Department of Radiation Oncology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA.
Med Phys. 2010 Oct;37(10):5228-37. doi: 10.1118/1.3483782.
The aim of the study was to quantify the effect of miniphantoms on in-air output ratio measurements, i.e., to determine correction factors for in-air output ratio.
Monte Carlo (MC) simulations were performed to simulate in-air output ratio measurements by using miniphantoms made of various materials (PMMA, graphite, copper, brass, and lead) and with different longitudinal thicknesses or depths (2-30 g/cm2) in photon beams of 6 and 15 MV, respectively, and with collimator settings ranging from 3 x 3 to 40 x 40 cm2. EGSnrc and BEAMnrc (2007) software packages were used. Photon energy spectra corresponding to the collimator settings were obtained from BEAMnrc code simulations on a linear accelerator and were used to quantify the components of in-air output ratio correction factors, i.e., attenuation, mass energy absorption, and phantom scatter correction factors. In-air output ratio correction factors as functions of miniphantom material, miniphantom longitudinal thickness, and collimator setting were calculated and compared to a previous experimental study.
The in-air output ratio correction factors increase with collimator opening and miniphantom longitudinal thickness for all the materials and for both energies. At small longitudinal thicknesses, the in-air output ratio correction factors for PMMA and graphite are close to 1. The maximum magnitudes of the in-air output ratio correction factors occur at the largest collimator setting (40 x 40 cm2) and the largest miniphantom longitudinal thickness (30 g/cm2): 1.008 +/- 0.001 for 6 MV and 1.012 +/- 0.001 for 15 MV, respectively. The MC simulations of the in-air output ratio correction factor confirm the previous experimental study.
The study has verified that a correction factor for in-air output ratio can be obtained as a product of attenuation correction factor, mass energy absorption correction factor, and phantom scatter correction factor. The correction factors obtained in the present study can be used in studies involving in-air output ratio measurements using miniphantoms.
本研究旨在量化微型体模对空气中输出比测量的影响，即确定空气中输出比的校正因子。
使用蒙特卡罗（MC）模拟分别模拟了由不同材料（PMMA、石墨、铜、黄铜和铅）制成的微型体模，以及在光子束为 6MV 和 15MV 时具有不同纵向厚度或深度（2-30g/cm2）的微型体模，在准直器设置范围从 3x3 到 40x40cm2 的情况下进行了空气中输出比测量。使用了 EGSnrc 和 BEAMnrc（2007）软件包。从线性加速器上的 BEAMnrc 代码模拟获得了与准直器设置相对应的光子能谱，并用于量化空气中输出比校正因子的组成部分，即衰减、质量能量吸收和体模散射校正因子。计算并比较了微型体模材料、微型体模纵向厚度和准直器设置作为函数的空气中输出比校正因子，并与以前的实验研究进行了比较。
对于所有材料和两种能量，空气中输出比校正因子随准直器开口和微型体模纵向厚度的增大而增大。在较小的纵向厚度下，PMMA 和石墨的空气中输出比校正因子接近 1。在最大准直器设置（40x40cm2）和最大微型体模纵向厚度（30g/cm2）下，空气中输出比校正因子达到最大值：6MV 为 1.008 +/- 0.001，15MV 为 1.012 +/- 0.001。空气中输出比校正因子的 MC 模拟验证了以前的实验研究。
本研究验证了可以将空气中输出比的校正因子作为衰减校正因子、质量能量吸收校正因子和体模散射校正因子的乘积获得。本研究中获得的校正因子可用于使用微型体模进行空气中输出比测量的研究。
