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銥-192近接治療射源之劑量計算與測量研究

The Study of Dose Calculation and Measurement for 192IR Brachytherapy Source

摘要


目的:現今的近接治療方式大多以高劑量率(high dose rate, HDR)的遙控後荷式近接治療(remote afterloading brachytherapy)為主,而國內醫院大多採用Nucletron的micro-Selectron系列,此系列採用活度10居禮(Ci)的銥-192(^192Ir)近接治療射源。近接治療的劑量計算皆以電腦治療計畫系統運算,與micro-Selectron配合的系統為PLATO(PLATO Brachytherapy - INSIGHT^TM module system),PLATO是一套以TG-43作計算基礎的電腦治療計畫系統,為探討銥-192近接治療射源的劑量分布,本實驗擬利用熱發光劑量計(thermoluminescent dosimeter, TLD)實測與西弗積分(Sievert integral)計算兩種方式來與TG-43的結果比較。材料與方法:以Varian 2100C型醫用直線加速器作為標準射源篩選TLD,將篩選過的TLD與needle applicator置於特製水假體中,測量不同位置下的劑量值,另外,配合當時的活度計算出Sievert integral與TG-43的劑量值。結果:TLD的測量值與TG-43的計算值的結果,誤差皆在8%內。而Sievert integral的結果,不論與TLD的測量值或TG-43的計算值都有明顯的差異,尤其在越接近射源的位置越顯著,最大誤差可達22%,隨著徑向距離增加,Sievert integral的誤差值有減少的趨勢,在徑向距離大於6 cm後,誤差會大幅減少,可在10%以內。結論:利用TLD測量與TG-43的計算值的差異最大為8%;但Sievert integral的結果與其他兩者的差距在10-20%左右,顯示斜散射的貢獻會導致劑量被低估,且斜散射的影響會隨距離增加而減少。對於近接治療特有的陡峭劑量梯度變化,體積小的TLD能提供較佳的空間解析度。另外,由誤差分析可知,射源與TLD的左右偏移是本實驗最大的誤差來源。

並列摘要


Purpose: High dose rate (HDR) remote afterloading brachytherapy is the most common methods used for brachytherapy. Most hospitals in Taiwan used the Nucletron microSelectron system with 10 Ci 192Ir source for brachytherapy. The theory of dose calculation of Nucletron microSelectron system was based on TG-43. The aim of this study was to compare the result of TLD measurement the calculation results using the Sievert integral and TG-43.Material and Method: The TLDs and Varian 2100C LINAC were used to build the calibration curve. The selected TLD and needle applicator were put in the special positions in solid water phantom. The result of measurement was compared with Sievert integral and TG-43.Result: The variation between the result of measurement and calculation with TG-43 are in 8%, but the result of calculation with Sievert integral have large variation up to 22% between the others, especial in the region near source. The differences between three methods were decreased with large radial distances.Conclusion: The variation between the result of measurement and calculation with TG-43 were within 8%. The underestimate of dose for Sievert integral might be due to the oblique scatting, and the influence of oblique scatting would be decreased with the increasing of radial distance. The most contribution of error in this study might be come from the shift of TLD positions and the brachytherapy source position.

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