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研究生:張力允
研究生(外文):Li-yun Chang
論文名稱:後荷治療機192Ir射源校正與模擬片重建驗證之創新技術
論文名稱(外文):Innovative Techniques of Ir192-Calibration and Film-Reconstruction-Verification for High Dose Rate Remote Afterloading Systems
指導教授:陳天送陳天送引用關係
指導教授(外文):Tainsong Chen
學位類別:博士
校院名稱:國立成功大學
系所名稱:醫學工程研究所碩博士班
學門:工程學門
學類:綜合工程學類
論文種類:學術論文
論文出版年:2009
畢業學年度:97
語文別:英文
論文頁數:81
中文關鍵詞:品質保證農夫型游離腔X光照相片銥-192 校正片子重建後荷治療
外文關鍵詞:radiographic filmquality assurancefarmer chamberbrachytherapyIr-192 calibrationfilm reconstruction
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增加癌症病人的腫瘤局部控制率一直以來都是放射治療科醫師長遠的目標。為了減少正常組織的副作用,病人腔體內腫瘤的加強照射通常會使用後荷治療機(內有放射性同位素銥-192)來完成。要提高腫瘤的局部控制率,最重要的就是給予正確的放射劑量到正確的腫瘤部位。

關於使用後荷治療機治療給予病人正確的劑量這一部分,我們提出了一個簡單、實用及省錢的技術來校正銥-192(192Ir)的空氣克馬強度(air kerma strength 簡稱AKS)。這個技術所使用到的設備,是使用作為直線加速器品質保證(QA)的一些基本配備-0.6 cc 農夫型游離腔(farmer chamber)、X光照相片(V-film)以及聚苯乙烯假體(polystyrene phantom)。我們使用了三種廠牌的農夫型游離腔來作校正:Exradin A19、PTW N30004及TM30001。我們將一片25.4´30.5平方公分的X光照相片貼上一片聚苯乙烯板,然後利用膠帶將後荷治療機專用治療管(Applicator,射源由後荷治療機進入此管停留,為治療病人或做射源校正)及農夫型游離腔固定在X光片的紙封套上。之後我們先將這張X光片接上後荷治療機,用192Ir暴露一秒鐘,再將它置於模擬射影X光機下照射。再來我們將片子小心抽出去沖洗,其餘保持不動置於5公分厚的聚苯乙烯假體上,然後將射源送入applicator開始校正。根據靜電計(electrometer)的讀值及從X光片上游離腔與applicator影像間的距離,我們可以利用我們的新技術來計算192Ir的AKS。

與許多廠牌的井型游離腔(well chamber)的測量值比較,實驗結果顯示此新技術測量出的AKS,其相對誤差在百分之1.5以內。相對於傳統的「七個距離技術」(7-distance measurement technique),我們所提出的方法是更為有效率的,因為用我們的方法僅僅需要量一個距離(射源至游離腔的距離)。總結來說,我們的方法是簡單而又省錢的,特別是為某些醫院沒有井型游離腔或農夫型游離腔專用校正支架(calibration jig),但須注意的是伴隨而來的潛在不確定因子。
關於使用後荷治療機治療病人正確的部位這一部分,為了節省經費,我們利用了一個校正直線加速器的標準工具來驗證後荷治療計劃系統的片子重建技術的整個程序是否正確。正如歐洲放射治療品質保證指引第八冊(ESTRO booklet 8)上所述:「每個單位都應該驗證用於臨床治療的片子重建技術的整個程序」。我們所使用的工具是「射束同心檢測器」(Isocentric Beam Checker),它的板子表面上鉗有小剛丸。我們將此檢測器置於模擬攝影X光機的床上,並將此檢測器的中心移至模擬攝影機的放射線(Beam)中心(isocenter)且此檢測器上其中的一條十字線平行於模擬攝影機的機臂(Gantry)旋轉軸。我們旋轉模擬攝影機機臂使得此檢測器上的小鋼珠以一個三維的方式呈現,來驗證正交片重建術(orthogonal-films reconstruction tech.)、兩任意角度片子重建術(2-films-with-variable-angles reconstruction tech.)及三任意角度片子重建術(3-films-with-variable-angles reconstruction tech.)。然後我們對著此檢測器照片子,並將片子上小鋼珠的影像輸入電腦治療計劃系統,由電腦治療計劃系統重建每顆小鋼珠在三度空間的位置後與其真正在空間中的位置來比較。

實驗結果告訴我們,誤差與物體到isocenter的距離有關而與物體之放大率無關。 平均誤差小於1 mm 是合於Roué et al. [Radiother. Oncol. 78, 78-83 (2006)]所定的容忍值。然而根據我們的誤差模型,我們發現到如果物體距isocenter 20 cm 以上,並且機頭(Collimator)與機臂(Gantry)旋轉計讀值誤差為0.5度,電腦重建出物體的位置誤差將大於2 mm。因此我們建議除了做好機頭與機臂旋轉計讀值的品質保證外,臨床治療病人的時候,應儘量將病人要治療的部位移至與isocenter愈近愈好。這個驗證技術適用於不同的治療系統,並且可讓人選擇出對自己臨床最適用的片子重建技術。
Improving the local control rate of cancer patients is always the goal of radiation therapy oncologists. To reduce complications to normal tissue, the cavity boost is usually accomplished with the use of high dose rate remote afterloading systems (HDR), which assemble the radionuclide, Ir-192. To improve the local control rate, the first step is giving the right dose to the right position.

