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多葉式準直儀弧形邊緣偏移修正對弧形調控放射治療之劑量影響

THE INFLUENCE OF ROUNDED LEAF CORRECTIONS FOR MLC IN VOLUMETRIC MODULATED ARC THERAPY

摘要


目的:Philips Pinnacle^3治療計畫系統提供多葉式準直儀葉片弧形邊緣偏移(rounded leaf offset, RLO)修正參數,可修正葉片位置與輻射照野邊緣位置之差異。本實驗將量測RLO修正量後,將此參數輸入治療計畫系統,藉此修正後葉片位置後重新計算臨床使用之弧形調控放射治療 (VMAT)計畫劑量分布,並評估結果是否改善。材料與方法:本實驗主要包括兩部份:(1)葉片弧形邊緣偏移修正測量:利用劑量系統測量不同葉片照野設定下,其輻射照野邊緣位置,藉以評估兩者間的差異;(2)葉片偏移量調整後之弧形調控放射治療之劑量評估:以測量之RLO修正參數輸入治療計畫系統後,計算弧形調控放射治療之劑量分布,此計算結果將與實際利用劑量測量系統在假體中量測到的結果比較。結果:偏移量測量結果顯示,在葉片位置越過中心射束軸其RLO介於-0.205 mm至-0.658 mm,除了在葉片位置-10 cm處時RLO與預設值差異約為1.6 mm以外,在-8 cm 至0 cm之RLO與預設值差異小於1 mm。而中心射束軸上測量到的修正項為0.538 mm,其餘在葉片位置 由2 cm至18 cm之修正量則介於0.655 mm 至-1.061 mm。經過RLO 調整後的治療計畫系統 計算弧形調控放射治療劑量皆比預設RLO 計算出的結果來的低。另外,實測5 個治療計畫之劑 量分布後,顯示修正後的點劑量比未修正所計算出的結果較接近實測值,其誤差減少約1.6%至3.4%;而利用二維陣列式游離腔測量劑量分布結果其加馬指標在3% / 3 mm 標準下介於94.4%至98.2%。結論:本研究以上述方式量測本科RLO修正值,由結果可看到與原預設值有明顯差異,經此方法 調整後的RLO修正值可提升治療計畫系統計算弧形調控放射治療劑量分布的準確度,如此也可確保病患接受放射治療時治療品質的保證。

並列摘要


Purpose : The purpose of the Rounded Leaf Offset (RLO) table was to take into account the differences between planning leaf positions and actual radiation leaf edges. Most users utilize the default RLO table supplied by the vendor. Our goal was to improve the dosimetric accuracy in Volumetric Modulated Arc Therapy (VMAT) by applying the measured RLO parameters. Materials and Methods : This study consists of two parts: (1)The radiation leaf edges at various positions were measured with ion chamber dosimetry system. The results were compared with the default table in the Treatment Planning System (TPS). (2) Five VMAT clinical plans were used to evaluate the overall dosimetric effects of RLO table. An ion chamber in a solid water phantom was used to determine the absolute dose at a reference point for each plan. The 2D dose distributions were measured with a 2D-Array dosimetry system. Results : The RLO measurements showed that the offsets were between -0.205 mm and 0.658 mm when the leaf positions crossed the central axis of the beam. The differences between the default and measured RLO values were less than 1 mm when the leaf positions were in the range of -8 cm and 0 cm, but at -10 cm position the difference was 1.6 mm. At the central beam axis, the measured correction was 0.538 mm and the others were in range of 0.655 mm and -1.061 mm for the leaf positions at 2 cm to 18 cm. After the modification of RLO table, the TPS calculated doses for the VMAT plans were lower than those calculated with default RLO table. Also, based on the measurements of 5 VMAT plans, the point doses improved about 1.6% to 3.4%, and the 2D dose distributions using 3% / 3 mm Gamma test achieved 94.4% to 98.2% passing rate. Conclusions : This study established the RLO correction values in our department. We found the default RLO values are obviously inaccurate for our machine. This adjustment of RLO table improves the dose calculation accuracy of TPS, in the meantime, ensures the quality of patients’treatments.

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