摘要

楔形濾器在傳統放射治療上運用普遍，透過楔形濾器，可達劑量分布更均勻。一般而言，經過楔形濾器射束會產生特性上改變，這些改變不易在實際測量中測得，透過模擬系統，得知各種特性變化。以使用者介面化之BEAM程式模擬瓦利安直線加速器在6百萬電子伏特之輸出，研究中，機頭下加入臨床使用之楔形濾器幾何元件(component modules, CMs)，透過實際量測各角度濾器尺寸及感應耦合電漿質譜儀（inductively coupled plasma mass spectrometry, ICP -MS）金屬材質鑑定，於BEAM程式中建立元件及作用截面（cross section），以符合實際機頭與楔形濾器環境。BEAM程式中，均勻照野（open field）及 楔形濾器照野在95公分處的相空間粒子檔（phase-space data）均被建立；透過程式DOSXYZnrc建立與實際量測劑量之自動化水箱一致的環境，模擬不同照野大小、深度等條件下，百分深度劑量（percentage depth dose, PDD）與劑量剖面（dose profile）的變化；使用BEAMDP 程式，可以分析出不同條件下相空間粒子檔的變化，程式模擬結果，與水箱量測值進行比較，建立可延續使用的機頭輸入檔。均勻照野及15、30、45、60度楔形濾器下照野已模擬完成，與水箱量測值有趨於一致的結果，運用BEAMDP分析不同條件下的相空間粒子檔，與均勻照野比較，證明射束穿過楔形濾器後產生射束硬化作用（beam hardening），分析不同條件下射束能譜（beam spectra）、平均能量（mean energy）、光子通量（photo fluence）及二次電子汙染（electron contamination）的變化。結果顯示，在30度楔形濾器照野因低能光子與濾器作用因素產生射束硬化，輸出平均能量較均勻照野增加20％，也因為減少了電子汙染，形成增建區（build-up region）之劑量下降3
％。本研究中，發現楔形濾器照野會產生射束特性變化，這證明粒子與介質作用後的變化，而蒙地卡羅程式是研究劑量變化的有效方法， 所建立的組件與程式輸入檔可運用在後續的研究上。

Abstract

Physical wedges (PWs) are commonly used as beam modifying devices in clinical to optimize the dose distribution. Some beam characteristics can not be evaluated by calculations or measurements. The user code ‘BEAM’ was used to simulate 6 MV photon beam generated by a linac. In this study, the new PWs component modules were established in linac head by BEAMnrc. The elements composition of PWs were determined by ICP-MS. The PWs and open fields phase-space data were also accumulated in BEAMnrc. Dose distributions in water phantom were performed by DOSXYZnrc. The code ‘BEAMDP’ was used to analyze these phase-space data for beam characterizations. The percentage depth dose (PDD) and dose profiles of simulations were verified with measurements, which were performed by water tank and ionization chamber. Relative doses were compared between the simulations and measurements. The present Monte Carlo simulations for PWs and open fields were validated first with the measurements for a 10x10cm2 field with all wedges. The results showed a good agreement between the simulated and measured dose distribution in PWs and open fields. The PDD curves for the open field were also included to demonstrate the effects of beam hardening due to the PWs. The BEAMDP calculation reveals that the effects of a PWs are different in affecting beam spectra, mean energy, photo fluence, and electron contamination. PWs may result in about 20% increase in mean photon energy due to beam hardening, which can also introduce about 3% dose reduction in the build-up region due to the reduction of contaminated electrons by the 30 degree PW. The Monte Carlo code is an accurate method to study dosimetric characteristics and the input datas can be used in further studies.

目錄

摘要...................................................I
Abstract..............................................II
圖目錄................................................III
表目錄..................................................V
第一章 緒論
1.1 研究背景.............................................1
1.2 現況分析.............................................2
1.3 文獻回顧.............................................3
1.4 研究目的.............................................4
1.5 研究架構.............................................4
第二章 材料與方法
2.1 臨床直線加速器自動化水箱劑量量測 .............................7
2.2 蒙地卡羅方法介紹..............................................8
2.2.1 蒙地卡羅程式模擬法..........................................8
2.2.2 蒙地卡羅模擬程.............................................9
2.2.3 EGS4 程式.................................................12
2.2.4 BEAMnrc user code.........................................14
2.2.5 BEAMnrc 子程式.............................................16
2.2.6 BEAMDP 子程式..............................................16
2.2.7 DOSXYZnrc 子程式 .........................................16
2.3 直線加速器模擬................................................19
2.3.1 模擬環境與對象 ............................................19
2.3.2 直線加速器構造簡介..........................................20
2.3.3 使用 BEAMnrc 程式模擬機頭...................................22
2.3.4 模擬產生之相位空間檔.........................................23
2.3.5 模擬均勻照野水假體劑量的分布 ................................23
2.3.6 使用 BEAMnrc 程式模擬楔形濾器照野............................23
2.3.7 使用 BEAMnrc 程式模擬楔1 形濾器模組...........................24
2.3.8 以 BEAMDP 分析在楔形濾器下之相位空間資訊檔.....................27
第三章 結果
3.1 直線加速器模擬照野與實測照野百分深度劑量之比對..................... 29
3.2 楔形濾器組件之元素百分比鑑定結果.................................. 29
3.3 以 BEAMnrc 建立楔形濾器組件與 EGS windows 軟體運用結果.............30
3.4 蒙地卡羅法模擬楔形濾器照野與實際量測之比較..........................32
3.5 蒙地卡羅法驗證楔形濾器下之射束劑量輸出特性
3.5.1 平均能量 (Mean energy) ........................................37
3.5.2 能譜分佈 (Spectral distribution) ..............................37
3.5.3 能通量分佈
3.5.3.1 光子能通量(photon energy fluence) ....................38
3.5.3.2 電子能通量(electron energy fluence) ....................39
3.5.4、角度分佈
3.5.4.1 光子角度分佈(photo angular distribution) .................40
3.5.4.2 電子角度分佈(electron angular distribution) ...............40
第四章 討論
4.1 模擬效率探討.................................................... 41
4.2 幾何模組之建立組件選擇 ..........................................41
4.3 研究價值.........................................................42
第五章 結論 ........................................................ 43
參考文獻