For giving the right dose to the patient, a simple, practical and economical technique was proposed to calibrate the air kerma strength of Ir-192 HDR. This technique makes use of the 0.6cc farmer chamber, radiographic film and polystyrene phantom. These tools are commonly used for the dosimetry quality assurance of the clinical linear accelerator. In this study, the Exradin A19, PTW N30004 and TM30001 farmer chambers were used for the calibration. To perform the calibration, a 25.4´30.5 cm2 radiographic film was taped onto a piece of polystyrene plate, and the applicator and farmer chamber were affixed to the film envelope. The film was irradiated by the 192Ir source, followed by exposure to the simulator X-ray beam. The film set with the film removed was then placed on a 5 cm thick polystyrene phantom for calibration measurement. Based on the electrometer reading from the farmer chamber irradiated by 192Ir and the measured source-to-chamber distance by means of the images on the developed film, we can calculate the air kerma strength of the 192Ir using the new technique. Our calibration results were compared with those of well chambers of several different brands and the data provided by the manufacturer. The differences were all within 1.5%. Compared to the “7-distance measurement technique” by Stump et al. [Med. Phys. 29, 1483–1488 (2002)], our method is more efficient (one instead of seven measurements). In summary, we have introduced a simpler and more cost effective method for those hospitals without a calibration jig or well chamber.

With regard to the dose delivering to the right position, we presented a method that employs standard linear accelerator QA tools to verify the accuracy of the film reconstruction algorithms used in the brachytherapy planning system. Verification of reconstruction techniques is important, as suggested in the ESTRO booklet 8: “The institution should verify the full process of any reconstruction technique employed clinically”. We also performed error modeling to analyze seed-position errors. The “Isocentric Beam Checker” device, which has a two-dimensional array of steel balls embedded on its surface, was used for this. The Checker was placed on the simulator couch with its center ball coincident with the simulator isocenter, and one axis of its cross marks parallel to the axis of gantry rotation. We rotated the gantry of the simulator to make the Checker behave like a three-dimensional array of balls. Three algorithms were used in the Abacus treatment planning system, and were tested: orthogonal-films, 2-films-with-variable-angles and 3-films-with-variable-angles. After exposing and digitizing the films, the position of each steel ball on the Checker was reconstructed and compared with its true position, which can be accurately calculated. The results showed that the error was dependent on the object-isocenter distance, but not the magnification of the object. The averaged errors were less than 1 mm within the tolerance level defined by Roué et al. [Radiother. Oncol. 78, 78-83 (2006)]. However, according to our error modeling, the theoretical error would be greater than 2 mm if the objects were located more than 20 cm away from the isocenter with a 0.5o reading error of the gantry and collimator angles. Thus, in addition to carefully performing the QA of the gantry and collimator angle indicators, we suggest that the patient, together with the applicators or seeds inside, should be placed as close to the isocenter as possible. This method could be used to test the reconstruction techniques of any planning system, and the most suitable one can be chosen for clinical use.
Chinese Abstract………………………………………………………i
Abstract…………………………………………………………………iv
Preface……………………………………………………………vii
Table of Contents…………………………………………………viii
List of tables………………………………………………………x
List of Figures………………………………………………….xiii
Nomenclature………………………………………………………….xvi

Chapter 1: Introduction……………………………………………1
1.1 Introduction of Brachytherapy by HDR…………………1
1.2 The aims of this study……………………………………3

Chapter 2: Materials and Methods…………………………………9
2.1 Dosimetry aspect……………………………………………9
2.1.1 Study of room scatter for farmer chambers…………11
2.1.2 Study of wall scatter for well chambers…………20
2.1.3 Modeling of the new technique…………………………28


2.1.4 Uncertainty evaluation of the new technique regarding the scatters………………………………………………………31
2.1.5 Application of the new technique…………………………33
2.2 Mechanical aspect…………………………………………35
2.2.1 Verification of film reconstruction techniques……35
2.2.2 Method to accurately calibrate the gantry angle indicators of the radiotherapy linear accelerators…………41
2.2.3 Error modeling for the orthogonal-films algorithm and the 2-films-with-variable-angles algorithm…………………55

Chapter 3: Results and Discussion………………………………59
3.1 Dosimetry aspect………………………………………………59
3.2 Mechanical aspect………………………………………………64

Chapter 4: Conclusion ..……………………………………70

References…………………………………………………………75
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