Chuter Robert W, Rixham Philip A, Weston Steve J, Cosgrove Vivian P
St James's University Hospital.
J Appl Clin Med Phys. 2016 Jan 8;17(1):112-120. doi: 10.1120/jacmp.v17i1.5686.
The feasibility of using portal dosimetry (PD) to verify 6 MV flattening filter-free (FFF) IMRT treatments was investigated. An Elekta Synergy linear accelerator with an Agility collimator capable of delivering FFF beams and a standard iViewGT amorphous silicon (aSi) EPID panel (RID 1640 AL5P) at a fixed SSD of 160 cm were used. Dose rates for FFF beams are up to four times higher than for conventional flattened beams, meaning images taken at maximum FFF dose rate can saturate the EPID. A dose rate of 800 MU/min was found not to saturate the EPID for open fields. This dose rate was subsequently used to characterize the EPID for FFF portal dosimetry. A range of open and phantom fields were measured with both an ion chamber and the EPID, to allow comparison between the two. The measured data were then used to create a model within The Nederlands Kanker Instituut's (NKI's) portal dosimetry software. The model was verified using simple square fields with a range of field sizes and phantom thicknesses. These were compared to calculations performed with the Monaco treatment planning system (TPS) and isocentric ion chamber measurements. It was found that the results for the FFF verification were similar to those for flattened beams with testing on square fields, indicating a difference in dose between the TPS and portal dosimetry of approximately 1%. Two FFF IMRT plans (prostate and lung SABR) were delivered to a homogeneous phantom and showed an overall dose difference at isocenter of ~0.5% and good agreement between the TPS and PD dose distributions. The feasibility of using the NKI software without any modifications for high-dose-rate FFF beams and using a standard EPID detector has been investigated and some initial limitations highlighted.
研究了使用门静脉剂量测定法（PD）来验证6兆伏无均整器（FFF）调强放射治疗（IMRT）的可行性。使用了一台配备Agility准直器的医科达Synergy直线加速器，该准直器能够提供FFF射束，以及一个标准的iViewGT非晶硅（aSi）电子射野影像装置（EPID）面板（RID 1640 AL5P），在160厘米的固定源皮距（SSD）下工作。FFF射束的剂量率比传统均整射束高出四倍，这意味着在最大FFF剂量率下采集的图像会使EPID饱和。发现800 MU/分钟的剂量率对于开放射野不会使EPID饱和。随后使用该剂量率来表征用于FFF门静脉剂量测定的EPID。使用电离室和EPID测量了一系列开放射野和模体射野，以便对两者进行比较。然后将测量数据用于在荷兰癌症研究所（NKI）的门静脉剂量测定软件中创建一个模型。使用具有一系列射野尺寸和模体厚度的简单方形射野对该模型进行了验证。将这些结果与使用Monaco治疗计划系统（TPS）进行的计算以及等中心电离室测量结果进行了比较。发现在方形射野测试中，FFF验证的结果与均整射束的结果相似，表明TPS和门静脉剂量测定之间的剂量差异约为1%。两个FFF IMRT计划（前列腺和肺部立体定向体部放疗（SABR））被照射到一个均匀模体上，结果显示等中心处的总体剂量差异约为0.5%，TPS和PD剂量分布之间具有良好的一致性。研究了在不对高剂量率FFF射束进行任何修改的情况下使用NKI软件以及使用标准EPID探测器的可行性，并突出了一些初始局限性。