附錄一 Varian 21EX 直線加速器楔形濾器材質元素百分比鑑定報告
附錄二 Varian 21EX 直線加速器楔形濾器材質元素百分比鑑定報告

圖目錄

圖 1-1、本研究之機頭模擬剖面示意圖(機頭模擬及驗證階段) ...............5
圖 1-2、模擬楔形濾器照野劑量驗證示意圖(照野模擬階段) .................6
圖 2-1、農夫型游離腔安裝圖示........................................7
圖 2-2、IBA 自動化水箱安裝圖示......................................7
圖 2-3、蒙地卡羅法與一般實驗方法之比較示意圖.........................8
圖 2-4、蒙地卡羅法基本架構..........................................9
圖 2-5、使用蒙地卡羅模擬程式追蹤輻射粒子遷移流程圖...................10
圖 2-6、EGS4 程式的運作結構圖......................................13
圖 2-7、BEAMnrc 程式使用者圖形化介面...............................15
圖 2-8、建構在 EGSnrc 基礎下的 BEAMnrc 系統結構示意圖...............15
圖 2-9、直線加速器主要結構.........................................21
圖 2-10、模擬治療機頭結構示意圖.....................................22
圖 2-11、設計之楔形濾器其結構與距離射源之相對距離示意圖...............24
圖 2-12、在49公分處 BEAMnrc 程式在模擬楔形濾器組件設定...............25
圖 2-13、以 BEAMnrc 程式在模擬45度楔形濾器組件設定輸入檔.............25
圖 2-14、使用 EGSnrc MP 使用者介面建立45 楔形濾器作用截面過 程........26
圖 2-15、將 BEAMnrc 程式 PEGS4 母檔加入45度楔形濾器作用截 面.........26
圖 2-16、BEAMnrc 程式模擬45°楔形濾器之輸入檔參數鍵入情形..............27
圖 2-17、BEAMDP GUI 使用者介面程式..................................28
圖 3-1、實測值與模擬值百分深度劑量之比較..............................29
圖 3-2、BEAMnrc 程式模擬(a)15° (b)30° (c)45° (d)60°楔形濾器剖面圖......31
圖 3-3、BEAMnrc 程式與 EGS windows 機頭及楔形濾器結構呈現...............32
圖 3-4、BEAMnrc 模擬(a)15° (b)30° (c)45° (d)60°楔形濾器10X10公分照野、深度5
公分處之劑量剖面. ..................................................33
圖 3-5：BEAMnrc 模擬各角度楔形濾器10X10公分照野、深度5公分處之劑量剖面圖綜合
比較...............................................................33
圖 3-6：以游離腔量實測(a)15° (b)30° (c)45° (d)60°楔形濾器10X10公分照野、深度5
公分處之劑量剖面圖..................................................34
圖 3-7：以游離腔量測各角度楔形濾器10X10公分照野、深度5公分處之劑量剖面圖綜合
比較...............................................................34
圖 3-8：15°楔形濾器 BEAMnrc 模擬值與游離腔量測在10X10公分照野、深度5公分處之劑量
剖面比較...........................................................35
圖 3-9：30°楔形濾器 BEAMnrc 模擬值與游離腔量測在10X10公分照野、深度5公分處之劑量
剖面比較...........................................................35
圖 3-10：45°楔形濾器 BEAMnrc 模擬值與游離腔量測在10X10公分照野、深度5公分處之劑量
剖面比較...........................................................36
圖 3-11：60°楔形濾器 BEAMnrc 模擬值與游離腔量測在10X10公分照野、深度5公分處之劑量
剖面比較...........................................................36
圖 3-12：模擬之均勻照野與楔形濾器照野平均能量分佈........................37
圖 3-13：模擬之均勻照野與楔形濾器照野光子與電子能譜分佈...................38
圖 3-14：模擬之均勻照野與楔形濾器照野光子能通量分佈情形...................39
圖 3-15：模擬之均勻照野與楔形濾器照野電子能譜分佈情形.....................39
圖 3-16：模擬之均勻照野與楔形濾器照野光子角度分佈情形.....................40
圖 3-17：模擬之均勻照野與楔形濾器照野電子角度分佈情形.....................41

表目錄
表一：楔形濾器材質鑑定結果............................................. 30

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