Carrasco P, Jornet N, Duch M A, Panettieri V, Weber L, Eudaldo T, Ginjaume M, Ribas M
Servei de Radiofísica i Radioprotecció, Hospital de la Santa Creu i Sant Pau, St. Antoni Maria-Claret 167, 08025 Barcelona, Spain.
Med Phys. 2007 Aug;34(8):3323-33. doi: 10.1118/1.2750972.
To evaluate the dose values predicted by several calculation algorithms in two treatment planning systems, Monte Carlo (MC) simulations and measurements by means of various detectors were performed in heterogeneous layer phantoms with water- and bone-equivalent materials. Percentage depth doses (PDDs) were measured with thermoluminescent dosimeters (TLDs), metal-oxide semiconductor field-effect transistors (MOSFETs), plane parallel and cylindrical ionization chambers, and beam profiles with films. The MC code used for the simulations was the PENELOPE code. Three different field sizes (10 x 10, 5 x 5, and 2 x 2 cm2) were studied in two phantom configurations and a bone equivalent material. These two phantom configurations contained heterogeneities of 5 and 2 cm of bone, respectively. We analyzed the performance of four correction-based algorithms and one based on convolution superposition. The correction-based algorithms were the Batho, the Modified Batho, the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system (TPS), and the Helax-TMS Pencil Beam from the Helax-TMS (Nucletron) TPS. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. All the correction-based calculation algorithms underestimated the dose inside the bone-equivalent material for 18 MV compared to MC simulations. The maximum underestimation, in terms of root-mean-square (RMS), was about 15% for the Helax-TMS Pencil Beam (Helax-TMS PB) for a 2 x 2 cm2 field inside the bone-equivalent material. In contrast, the Collapsed Cone algorithm yielded values around 3%. A more complex behavior was found for 6 MV where the Collapsed Cone performed less well, overestimating the dose inside the heterogeneity in 3%-5%. The rebuildup in the interface bone-water and the penumbra shrinking in high-density media were not predicted by any of the calculation algorithms except the Collapsed Cone, and only the MC simulations matched the experimental values within the estimated uncertainties. The TLD and MOSFET detectors were suitable for dose measurement inside bone-equivalent materials, while parallel ionization chambers, applying the same calibration and correction factors as in water, systematically underestimated dose by 3%-5%.
为了评估两种治疗计划系统中几种计算算法预测的剂量值，在具有水等效和骨等效材料的非均匀层体模中进行了蒙特卡罗（MC）模拟以及使用各种探测器进行测量。使用热释光剂量计（TLD）、金属氧化物半导体场效应晶体管（MOSFET）、平面平行电离室和圆柱形电离室测量百分深度剂量（PDD），并使用胶片测量射野轮廓。用于模拟的MC代码是PENELOPE代码。在两种体模配置和一种骨等效材料中研究了三种不同的射野尺寸（10×10、5×5和2×2 cm²）。这两种体模配置分别包含5 cm和2 cm厚的骨不均匀性。我们分析了四种基于校正的算法和一种基于卷积叠加算法的性能。基于校正的算法包括巴托算法、改进的巴托算法、Cadplan（瓦里安）治疗计划系统（TPS）中实现的等效组织空气比（TAR）算法以及Helax-TMS（核通）TPS中的Helax-TMS笔形束算法。卷积叠加算法是Helax-TMS中实现的坍缩圆锥算法。与MC模拟相比，所有基于校正的计算算法在18 MV时均低估了骨等效材料内的剂量。就均方根（RMS）而言，对于骨等效材料内2×2 cm²的射野，Helax-TMS笔形束算法（Helax-TMS PB）的最大低估约为15%。相比之下，坍缩圆锥算法得出的值约为3%。对于6 MV，发现了更复杂的情况，坍缩圆锥算法的表现较差，高估了不均匀性内3% - 5%的剂量。除了坍缩圆锥算法外，任何计算算法均未预测到骨 - 水界面处的剂量重建以及高密度介质中的半值层收缩，并且只有MC模拟在估计的不确定性范围内与实验值匹配。TLD和MOSFET探测器适用于骨等效材料内的剂量测量，而平行电离室在应用与水中相同的校准和校正因子时，系统地低估剂量3% - 5%。
