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    Please use this identifier to cite or link to this item: http://nthur.lib.nthu.edu.tw/dspace/handle/987654321/85130


    Title: 被動式輻射劑量計應用於強子治療之研究
    Authors: 蕭明城
    Hsiao, Ming Chen
    Description: GH029813801
    博士
    核子工程與科學研究所
    Date: 2015
    Keywords: 輻染膠片;丙氨酸;強子治療;混合輻射場;相對效應
    Radiochromic Film;Alanine;Hadron Therapy;Mixed Radiation Field;Relative Effectiveness
    Abstract: 輻染膠片和丙氨酸兩種被動式輻射劑量計已被廣泛例行用於醫療X光及加馬射線的光子輻射場劑量測量。本論文研究此兩種劑量計在次熱中子硼中子捕獲治療(BNCT)、質子治療和碳離子治療此三種強子治療射束的劑量測量。此兩種劑量計對於各種不同粒子的響應與光子相比均不相同,因此本論文重點即在探討這兩種劑量計對各種不同粒子與光子響應相較之相對效應(RE),使這兩種劑量計可用於各種不同強子治療射束場合並測得正確劑量值。
    在次熱中子BNCT射束中,中子與劑量計作用會產生各種帶電粒子在劑量計中造成劑量,另外還有加馬射線與物質作用產生的電子亦會在劑量計中造成劑量,因此BNCT射束為一複雜的混合輻射場。本論文分別將輻染膠片與丙氨酸置於PMMA假體中,在本校清華水池式反應器(THOR)的次熱中子BNCT射束從事測量,並利用MCNPX蒙地卡羅程式執行計算,計算係利用已校正的中子監測系統和中子與加馬射線源項,將MCNPX計算而得的中子劑量和加馬射線劑量作為輔助,進而求得輻染膠片和丙氨酸劑量計在次熱中子BNCT射束之中子RE。
    又因輻染膠片為一種二維劑量計,本論文於是將大張輻染膠片置於PMMA假體內,在THOR的次熱中子BNCT射束嘗試測量二維加馬射線劑量分布。中子對輻染膠片造成的劑量貢獻由下述三個步驟進行測定:首先係用銅片間接中子照相法(INR)結合影像板(IP)測量活化銅衰變輻射,取得二維之銅活度分布,接著利用MCNPX配合中子監測系統之讀數計算中子對輻染膠片軸線處所造成的劑量,最後則採用INR和IP所測得的二維銅活度分布將中子對輻染膠片軸線處所造成的劑量進行二維展開。將輻染膠片測得之二維光子等效劑量分布扣除經中子RE修正之中子劑量二維分布即得二維加馬劑量分布,其與MCNPX計算結果相比吻合情況良好。此外若次熱中子BNCT射束源頭之中子與加馬射源成份固定,則射束中中子和加馬射線通率之比例亦不致改變,因此輻染膠片所測得的光子等效劑量分布應可直接用於射束均勻度之品保作業。為此本論文亦嘗試制定品保測量可否通過之判定條件。
    在質子和碳離子治療射束的測量中,本論文主要探討輻染膠片和丙氨酸在測量位置的RE。本論文首先由文獻回顧整理出這兩種劑量計對於質子、碳離子及其它粒子的RE隨粒子能量變化的函數,其中輻染膠片劑量計的粒子RE隨能量變化函數均來自測量結果,而丙氨酸數據則來自徑跡結構理論模式計算結果。由於此兩種劑量計對於各種粒子的RE均與粒子能量成依存關係,因此若要求得劑量計在測量位置的RE,必須先知道測量位置的粒子能量分布。本論文利用MCNPX計算兩種劑量計分別在粒子進入水中不同深度即布拉格曲線不同位置之粒子通率能量分布,再利用對應的RE能量函數,以劑量加權方式求出劑量計在該位置之RE。由於體外離子治療時均利用擴展布拉格峰(SOBP)區域劑量從事治療,因此本論文亦以MCNPX計算各種能量粒子在水中之布拉格曲線,並參考文獻模式求出各布拉格曲線之加權因數,相加後之布拉格曲線即為SOBP。SOBP區域內各位置之RE係利用各布拉格曲線上各位置之RE值求得。研究結果顯示在質子治療射束SOBP區域中,輻染膠片和丙氨酸劑量計之RE值幾乎均為定值,輻染膠片RE值約為0.96而丙氨酸RE值則趨近於1。碳離子治療射束在布拉格峰附近,除了碳離子外還會有相當數量的核反應碎片,其數量與入射碳離子能量及測量位置互成依存關係,因此需先獲得各種粒子的RE隨粒子能量的變化函數,再修正輻染膠片及丙氨酸在特定能量碳離子治療射束及特定位置產生的各種粒子RE後才可能正確測量劑量,惟如此將相當複雜而不易使用。
    據此,本論文已研究上述兩種被動式輻射劑量計應用於三種強子治療射束之粒子RE,及其在強子治療射束場合的實際應用。
    Two types of passive radiation dosimeters of the radiochromic film and the alanine have been extensively and regularly applying for dose measurements in photon fields of medical X-rays and gamma-rays. This dissertation studies these two passive dosimeters for dose measurement in therapeutic beams of boron neutron capture therapy (BNCT), proton therapy, and carbon ion therapy. Responses of these two dosimeters irradiated with various particles differ from their photon responses. Therefore, this dissertation mainly investigates the relative effectiveness (RE) values of these two dosimeters for various particles in relation to photons, so as to make these dosimeters be used for dose measurement in the context of hadron therapy.
