電子射束遮擋照野劑量輸出之研究

目的：電子射束不規則形遮檔照野的劑量輸出尚無法以任何數學公式予以正確預估及計算，本篇針對Varian CL-1800 sn:164直線加速器各電子限光筒，各能量電子射束分別測量由全開到1×1cm^2正方形照野的輸出，觀察其輸出與遮擋面積之變化關係，進而建議臨床電子射束不規則形照野劑量之修正方法。材料與方法：採用CERROBEND鉛合金製作面積不同，範圍為1×1cm^2至10×10cm^2，step size=1cm及15×15cm^2之正方形遮檔鉛塊，置於Varian CL-1800 sn:164之限光筒底部，以N.E. 0.03 c.c.平板型游離腔(Type:2534)置於RMI PMMA固態假體射束中心軸最高劑量點深度，對20×20cm^2，15×15cm^2, 10×10cm^2及6×6cm^2電子限光筒及能量為4,6,9,12,15 MeV的電子射束測量其輸出，並將其數值歸一到全開照野的輸出，以觀察電子射線在同一限光筒中，因照野縮小後其劑量的變化惰形；並以數塊臨床病患之鉛塊經測量及計算互相比對其誤差。結果：20×20及15×15cm^2的限光筒，對各能量而言，照野面積小於5×5cm^2以下時輸出明顯降低；10×10cm^2的限光筒，照野面積小於4×4cm^2以下；6×6cm^2的限光筒，照野面積小於3×3cm^2以下輸出亦明顯降低，若遮擋照野不小於以上數值則其輪出不必特別修正。結論：不規則形照野電子射束劑量輸出的變化無法如光子射束一般可用SAR值及Clarkson法子以精確計算，由本實驗的結果可知：若遮擋面績不超過劑量衰減點則其輸出不必特別修正：若遮擋面續超過劑量衰減點則可用 M D Mills所提出的方法進行劑量計算。

關鍵字

電子射束；小照野劑量輸出；劑量計算

並列摘要

Purpose: Computer summation algorithms for irregular electron fields are not widely available. In this study, we measured the output factors of each electron energy and each electron cone (except 25× 25 cm^2 applicator) of Varian Cl-l800 sn: 164 linear accelerator. The cut-out size is ranging from l×1 cm^2 to l0×10 cm^2 stepped by 1 cm for l0×10 cm^2 and 6×6 cm^2 electron cones and 15×15 cm^2 for 20×20 cm^2 cone. According to the measured data, we will observed the variation of the output factors and suggest the method for irregular electron field output correction. Materials & Methods: Cut-out BSBs(Beam-Shaping-Blocks) were made of CERROBEND alloy to put into the frame of each applicator. The output was measured with 0.0 3cc parallel plate ionization chamber be setted at the dmax of RMI PMMA solid phantom and connected to lonex Dosemaster 2590 electrometer. The readings were corrected with temperature, pressure and be normalized to its full open cone output. Results: For each energy and 15×15 cm^2, 20×20 cm^2 electron cones, the output factors decreases rapidly when the cut-out size is less than 5×5 cm^2. For 10 ×10 cm^2 cone, the output is not decrease when the cut-out size is larger than 4×4 cm^2. For 6×6 cm^2 cone, the output is the same when the cut-out size is not less than 3×3 cm^2. Conclusion: There is no simple mathematical function just like the SAR method and Clarkson mehtod that allows us to predict the output for a given field size and electron energy. According to the results of this measurement, the output of the irregular field can be corrected by Mills method if the cutout area less than the point of curve decreasing.

並列關鍵字

Electron beam ； Cutout field output ； Dosimetry

國際替代計量

電子射束遮擋照野劑量輸出之研究

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