Sudhyadhom A, Kirby N, Faddegon B, Chuang C F
Department of Radiation Oncology, University of California, San Francisco, San Francisco, California 94115.
Department of Radiation Oncology, University of California, San Francisco, San Francisco, California 94115 and Department of Radiation Oncology and Radiology, UTHSCSA, San Antonio, San Antonio, Texas 78229.
Med Phys. 2016 Mar;43(3):1507-13. doi: 10.1118/1.4941691.
High dose rate flattening filter free (FFF) beams pose new challenges and considerations for accurate reference and relative dosimetry. The authors report errors associated with commonly used ion chambers and introduce simple methods to mitigate them.
Dosimetric errors due to (1) ion recombination effects of high dose per pulse (DPP) FFF beams and (2) volume-averaging effects of the radial profile were examined on a TrueBeam STx. Four commonly used cylindrical ion chambers spanning a range of lengths (0.29-2.3 cm) and volumes (0.016-0.6 cm(3)) were used to determine the magnitude of these effects for 6 and 10 MV unflattened x-ray beams (6XFFF and 10XFFF, respectively). Two methods were used to determine the magnitude of ion collection efficiency: (1) direct measurement of the percent depth dose (PDD) for the clinical, high DPP beam in comparison to that obtained after reducing the DPP and (2) measurement of Pion as a function of depth. Two methods were used to quantify the magnitude of volume-averaging: (1) direct measurement of volume-averaging via cross-calibration and (2) calculation of volume-averaging from radial profiles of the beam. Finally, a simple analytical expression for the radial profile volume-averaging correction factor, Prp = OAR(0.29L), or the inverse of the off-axis ratio of dose at 0.29L, where L is the length of the chamber's sensitive volume, is introduced to mitigate the volume-averaging effect in Farmer-type chambers.
Errors in measured PDD for the clinical beams were 1.3% ± 0.07% and 1.6% ± 0.07% at 35 cm depth for the 6XFFF and 10XFFF beam, respectively, using an IBA CC13 ion chamber, due to charge recombination with a high DPP. Volume-averaging effects were 0.4% and 0.7% for the 6XFFF and 10XFFF beam, respectively, when measured with a Farmer-type chamber. For the application of TG-51, these errors combine when using a CC13 to measure the PDD and a Farmer for absolute output dosimetry for a total error of up to 2% at dmax for the 10XFFF beam.
Relative and absolute dosimetry in high DPP, unflattened x-ray beams of 10 MV or higher requires corrections for charge recombination and/or volume-averaging when dosimeters with certain geometries are used. Chambers used for PDD measurement are available that do not require a correction for charge recombination. A simple analytical expression of the correction factor Prp was introduced in this work to account for volume-averaging effects in Farmer chambers. Choice of an appropriate dosimeter coupled with application of the established correction factors Pion and Prp reduces the uncertainty in the PDD measurement and the reference dose measurement.
高剂量率无均整器（FFF）射束给精确的参考剂量测定和相对剂量测定带来了新的挑战和需要考虑的因素。作者报告了与常用电离室相关的误差，并介绍了减轻这些误差的简单方法。
在TrueBeam STx上研究了由于（1）高剂量每脉冲（DPP）FFF射束的离子复合效应和（2）径向剂量分布的体积平均效应导致的剂量测定误差。使用四个长度范围为0.29 - 2.3 cm、体积范围为0.016 - 0.6 cm³的常用圆柱形电离室，来确定6 MV和10 MV未均整X射线射束（分别为6XFFF和10XFFF）的这些效应的大小。使用两种方法来确定离子收集效率的大小：（1）直接测量临床高DPP射束的百分深度剂量（PDD），并与降低DPP后获得的结果进行比较；（2）测量Pion随深度的变化。使用两种方法来量化体积平均的大小：（1）通过交叉校准直接测量体积平均；（2）根据射束的径向剂量分布计算体积平均。最后，引入了一个简单的解析表达式，用于计算径向剂量分布体积平均校正因子Prp = OAR(0.29L)，即剂量在0.29L处的离轴比的倒数，其中L是电离室灵敏体积的长度，以减轻Farmer型电离室中的体积平均效应。
使用IBA CC13电离室时，对于6XFFF和10XFFF射束，在35 cm深度处临床射束测量的PDD误差分别为1.3% ± 0.07%和1.6% ± 0.07%，这是由于高DPP下的电荷复合所致。当使用Farmer型电离室测量时，6XFFF和10XFFF射束的体积平均效应分别为0.4%和0.7%。对于TG - 51的应用，当使用CC13测量PDD并使用Farmer电离室进行绝对输出剂量测定时，这些误差会叠加，对于10XFFF射束，在dmax处总误差高达2%。
在10 MV或更高能量的高DPP、未均整X射线射束中进行相对和绝对剂量测定时，当使用具有特定几何形状的剂量计时，需要对电荷复合和/或体积平均进行校正。有可用于PDD测量的电离室，不需要对电荷复合进行校正。在这项工作中引入了校正因子Prp的简单解析表达式，以考虑Farmer电离室中的体积平均效应。选择合适的剂量计并应用已确定的校正因子Pion和Prp，可降低PDD测量和参考剂量测量的不确定性。
