Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia.
Phys Med Biol. 2010 Feb 7;55(3):783-97. doi: 10.1088/0031-9155/55/3/016. Epub 2010 Jan 14.
Backscatter factors are important parameters in the determination of dose for kilovoltage x-ray beams. However, backscatter factors are difficult to measure experimentally, and tabulated values are based largely on Monte Carlo calculations. In this study we have determined new backscatter factors by both experimental and Monte Carlo methods, and compared them with existing backscatter factors published in the AAPM TG-61 protocol. The purpose of this study is twofold: (1) to evaluate the overall effectiveness of using Gafchromic EBT film for backscatter factor measurements and (2) to determine whether existing Monte Carlo-calculated backscatter factors need to be updated. We measured backscatter factors using Gafchromic EBT film for three field sizes (2, 4 and 6 cm diameter cones) and three kilovoltage beam qualities, including 280 kVp for which similar measurements have not previously been reported. We also present new Monte Carlo-calculated backscatter factors obtained using the EGSnrc/BEAMnrc code system to simulate the Pantak kilovoltage x-ray unit used in our measurements. The results were compared with backscatter factors tabulated in the AAPM TG-61 protocol for kilovoltage x-ray dosimetry. The largest difference between our measured and calculated backscatter factors and the AAPM TG-61 values was found to be 2.5%. This agreement is remarkably good, considering that the AAPM TG-61 values consist of a combination of experimental and Monte Carlo calculations obtained over 20 years ago using different measurement techniques, as well as older Monte Carlo code and cross-section data. Furthermore, our Monte Carlo-calculated backscatter factors agree within 1% with the AAPM TG-61 values for all beam qualities and field sizes. Our Gafchromic film measurements had slightly larger differences with the AAPM TG-61 backscatter factors, up to approximately 2% for the 6 cm diameter cone at a beam quality of 50 kVp. The largest difference in backscatter factors, of 2.5%, was found between Monte Carlo-calculated and Gafchromic film-measured data for the 100 kVp x-ray beam with the 4 cm diameter cone. The differences in backscatter factors between the three data sets (measurements, calculations and published values) are all within the uncertainties from our Gafchromic film measurements and Monte Carlo calculations. Our results demonstrate the suitability of using Gafchromic EBT film to measure equipment-specific backscatter factors for kilovoltage x-ray beams over the entire energy range and also confirm that backscatter factors published in kilovoltage dosimetry protocols still remain valid.
反散射因子是确定千伏 X 射线束剂量的重要参数。然而，反散射因子很难通过实验测量，并且表列值主要基于蒙特卡罗计算。在这项研究中，我们通过实验和蒙特卡罗方法确定了新的反散射因子，并将其与 AAPM TG-61 协议中公布的现有反散射因子进行了比较。本研究的目的有两个：（1）评估使用 Gafchromic EBT 胶片测量反散射因子的整体效果，（2）确定是否需要更新现有的蒙特卡罗计算反散射因子。我们使用 Gafchromic EBT 胶片测量了三个射野大小（2、4 和 6 cm 直径圆锥体）和三种千伏射线质量的反散射因子，包括以前没有报道过的 280 kVp。我们还使用 EGSnrc/BEAMnrc 代码系统计算了新的蒙特卡罗计算的反散射因子，以模拟我们测量中使用的 Pantak 千伏 X 射线装置。结果与 AAPM TG-61 协议中为千伏 X 射线剂量学列出的反散射因子进行了比较。我们测量和计算的反散射因子与 AAPM TG-61 值之间的最大差异为 2.5%。考虑到 AAPM TG-61 值是使用不同的测量技术在 20 多年前获得的，并且使用了较旧的蒙特卡罗代码和横截面数据，这一结果非常好。此外，我们的蒙特卡罗计算的反散射因子与 AAPM TG-61 值在所有射线质量和射野大小下的差异都在 1%以内。我们的 Gafchromic 胶片测量值与 AAPM TG-61 反散射因子的差异略大，在 50 kVp 射线质量下，6 cm 直径圆锥体的差异最大，约为 2%。在 100 kVp X 射线和 4 cm 直径圆锥体的情况下，最大的反散射因子差异（2.5%）是在蒙特卡罗计算和 Gafchromic 胶片测量数据之间发现的。三个数据集（测量值、计算值和公布值）之间的反散射因子差异都在我们的 Gafchromic 胶片测量值和蒙特卡罗计算值的不确定度范围内。我们的结果表明，使用 Gafchromic EBT 胶片测量整个能量范围内千伏 X 射线束的设备特定反散射因子是合适的，并且还证实了在千伏剂量学协议中公布的反散射因子仍然有效。
