Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study
Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study
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DOI：
10.1118/1.4868695
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发表时间：
2014-04-01
期刊：
影响因子：
3.8
通讯作者：
Andreo, Pedro
中科院分区：
文献类型：
--
作者：
Benmakhlouf, Hamza;Sempau, Josep;Andreo, Pedro
目的：测定来自两家制造商的空气和液体电离室、硅二极管和金刚石探测器在6 MV小光子光束中的特定输出校正因子k(Qclin,Qmsr)（fclin,fmsr）。方法：根据Alfonso等人发表的国际形式定义的场输出因子[Med. Phys. 35, 5179-5186(2008)]，将小光子光束的剂量学与机器特定参考场的剂量学联系起来；它们包括对探测器读数测量比率的修正，通常用作宽光束的输出因子。输出校正因子用统计不确定度（a型）为0.15%或更低的PENELOPE蒙特卡罗（MC）系统计算。探测器的几何形状使用制造商提供的蓝图进行编码，并使用瓦里安Clinac iX 6 MV直线加速器作为源，用于0.5 x 0.5 cm(2)和10 x 10 cm(2)之间的场大小的相空间文件。在每个小场和参考光束为10 × 10 cm的情况下，在10 cm的深度，对探测器内的吸收剂量和没有探测器时的小体积进行评分，确定输出校正因子(2)。结果：用蒙特卡罗计算得到的液体电离室和金刚石探测器的输出校正因子，即使在最小的场尺寸下，误差也在+/- 1%以内。正如预期的那样，由于空腔尺寸的原因，发现对小型空气电离室的修正是重要的。硅二极管的校正系数随探测器类型（屏蔽或非屏蔽）而变化，证实了其他作者的发现；对两家厂家的探测器进行了不同的修正。对各种探测器计算因子的差异进行了彻底的分析，并尽可能将结果与已发表的数据进行比较，这些数据通常是针对不同的加速器和使用EGSnrc MC系统计算的。差异被用来估计修正因子的b型不确定度。结合蒙特卡罗计算的a型不确定度，对综合标准不确定度进行了估计，并将其分配给各种估计的平均修正系数。结论：本研究为瓦里安Clinac iX 6毫伏加速器中多种探测器的输出校正因子提供了一组一致且具体的数据，有助于提高对小光子束物理学的理解。对于任何探测器，校正因子一般都不能被忽略，并且正如预期的那样，校正因子的大小随着场尺寸的减小而增大。由于目前可用的临床加速器类型数量减少，建议针对直线加速器模型和场大小专门给出探测器输出校正因子，而不是由于每种加速器模型使用不同的电子光斑尺寸和光子准直系统而必然随加速器类型和场大小而变化的光束质量指示符。(C) 2014年美国医学物理学家协会。
Purpose: To determine detector-specific output correction factors, k(Qclin,Qmsr)(fclin,fmsr) in 6 MV small photon beams for air and liquid ionization chambers, silicon diodes, and diamond detectors from two manufacturers.Methods: Field output factors, defined according to the international formalism published by Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)], relate the dosimetry of small photon beams to that of the machine-specific reference field; they include a correction to measured ratios of detector readings, conventionally used as output factors in broad beams. Output correction factors were calculated with the PENELOPE Monte Carlo (MC) system with a statistical uncertainty (type-A) of 0.15% or lower. The geometries of the detectors were coded using blueprints provided by the manufacturers, and phase-space files for field sizes between 0.5 x 0.5 cm(2) and 10 x 10 cm(2) from a Varian Clinac iX 6 MV linac used as sources. The output correction factors were determined scoring the absorbed dose within a detector and to a small water volume in the absence of the detector, both at a depth of 10 cm, for each small field and for the reference beam of 10 x 10 cm(2).Results: The Monte Carlo calculated output correction factors for the liquid ionization chamber and the diamond detector were within about +/- 1% of unity even for the smallest field sizes. Corrections were found to be significant for small air ionization chambers due to their cavity dimensions, as expected. The correction factors for silicon diodes varied with the detector type (shielded or un-shielded), confirming the findings by other authors; different corrections for the detectors from the two manufacturers were obtained. The differences in the calculated factors for the various detectors were analyzed thoroughly and whenever possible the results were compared to published data, often calculated for different accelerators and using the EGSnrc MC system. The differences were used to estimate a type-B uncertainty for the correction factors. Together with the type-A uncertainty from the Monte Carlo calculations, an estimation of the combined standard uncertainty was made, assigned to the mean correction factors from various estimates.Conclusions: The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model. (C) 2014 American Association of Physicists in Medicine.