預防動靜脈瘻管阻塞之狹長型電子射束劑量分析

目的：本研究是想瞭解電子射束在狹長型照野中所產生的劑量特性，量測其 (1) 中心軸百分深度劑量值（percent depth dose of central axis）、(2) 照野因子（output factor）、(3) 劑量分佈（isodose distribution），提供一套劑量參考系統，不僅對於未來動靜脈管放射線預防阻塞上能有所助益，也可提供小範圍的皮膚或頭頸部病灶之電子射線治療確切的劑量遵循參考。材料與方法：針對臨床上較常使用的八種照野來做量測與分析，包含2×5、3×5、4×5、5×5、2×10、3×10、4×10、5×10㎝2，所選擇的能量為動靜脈管較常使用的三種能量，包含6、8、10 MeV，而使用的直線加速器為Siemens KDS-2並搭配95㎝的10×10㎝2電子錐筒（electron cone），而射源到假體表面距離（source-to-surface distance, SSD）為100㎝。量測其 (1) 中心軸百分深度劑量值（percent depth dose of central axis）、(2) 照野因子（output factor）、(3) 劑量分佈（isodose distribution）。為了產生狹小照野，必須利用電子射束遮擋塊（electron block）來達到目的，而遮擋塊放置的位置會影響電子射束的劑量分佈，所以在本研究中也比較兩種遮擋塊位置（置於電子錐筒及假體表面）不同所造成的劑量分佈、百分深度劑量值及照野因子的改變，以提供日後臨床使用時參考。結果與討論：在百分深度劑量方面可以觀察出當深度小於dmax時，狹長型照野相對於正方形照野有較高的百分深度劑量值；而深度大於dmax時，卻有相反的趨勢。此外，針對遮擋塊位於電子錐筒下方與假體表面的研究顯示，當遮擋塊位於電子錐筒下方時有較少的低能量散射電子，使得百分深度劑量值的表現介於正方形照野與遮擋塊位於假體表面二者之間。在照野因子方面，當照野較小時，照野因子也較小，此外，當遮擋塊位於電子錐筒下方時，照野因子比沒有遮擋鉛塊有更劇烈的變化。在等劑量曲線分佈方面當遮擋塊位於電子錐筒下方時，等劑量曲線呈現較為鬆散的分佈。因此臨床上，遮擋塊的位置應儘可能的接近體表。等劑量曲線在高劑量區有明顯內縮情形，當能量愈大及照野愈小時，其內縮情況愈嚴重。因此，由本實驗可提供一套針對小而狹長型電子射束照野的劑量參考系統，不僅對於未來動靜脈管放射線預防再阻塞上能有所助益，也可應用於小範圍的皮膚照射或頭頸部病灶之電子射線治療劑量計算所參考。

關鍵字

電子射線；動靜脈瘻管；狹長型照野

並列摘要

Purpose : The purpose of this study is to evaluate dose distribution of small and elongated electron beam fields for further reference in arterio-venous fistula treatment. The percent depth dose, isodose curve, and output factor were measured as a function of electron energies and field sizes to provide a practical reference dose system. Material and Methods : According to clinical practices , eight field sizes were measured (including 2 ×5, 3 ×5, 4 ×5, 5 ×5, 2 ×10, 3 ×10, 4 ×10, 5 ×10 cm2) and energy of 6, 8, 10 MeV generated by SIEMENS KDS-2 Linac were used in this study. The distance from source to surface (SSD) is 100cm. To generate small and elongated field, the electron blocks were used in our study, and percent depth dose, isodose curves, and output factor were also compared with blocks on electron cone and on phantom surface. Result and Discussion : The percent depth dose, isodose curve, and output factor were measured as a function of electron energies and field sizes. The Dmax shifts towards the surface as the electron beam field size reduced. The smaller the field sizes, the greater the surface dose, and the dose fall-off region becomes more gradual. If the mean size of arterio-venous fistula is 1.5 cm in length and 5mm in width, the safe margin should add 8mm more for long and short axis to cover treatment target adequately. The closer between cerrobend blocks and skin, the better the dose distribution will be. Conclusion : The dialysis accesses (arterio-venous fistula) are suitable for electron-beam therapy because of their superficial location. The electron fields for arterio-venous fistula are usually small and elongated, dose distribution are usually unpredictable by conventional calculation table. The study provides a practical reference dose system for small and elongated field size electron beam dose calculation not only for arterio-venous fistula therapy, but also for skin, head and neck area treatment.

並列關鍵字

Electron beam ； Arteriovenous fistula ； Small and elongated fields

國際替代計量

預防動靜脈瘻管阻塞之狹長型電子射束劑量分析

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