臺灣博碩士論文加值系統

使用蒙地卡羅法計算病患體內劑量分佈，首先最重要是獲得輻射射束入射病患之前，空氣中的分布資訊，包含粒子的能譜分布、通量分布等。然而由於這些粒子分布資訊不易由測量獲得，因而使用蒙地卡羅法來模擬直線加速器機頭結構並計算射束中粒子分布的資訊。本研究欲模擬Siemens Primus M3375醫用直線加速器輸出之光子射束分布，獲得其相位空間檔(phase space file)以作為後續病患劑量分布的計算基礎。
本研究之模擬分成三部分，使用BEAM程式之BEAMnrc、BEAMDP、以及DOSXYZnrc三個子程式來進行。第一部分模擬直線加速器機頭結構，針對Siemens Primus M3375 直線加速器輸出之六百萬伏特(6MV)與十百萬伏特(10MV)光子射束，分別於均勻照野與楔形濾器照野下，以BEAMnrc子程式模擬得到距離射源一公尺(SSD=100cm)處，空氣中的粒子分布資訊相位空間檔。第二部分將此相位空間檔以BEAMDP子程式分析其粒子通量分布、能譜分布與角度分布情形。接著以此相位空間檔作為DOSXYZnrc子程式之輸入射源，模擬計算均勻水假體深度百分率劑量與特定深度劑量剖面；同時於相同設定下，以游離腔測量校正用水箱內深度百分率劑量與特定深度劑量剖面。比較測量數據與模擬結果，調整模擬之幾何結構與參數設定值，以使模擬結果與測量數據吻合。其中楔型濾器照野的模擬過程如同均勻照野，除了必須於機頭結構中加入楔形濾器，最後同樣以水箱測量數據驗證模擬結果。第三部分將驗證完成之各照野模擬結果，採距離射源九十公分(SSD=90cm)處的相位空間檔，以三個仿照臨床照射方式，計算均勻水假體(尺寸20x20x20立方公分)內，中央軸面之等劑量曲線分佈，並與臨床治療計畫系統(ADAC Pinnacle)計算之等劑量曲線分佈做比較，觀察其劑量差異情形。
使用BEAMnrc模擬所得之粒子分布相位空間檔，以BEAMDP分析其通量分布、能譜分布與角度分布，檢視各照野之結果均合理。以此相位空間檔輸入DOSXYZnrc計算各照野垂直入射水假體之劑量分布，與實際於水箱中測量的結果比較，吻合度皆良好；於最大劑量深度之後，深度百分率劑量差異皆低於2%；於80%照野內之橫向劑量剖面劑量差異皆低於3%。以驗證完成之相位空間檔，計算三個仿臨床照射方式之假體內等劑量曲線分布，與治療計畫系統計算結果比較，劑量差異皆少於3%。
本研究之劑量模擬結果與實際測量之劑量差異皆落於合理範圍內，顯示由BEAMnrc模擬所得的光子射束分佈合理，可作為未來臨床治療劑量計算的基礎資訊。

Monte Carlo calculation is presently the most accurate method to calculate dose distributions in patients treated with radiation. A prerequisite for such calculations is to have accurate information of the input data, i.e., the phase space data. The primary task of this study is thus using Monte Carlo simulation to obtain the phase space data of radiation beams generated from a medical linear accelerator. The phase space data contain information about the particles’ position, energy, and direction; these are correspondingly represented as the particle fluence, energy fluence, spectrum, and angular distribution.
In our simulation of a Siemens Primus M3375 linear accelerator, the BEAM code subroutines of BEAMnrc, BEAMDP and DOSXYZnrc were used respectively, for treatment head simulation, phase space analysis, and phantom dose calculation. Our simulation comprised three parts: First, the treatment head was simulated and the phase space data within a 50cm field, at 100cm distance from the source were saved in files. These simulations were performed for 6MV and 10MV photons, for both open fields and wedged fields of 15�a, 30�a, 45�a, and 60�a physical wedges. Second, the phase space files were analyzed using BEAMDP for particles’ distribution. Finally, the phase space files were taken as the input sources for dose calculation in a water phantom using the DOSXYZnrc subroutine. Depth doses and lateral dose profiles were calculated and compared with measured data for validation.
Measured data were acquired under standard conditions for quality assurance using the IBA water tank and Wellhofer farmer type ion chamber. These measured dose distributions were compared with the simulated results, and free simulation parameters such as the initial electron energy and the spot size can be adjusted to make the simulated results agree with the measured ones. For clinical application, the validated phase space files were used to calculate isodose distributions with three cases of typical radiation technique. The calculated isodose distributions were compared with those computed by the clinical treatment planning system.
The results showed that the dose differences between measurements and simulations were less than 2% for depth doses, and less than 3% for dose profiles inside the 80% field size. For clinical cases, comparisons between simulation results and treatment planning system calculations showed that the dose differences were within 3%. It demonstrated that the simulated phase space data were reliable and could be used for clinical patient dose calculations for future investigations.

摘要 iii
Abstract v
致謝 vii
目次 viii
表次 x
圖次 xi
第一章　緒論 1
1.1研究背景 1
1.2文獻回顧 2
1.3研究目的 4
1.4論文架構 4
第二章　材料與方法 6
2.1蒙地卡羅法 6
2.2蒙地卡羅模擬程式 7
2.2.1 EGS4程式 10
2.2.2 EGSnrc/BEAMnrc程式 12
2.3直線加速器模擬 20
2.3.1直線加速器構造 21
2.3.2 BEAMnrc模擬直線加速器機頭 25
2.3.3 BEAMnrc模擬參數設定 27
2.3.4 BEAMDP分析相位空間資訊檔 29
2.3.5水假體劑量的模擬計算 30
2.4 仿臨床照射方式之水假體劑量模擬計算 33
2.5 水箱劑量測量 36
第三章　結果 38
3.1分析相位空間粒子資訊 38
3.1.1通量分布 38
3.1.2能譜分布 43
3.1.3角度分佈 45
3.2水假體內劑量分布 47
3.2.1 6MV光子均勻照野下 47
3.2.2 10MV光子均勻照野下 50
3.2.3 6MV光子照野下加入楔形濾器 52
3.2.4 10MV光子照野下加入楔形濾器 56
3.2.5 仿臨床照射方式之等劑量剖面分布 60
第四章　討論 65
4.1模擬時間與效率評估 65
4.2模擬參數對模擬結果的影響 66
4.3測量與模擬結果的比較 68
4.4研究限制 71
第五章　結論 72
參考文獻 73

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