McCowan P M, McCurdy B M C
Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada and Medical Physics Department, CancerCare Manitoba, 675 McDermot Avenue, Winnipeg, Manitoba R3E 0V9, Canada.
Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada; Medical Physics Department, CancerCare Manitoba, 675 McDermot Avenue, Winnipeg, Manitoba R3E 0V9, Canada; and Department of Radiology, University of Manitoba, 820 Sherbrook Street, Winnipeg, Manitoba R3A 1R9, Canada.
Med Phys. 2016 Jan;43(1):254. doi: 10.1118/1.4938413.
The in vivo 3D dose delivered to a patient during volumetric modulated arc therapy (VMAT) delivery can be calculated using electronic portal imaging device (EPID) images. These images must be acquired in cine-mode (i.e., "movie" mode) in order to capture the time-dependent delivery information. The angle subtended by each cine-mode EPID image during an arc can be changed via the frame averaging number selected within the image acquisition software. A large frame average number will decrease the EPID's angular resolution and will result in a decrease in the accuracy of the dose information contained within each image. Alternatively, less EPID images acquired per delivery will decrease the overall 3D patient dose calculation time, which is appealing for large-scale clinical implementation. Therefore, the purpose of this study was to determine the optimal frame average value per EPID image, defined as the highest frame averaging that can be used without an appreciable loss in 3D dose reconstruction accuracy for VMAT treatments.
Six different VMAT plans and six different SBRT-VMAT plans were delivered to an anthropomorphic phantom. Delivery was carried out on a Varian 2300ix model linear accelerator (Linac) equipped with an aS1000 EPID running at a frame acquisition rate of 7.5 Hz. An additional PC was set up at the Linac console area, equipped with specialized frame-grabber hardware and software packages allowing continuous acquisition of all EPID frames during delivery. Frames were averaged into "frame-averaged" EPID images using matlab. Each frame-averaged data set was used to calculate the in vivo dose to the patient and then compared to the single EPID frame in vivo dose calculation (the single frame calculation represents the highest possible angular resolution per EPID image). A mean percentage dose difference of low dose (<20% prescription dose) and high dose regions (>80% prescription dose) was calculated for each frame averaged scenario for each plan. The authors defined their unacceptable loss of accuracy as no more than a ±1% mean dose difference in the high dose region. Optimal frame average numbers were then determined as a function of the Linac's average gantry speed and the dose per fraction.
The authors found that 9 and 11 frame averages were suitable for all VMAT and SBRT-VMAT treatments, respectively. This resulted in no more than a 1% loss to any of the dose region's mean percentage difference when compared to the single frame reconstruction. The optimized number was dependent on the treatment's dose per fraction and was determined to be as high as 14 for 12 Gy/fraction (fx), 15 for 8 Gy/fx, 11 for 6 Gy/fx, and 9 for 2 Gy/fx.
The authors have determined an optimal EPID frame averaging number for multiple VMAT-type treatments. These are given as a function of the dose per fraction and average gantry speed. These optimized values are now used in the authors' clinical, 3D, in vivo patient dosimetry program. This provides a reduction in calculation time while maintaining the authors' required level of accuracy in the dose reconstruction.
在容积调强弧形治疗（VMAT）过程中，输送给患者的体内三维剂量可通过电子射野影像装置（EPID）图像来计算。这些图像必须以电影模式（即“动态”模式）采集，以便获取随时间变化的输送信息。在弧形照射期间，每个电影模式EPID图像所对的角度可通过图像采集软件中选择的帧平均次数来改变。较大的帧平均次数会降低EPID的角分辨率，并导致每个图像中所含剂量信息的准确性下降。另外，每次照射采集的EPID图像较少会减少整体三维患者剂量计算时间，这对于大规模临床应用很有吸引力。因此，本研究的目的是确定每个EPID图像的最佳帧平均值，即对于VMAT治疗，在三维剂量重建准确性没有明显损失的情况下可以使用的最高帧平均次数。
将六个不同的VMAT计划和六个不同的立体定向体部放疗 - VMAT计划输送到一个人体模型。在配备有aS1000 EPID且帧采集速率为7.5 Hz运行的瓦里安2300ix型直线加速器（直线加速器）上进行照射。在直线加速器控制台区域设置了一台额外的个人计算机，配备专门的帧采集硬件和软件包，以便在照射期间连续采集所有EPID帧。使用Matlab将帧平均为“帧平均”EPID图像。每个帧平均数据集用于计算患者的体内剂量，然后与单帧EPID体内剂量计算结果进行比较（单帧计算代表每个EPID图像的最高可能角分辨率）。针对每个计划的每个帧平均情况，计算低剂量（<20%处方剂量）和高剂量区域（>80%处方剂量）的平均剂量百分比差异。作者将不可接受的准确性损失定义为高剂量区域的平均剂量差异不超过±1%。然后根据直线加速器的平均机架速度和每次分割剂量确定最佳帧平均次数。
作者发现，9次和11次帧平均分别适用于所有VMAT和立体定向体部放疗 - VMAT治疗。与单帧重建相比，这导致任何剂量区域的平均百分比差异损失不超过1%。优化后的次数取决于治疗的每次分割剂量，对于12 Gy/分割（fx）确定高达14次，8 Gy/fx为15次，6 Gy/fx为11次，2 Gy/fx为9次。
作者确定了多种VMAT类型治疗的最佳EPID帧平均次数。这些次数是作为每次分割剂量和平均机架速度的函数给出的。这些优化值现在用于作者的临床三维体内患者剂量测定程序。这在保持作者所需剂量重建准确性水平的同时减少了计算时间。
