Field Size and Depth Dependence of Wedge Factors

放射治療的方法上，時常會用到楔形濾片來作為修補病患體表曲度起伏所造成的組織落差，或作為改變劑量分布曲線，使多重射束所形成的靶區劑量分佈更均勻。但是當射束路徑中加上了楔形濾片，射束的輸出量就將受到影響。此影響包括：1．射束平均能量的改變；穿透力較大。2．散射量的變化；散射量隨照射到濾片的面積大小而變。但是，一般放射治療單位僅使用在參考照野和參考深度下所量出來的單一楔形濾片係數值，而忽略此係數值隨照野大小和治療深度所變化的量。本篇報告針對指稱能量6MV和10MV的光子射束，在Siemes KDS-2直線加速器外插式15°、30°、45°和60°楔形濾片的研究顯示，若僅用單一濾片修正係數，在6MV射束；45°濾片下，將可能在30×25平方公分的照野和15cm深的位置出現高達10.6%的劑量誤差。此外，在我們研究也顯示：較薄的濾片和較高的射束能量所表現出來的誤差較小。另外，針對電腦治療計劃系統的使用，我們也求出了隨照野和深度而變化的濾片係數值修正函數，其計算值與測量值的差異值在2%以內。

關鍵字

劑量學；直線加速器；放射治療學

並列摘要

The radiation output in the presence of wedge filters is characterized by the transmission factor and open beam field size factors. Conventionally, the wedge factor for high-energy photons is measured in a water phantom at a reference depth and a reference field size. However, experimental measurements on different wedges indicate that the wedge factors are a function of field size and depth. Therefore in the presence of a wedge in the beam, field size and depth dependent wedge factors may be necessary in the radiation treatment calculation. The wedge filters on a Siemens KDS-2 linear accelerator, with nominal energy 6 and 10MV, have nominal wedge angles of 15°, 30°, 45° and 60°. For 6MV with a 45° wedge, the use of a single wedge factor measured for 10×10cm^2 field at 5cm depth introduces errors of up to 10.6% for a maximum wedge field at 15cm depth. The use of a thinner wedge at higher energy exhibit less differences. A computer treatment planning system requires a correction function for a more accurate dose calculation in wedged fields. In this study, correction functions were established for each wedge angle and each beam energy. The differences of wedge factors between the measured data and correction function data are within 2%.

並列關鍵字

Dosimetry ； Linear accelerator ； Therapeutic radiology

國際替代計量

Field Size and Depth Dependence of Wedge Factors

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