.5 and 20 cm. Lateral position of 90% field size was used for OAR evaluation and κα tests for lateral profiles were performed within the center 90% field width.

RESULTS
Penetration of percentage depth curves increases with increasing mean energy of initial electrons, especially for 10 10 cm2 field but insensitive to energy spread and almost independent of radial spread.
OARs of lateral profiles decrease with increasing mean energy of initial electrons. Radial spread had great impact on OAR especially for 40 40 cm2 field but insensitivity to energy spread.
Energy distribution of the initial electrons was concluded to be a Gaussian distribution with mean energy equal to 6.2 MeV and FWHM setting 0.235 MeV. The optimal intensity distribution obtained in this work is a radially symmetric Gaussian distribution with FWHM equal to 1 mm.

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