強度調控式放射治療之劑量驗證

Dose Verification for Intensity Modulated Radiation Therapy

梁基安(Ji-An Liang) ； 蕭安成(An-Cheng Shiau) ； 蘇經雄(Ching-Hsiung Su) ； 施文彬(Wen-Pin Shih) ； 潘佳穎(Jia-Ying Pan)

《放射治療與腫瘤學》 12卷3期 (2005/09) Pp. 227-233

https://doi.org/10.6316/TRO/200512(3)227

摘要

目的：強度調控式放射治療(Intensity-modulated radiation therapy，IMRT)是一項全新的治療技術，藉由逆向式電腦治療計畫系統及多葉式準直儀的密切配合，可以製造出前所未有的治療劑量分佈，使危急器官在放射治療的過程中接受到較低的劑量。此外，在保護正常組織的同時，並不會影響到腫瘤所該接受的劑量。但是在此複雜的技術下，劑量分佈的驗證便成為極為關鍵的工作。本研究的目的便是規劃一套劑量驗證的方法，以確保治療的品質。
材料與方法：本研究使用Varian 21EX直線加速器，搭配120片多葉式準直儀，可產生6 MV及10 MV射束。治療計畫系統是使用Varian Eclipse system (Ver. 7.1)。劑量驗證程序分三階段進行，一、病患治療計畫完成後利用電腦治療計畫系統的驗證計畫功能(create verification plan)轉植到固態水假體（30×30×20公分）。分為點劑量驗證及平面劑量分佈驗證。點劑量驗證與實際計畫有相同的射束安排，而平面劑量分佈驗證則將所有射束皆設定在垂直向下的角度來給予劑量。二、分析此計畫在假體內的劑量分佈，選擇劑量變化梯度較為緩和的區域作為點劑量量測的參考點。平面劑量分佈驗證的參考平面則選擇在等中心點的位置。三、以游離腔劑量量測系統實際量測驗證計畫的參考點劑量值，並且與電腦計算值做比較。以膠片、雷射掃瞄儀及Doselab膠片劑量分析軟體，分析及比較電腦治療計畫計算的二度空間平面劑量分佈與實際的劑量分佈的差異。
結果：在點劑量量測方面，分析124位病患的資料，與電腦治療計畫系統比較，其差異皆在±6%之內，其中89%的病患劑量差異小於±4%。在二度空間平面劑量分佈方面，每單－照野的等劑量曲線分佈，測量值與計算值皆有良好吻合。
結論：單點劑量量測可以提供準確的劑量差異比較，確保治療劑量的準確。二度空間平面劑量分佈的比較，可以確認IMRT治療在射束平面強度的分佈。本研究的結果證實所使用的方法可以用於校驗IMRT治療計畫，但是對於體內不均質組織，如肺臟、空腔及骨骼等部位之劑量，尚有待進一步發展劑量驗證之方法。

關鍵字

IMRT ； 治療計畫驗證 ； 品質保證 ； 膠片劑量量測

並列摘要

Purpose: Intensity modulated radiation therapy (IMRT) represents one of the most significant technical advances in radiation therapy. By using the inverse treatment planning system and mulitileaf collimator, it allows the clinical implementation of highly conformal dose distributions for target coverage and adjacent normal tissues sparing. However, these advances do not come without a risk. The clinical success or failure of each institution's IMRT program depends on the correct delivery of 3D dose distributions calculated by the planning system to the correct location in the patient. The objective of this study is to establish a dose verification procedure for IMRT planning.
Materials and Methods: A Varian 21EX linac with the 120 leaf multileaf collimator operated in 6 MV and 10 MV x-ray modes using dynamic and step and shoot delivery at 300 MU/min was used for all irradiations. Varian Eclipse planning system (Ver. 7.1) was used also. The procedure was designed for 3 steps. Step 1. Hybridize the patient plan to a solid water phantom using the ”create verification plan” in the Eclipse. Step 2. Recalculate the dose distribution in the phantom. Find area of low dose gradient and select points in this area for point dose verification. 2D dose distribution at isocenter for each portal are calculated for comparison. Step 3. The phantom is then irradiated according to this plan. The point doses are measured using CC01 ion chamber and the 2D dose distributions are measured using Kodak XV film or EDR2 film. Kodak LS75 laser scanner and DoseLab film analysis system are used for 2D dose comparison.
Results: The variations of point dose of 124 patients are within 6%. The calculated dose distributions are consistent to the measurements for the 2D dose comparison.
Conclusions: CC01 chamber is suitable for IMRT point dose verification. Film dosimetry system in this study can be used for IMRT 2D dose distribution comparison. IMRT dose verification can be done by this procedure.

並列關鍵字

IMRT ； Treatment verification ； Quality assurance ； Film dosimetry