強度調控放射治療之表面劑量測量

Surface Dose Measurement in Intensity Modulated Radiotherapy

劉國基(Kuo-Chi Liu) ； 蕭安成(An-Cheng Shiau) ； 趙敏(Max-Ming Chao) ； 梁基安(Ji-An Liang) ； 楊世能(Shin-Neng Yang) ； 蘇經雄(Ching-Hsiung Su) ； 施文彬(Wen-Pin Shih) ； 陳信雄(Hsin-Hsiung Chen)

《放射治療與腫瘤學》 14卷2期 (2007/06) Pp. 113-121

https://doi.org/10.6316/TRO/200714(2)113

摘要

目的：本文主要評估熱發光劑量計在高能光子射束增建區劑量量測的準確性， 以建立臨床上量測強度調控放射治療（Intensity Modulated Radiotherapy, IMRT ） 頭頸部病患表面劑量之評估方法。材料與方法：本實驗主要包括三個部份：(1) 平行板游離腔於假體中增建區劑量測量；(2) 熱發光劑量計於假體中增建區劑量測量；(3) 熱發光劑量計應用於擬人形假體頭頸部表面劑量之測量。實驗使用Varian 21EX 直線加速器所產生的6 MV 光子射束。量測設備包括Markus 游離腔，以及厚度分別為0.89 mm、0.38 mm 及0.1 mm的熱發光劑量計。在垂直入射光子射束量測方面，Markus 游離腔增建區劑量量測數據， 均作適當修正，以得到較準確的百分深度劑量值，隨後與不同厚度之熱發光劑量計量測值做評估比較，以確認其不確定度。傾斜入射光子射束之假體表面量測方面，則以0.1mm厚度之熱發光劑量計與Markus 游離腔進行測量及評估比較。在擬人形假體頭頸部表面劑量的量測方面， 採用0.1 mm 厚度之熱發光劑量計之測量值與治療計劃結果進行比較。結果：增建區劑量量測方面，0.1 mm厚度之熱發光劑量計之測量結果與Markus游離腔（經過修正後之測量值）有較好的吻合度，適用於臨床表面劑量的量測。實際量測IMRT 表面劑量方面，治療計劃計算值與超薄型熱發光劑量計量測值的比較發現，大部份計算值在表面部位有明顯低估的情形。討論：不同厚度之熱發光劑量計應用於增建區劑量量測結果差異明顯，超薄型熱發光劑量計適用於表面劑量測量，臨床上，其測量值可藉以作為治療計劃之比較參考。

並列摘要

Purpose : The purpose of this study is to evaluate the accuracy of dose measurements in buildup region for high energy photon beams by using the thermoluminescence dosimeters. The method in evaluation of surface dose in intensity modulated radiotherapy for head and neck cancer has also been developed in this study. Materials and Methods : The measurements included: (1)measurements of buildup dose in phantom by plane-parallel ionization chamber; (2) measurements of buildup dose in phantom by thermoluminescence dosimeters; (3) measurements of surface dose on head and neck Rando phantom by using thermoluminescence dosimeters. Experiments were performed for 6 MV x-ray beams generated by a Varian 21EX linear accelerator. The instruments included the Markus plane- parallel ionization chamber and thermoluminescence dosimeters with 0.89 mm, 0.38 mm, and 0.1
mm in thickness, respectively. For normal incident beams, the dose measured in buildup region were corrected with suitable correction equations for plane-parallel ionization chamber and then evaluated the accuracy of the dose measurements in buildup region by using various thickness thermoluminescence dosimeters. The dose at phantom surface for obliquely incident beams were also measured and evaluated by the ultra-thin (0.1 mm) thermoluminescence dosimeters and plane-parallel ionization chamber for small field size (5 cm × 5 cm). For surface dose on the head and neck Rando phantom, the ultra-thin thermoluminescence dosimeters measurements were compared with the treatment planning system calculations.
Results : The dose measured in buildup region by using the ultra-thin thermoluminescence dosimeters and Markus chamber are in good agreement and the former is suitable for surface dose measurements in clinical application. The underestimation of surface dose was found in treatment planning system calculation results in comparison with ultra-thin thermoluminescence dosimeters measurements on head and neck Rando phantom.
Discussion : The dose measured in buildup region is markedly different by using TLDs in different thickness. The ultra-thin TLD is suitable for surface dose measurement, and provides the comparable reference in clinical treatment planning.