Xu Zhigang, Kim Jinkoo, Han James, Hsia An Ting, Ryu Samuel
Department of Radiation Oncology, Stony Brook University Hospital, Stony Brook, NY, USA.
J Appl Clin Med Phys. 2018 Jul;19(4):141-147. doi: 10.1002/acm2.12358. Epub 2018 May 21.
In this study we investigated the dose rate response characteristics of the Digital Megavolt Imager (DMI) detector, including panel saturation, linearity, and imager ghosting effects for flattening filter-free (FFF) beams. The DMI detector dose rate response characteristics were measured as a function of dose rate on a Varian TrueBeam machine. Images were acquired at dose rates ranging from 400 to 1400 MU/min for 6XFFF and 400 to 2400 MU/min for 10XFFF. Line profiles and central portal doses derived from the images were analyzed and compared. The linearity was verified by acquiring images with incremental Monitor Unit (MU) ranging from 5 to 500 MU. Ghosting effects were studied at different dose rates. Finally, for validation, test plans with optimal fluence were created and measured with different dose rates. All test plans were analyzed with a Gamma criteria of 3%-3 mm and 10% dose threshold. Our study showed that there was no panel saturation observed from the profile comparison even at the maximum dose rate of 2400 MU/min. The central portal doses showed a slight decrease (1.013-1.008 cGy/MU for 6XFFF, and 1.020-1.009 cGy/MU for 10XFFF) when dose rate increased (400-1400 MU/min for 6XFFF, and 400-2400 MU/min for 10XFFF). The linearity of the DMI detector response was better than 0.5% in the range of 20-500 MU for all energies. The residual image was extremely small and statistically undetectable. The Gamma index measured with the test plans decreased from 100% to 97.8% for 6XFFF when dose rate increased from 400 to 1400 MU/min. For 10XFFF, the Gamma index decreased from 99.9% to 91.5% when dose rate increased from 400 to 2400 MU/min. We concluded that the Portal Dosimetry system for the TrueBeam using DMI detector can be reliably used for IMRT and VMAT QA for FFF energies.
在本研究中，我们调查了数字兆伏成像仪（DMI）探测器的剂量率响应特性，包括平板饱和、线性度以及无均整器（FFF）射束的成像仪重影效应。在瓦里安TrueBeam机器上，测量了DMI探测器剂量率响应特性随剂量率的变化情况。对于6XFFF射束，在400至1400 MU/分钟的剂量率范围内采集图像；对于10XFFF射束，在400至2400 MU/分钟的剂量率范围内采集图像。对从图像中得出的线轮廓和中心射野剂量进行了分析和比较。通过采集增量监测单位（MU）范围为5至500 MU的图像来验证线性度。在不同剂量率下研究了重影效应。最后，为进行验证，创建了具有最佳注量的测试计划，并在不同剂量率下进行测量。所有测试计划均采用3%-3毫米的伽马标准和10%的剂量阈值进行分析。我们的研究表明，即使在2400 MU/分钟的最大剂量率下，从轮廓比较中也未观察到平板饱和现象。当剂量率增加时（6XFFF为400至1400 MU/分钟，10XFFF为400至2400 MU/分钟），中心射野剂量略有下降（6XFFF为1.013 - 1.008 cGy/MU，10XFFF为1.020 - 1.009 cGy/MU）。对于所有能量，DMI探测器响应的线性度在20至500 MU范围内优于0.5%。残留图像极小，在统计学上无法检测到。当剂量率从400增加到1400 MU/分钟时，6XFFF测试计划的伽马指数从100%降至97.8%。对于10XFFF，当剂量率从400增加到2400 MU/分钟时，伽马指数从99.