    In the epithermal neutron beam used for BNCT, the neutron dose to a dosimeter is induced by the charged particle emission of neutron reactions, and the gamma-ray dose to a dosimeter is also caused by electrons that are produced from photon interactions in a matter. The BNCT beam is therefore a complex mixed field. This dissertation places the radiochromic film and the alanine in a PMMA phantom irradiated with the BNCT beam at the Tsing Hua Open-pool Reactor (THOR) for dose measurement, and uses a Monte Carlo code of MCNPX to perform calculations. In order to make the calculated results an auxiliary, those calculations adapted a calibrated neutron monitor and the general source definitions of neutrons and gamma-rays to determine the neutron REs of both dosimeters for the BNCT beam.
    Since the radiochromic film is a two-dimensional (2D) dosimeter, this dissertation places a large piece of film in the PMMA phantom to measure the 2D distributions of the gamma-ray dose component of the BNCT beam. Regarding the determination of the neutron dose to the film, it can be divided into three steps: first was to measure the disintegrated radiations of the activated copper using the combination of the indirect neutron radiography (INR) and the image plate (IP), to retrieve the 2D distributions of the radioactivity over the activated copper, then use both MCNPX and the readings of the neutron monitor to calculate the neutron doses to the films on the central axis; lastly, the results of INR plate measurements were applied to expand the neutron doses to the film over the film’s area. Subtract the 2D distributions of the neutron doses from those of the photon equivalent doses to have the 2D distributions of the gamma-ray doses to the films. The measured results were generally in a good agreement with the calculated results of MCNPX. Meanwhile, if the composition of the neutron and gamma-ray sources is constant in the BNCT beam, the fractions of neutron and gamma-ray fluxes would not vary too much. Thus the distributions of the photon equivalent doses measured using the radiochromic films can be directly applied to the quality assurance (QA) procedure of the beam uniformity of the beam line. This dissertation also attempts to set up the criterion for the QA measurement.
    Of the measurements in therapeutic beams of proton therapy and carbon ion therapy, this dissertation mainly discusses the RE values of both dosimeters located at the measured positions. First was to review and conclude the RE functions of energy of both dosimeters for protons, carbon ions, and other particles. In which the RE functions of energy of the film for particles were originated from related experiments, while those of the alanine were modeled and calculated using the track structure theory. Because the REs of these two dosimeters for particles depend on particle energy; hence, in order to determine the RE of the dosimeter at a specific measured position, the energy distribution of a certain particle must be known. This dissertation uses MCNPX to calculate the energy distributions of particle fluxes at different positions along the Bragg curves. Then the RE of the dosimeter placed at the certain position can be determined by using the dose weighted method with the energy function of RE. Owing to that the external ion beam therapy utilizes the doses in the region of the spread-out Bragg peak (SOBP) to treat, this dissertation therefore uses MCNPX to calculate Bragg curves of multiple energies in liquid water, and generates several SOBPs by sets of weighting factors that were referred to a literature. The RE of dosimeters at each position in the SOBP region was determined using the RE distribution along each Bragg curve. Results showed that the proton REs of both the radiochromic film and the alanine dosimeter are fixed values, which are around 0.96 for the film and almost unity for the alanine. As for the measurement in therapeutic carbon ion beams, not only the carbon ions but the sufficient nuclear fragments are present in the vicinity of the Bragg peaks. The magnitude of fragments depends on the incident carbon ion energy and measured position, thus it has to acquire the RE functions of energy for multiple particles to correct the REs of both dosimeters for specific position and incident energy before these two dosimeters can be used to measure the delivered dose of carbon ion therapy. However, it is so complicated that the dose measurement using these dosimeters will not be viable.
    Accordingly, this dissertation has studied the RE values and practical usages of the aforesaid passive dosimeters applied to three therapeutic beams of hadron therapy.
    URI: http://nthur.lib.nthu.edu.tw/dspace/handle/987654321/85130
    Source: http://thesis.nthu.edu.tw/cgi-bin/gs/hugsweb.cgi?o=dnthucdr&i=sGH029813801.id
    Appears in Collections:[核子工程與科學研究所] 博碩士論文

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