多葉式準直儀弧形邊緣之參數設定對強度調控放射治療劑量之影響

The influence of rounded leaf end-modeling of MLC on dose delivery in IMRT

劉玉娟(Yu-chuan Liu)

指導教授 ： 陳信雄

中臺科技大學/健康科學院/放射科學研究所/碩士(2011年)

摘要

本研究主要目的在探討當電腦治療計畫接收測試時，在多葉式準直儀系統的參數設定中，會因弧型邊緣葉片設計的關係，導致電腦治療計畫中運算治療用的監測單位的參考點與實際治療的輻射位置點會產生些微差異，使得電腦治療計畫無法選取正確劑量剖面圖的位置，導致計算值與臨床劑量的實測值不符合。此情況在半照野的情況下最為明顯。故本實驗運用電腦治療計畫內之多葉式準直儀偏移量的設定，讓治療計劃的運算位置修正接近至最接近實際治療的位置點。
實驗的設備有Elekta Precise直線加速器，能量為6 MV，10 MV兩種光子能量，電腦治療計畫系統為ADAC Pinnacle3，version 7.6。使用水假體與輻射變色膠片兩種驗證工具做為半照野輻射劑量的測量。另外分析由直線加速器治療機頭到多葉式準直儀葉片的幾何關係，計算當射源經多葉式準直儀弧形邊緣中心點(Ymlc)和中心軸截面的交會點與實際測量的輻射劑量50%位置點到等中心點的差異，此實際輻射位置點與幾何關係計算出的位置差值，就是在電腦治療計畫中必須做運算位置修正的偏移修正量。最後，使用二維陣列(seven 29)游離腔測量平面劑量的驗證，法墨式游離腔與固態水假體作點的臨床劑量驗證。
實驗結果顯示，在半照野進行水假體的實驗測量，在6 MV光子能量的50 %輻射劑量處為1.12 cm，10 MV光子能量的50 %輻射劑量處為1.45 cm；輻射變色膠片的實驗測量結果為6 MV光子能量的50 %輻射劑量處為1.13 cm，10 MV光子能量的50 %輻射劑量處為1.4 cm；這也就是當電腦治療計劃使用多葉式準直儀的幾何位置當做運算的位置點時，會與實測的結果出現差異的原因。經過偏移量的修正後，臨床劑量的驗證從未修正前的-5.5~-7 %，減少差異為1.0 %內。

關鍵字

多葉式準直儀弧形邊緣 ； 偏移量 ； 電腦治療計畫

並列摘要

Abstract
In this work we address an experimental calibration and quality assurance of MLC system in conjunction with treatment planning system. For rounded leaf end MLCs, it is generally not possible to make both the light and radiation field edges agree with the digital readout, offset calibrations are illustrated in this work using Elekta’s MLC system.
If the isocenter is splitted by MLC, the dose verification result would be affected significantly by the round leaf end position.
Measurements were performed with Elekta Precise medical linear accelerator at photon energy of 6 and 10 MV. EBT Gaf Chromic film embedded in solid water phantom was used for the measurement of dose profiles. Profiles were also done by water phantom at the same depth of 10 cm. A calibration to convert from raw scanner-signal to dose was done. Epson XL 10000 scanner and associated software was employed for imaging the films. There are three procedures to perform MLC offset correction in ADAC Pinnacle3, version 7.6 treatment planning system. First, light field adjustment, second, implemented dose profiles, and then offset correction.
With offset correction of the MLC split dose profile in the treatment planning, the dose error could be cut down from several percent to be within -1~2 %.
The default offset correction in treatment planning system provided only for reference. This study shows the dose delivery affected by round leaf end position in IMRT if correction has not be dose corrected.

並列關鍵字

offset ； rounded leaf-end of MLC ； treatment planning

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