電子射束的有效射源至表面距離之特性研究

Quality Characteristics for Effective Source to Surface Distance of LINAC Electron Beam

鐘兆麒(CHAO-CHI CHUNG)

指導教授 ： 潘榕光

中臺科技大學/健康科學院/醫學影像暨放射科學系暨研究所/碩士(2017年)

摘要

本研究主要評估直線加速器電子射束有效射源至表面距離(effective source to surface distance, SSDeff)測量之相關特性研究。因為在大多數臨床的病例中，劑量輸出可利用有效射源位置以距離平方反比定律準確地預測。本研究是使用Elekta全功能Axesse 6D直線加速器各個不同電子能量(5, 6, 8, 9, 12, 14 MeV)的電子射束特性與劑量分布，及不同電子錐筒照野(6×6, 6×10,和10×10 cm2)，搭配使用Sun Nuclear Corporation (SNC)自動化三度空間水箱掃描系統，快速精準的將游離腔置於水箱中不同深度或不同的隙距(gap)進行量測，探討SSDeff與電子錐筒照野和電子能量各種組合的依存性。其中，四個相關因子: 電子能量、電子錐筒照野、最大劑量深度及隙距，分別具有3-26階，所以共有6×3×6×26=2808種組合模式。故本研究依據田口實驗設計方法，將其濃縮至207組進行實際測量，每組量測重複三次，以減少誤差並確保再現性。結果顯示，電子射束的有效射源至表面距離若未經修正，可能會造成實際劑量計算超過5 %以上的誤差。利用自動化3D水箱精準量測不同的電子錐筒照野與電子能量組合，做有效率之電子射束特性之量測，並將導出的數據整合為具有11個係數的一階非線性半經驗公式組合，可做廣泛、準確之預測及隨機驗證，並大幅降低使用固態水假體之工作負荷，以達提升工作之效能。

關鍵字

電子射束 ； 有效射源至表面距離(SSDeff) ； 半經驗公式

並列摘要

Quality characteristics for effective source to surface distance (SSD) of LINAC electron beam were evaluated in this work. In some cases of extended SSD, output cannot be accurately predicted by using the inverse square law from the nominal source position (X-ray target) because the electrons scatter from various locations such as the scattering foils, the X-ray jaws, and the electron applicator. For most cases of clinical significance, however, the output can be accurately predicted by using the inverse square law from an effective source position. The distance from this effective source position to the patient surface is called the "effective SSD" (SSDeff) and is well known to be a function of beam energy and cone size. We had determined the dependence of SSDeff on various combinations of cone size, and beam energy. An Axesse 6D LINAC was adopted and the four operated factors: beam energy, cone size, dmax, and gap were preset into 3-26 levels, respectively in this work. Thus a total of 6×3×6×26=2808 combinations was considered and condensed to only 207 trials of effective measurements according to Taguchi’s optimal recommendation. Each trial was measured three times to reduce the error and ensure the reproducibility. The derived data were then integrated altogether to compose a 1st-order nonlinear semi-empirical formula with 11 coefficients. The summarized uncertainties were suppressed to below 5 % for the estimation. The comparison between theoretical and practical evaluations was depicted and the correlated discussion were also emphasized point by point to imply the importance of this revised evaluation in clinical field.

並列關鍵字

electron beam ； effective source-to-surface distance (SSDeff) ； semi-empirical formula

參考文獻

[1] F.M. Khan : The Physics of Radiation Therapy. 5nd ed. 2013: 265-308

[2] G.C. Bentel, McGraw-Hill : Radiation therapy planning. 2nd ed. , 1996; 219-259.

[3] 葉世安、吳嘉明、蕭光吟、趙敏、陳宗哲：放射治療電子射線虛擬射源位置點對輻射劑量計算及分佈影響之探討。台灣應用輻射與同位素雜誌；2009；5(4)：793-804

[4] I. Chetty, J.J. DeMarco, T.D. Solberg.:A virtual source model for Monte Carlo modeling of arbitrary intensity distributions, Medical Physics, 2000; 27(1), 166–172

[5] 邱詠翔、陳儀男、林招膨、馬玉麟：電子射束有效SSD位置量測與劑量計算之研究。台灣應用輻射與同位素雜誌；2010；6(2)：857- 862