Li Yuting, Netherton Tucker, Nitsch Paige L, Balter Peter A, Gao Song, Klopp Ann H, Court Laurence E
Department of Radiation Physics, Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, USA.
Department of Radiation Oncology, Ohio State University Medical Center, Columbus, OH, USA.
J Appl Clin Med Phys. 2018 May;19(3):52-57. doi: 10.1002/acm2.12276. Epub 2018 Mar 3.
The aim of this study was to measure and compare the mega-voltage imaging dose from the Halcyon medical linear accelerator (Varian Medical Systems) with measured imaging doses with the dose calculated by Eclipse treatment planning system.
An anthropomorphic thorax phantom was imaged using all imaging techniques available with the Halcyon linac - MV cone-beam computed tomography (MV-CBCT) and orthogonal anterior-posterior/lateral pairs (MV-MV), both with high-quality and low-dose modes. In total, 54 imaging technique, isocenter position, and field size combinations were evaluated. The imaging doses delivered to 11 points in the phantom (in-target and extra-target) were measured using an ion chamber, and compared with the imaging doses calculated using Eclipse.
For high-quality MV-MV mode, the mean extra-target doses delivered to the heart, left lung, right lung and spine were 1.18, 1.64, 0.80, and 1.11 cGy per fraction, respectively. The corresponding mean in-target doses were 3.36, 3.72, 2.61, and 2.69 cGy per fraction, respectively. For MV-MV technique, the extra-target imaging dose had greater variation and dependency on imaging field size than did the in-target dose. Compared to MV-MV technique, the imaging dose from MV-CBCT was less sensitive to the location of the organ relative to the treatment field. For high-quality MV-CBCT mode, the mean imaging doses to the heart, left lung, right lung, and spine were 8.45, 7.16, 7.19, and 6.51 cGy per fraction, respectively. For both MV-MV and MV-CBCT techniques, the low-dose mode resulted in an imaging dose about half of that in high-quality mode.
The in-target doses due to MV imaging using the Halcyon ranged from 0.59 to 9.75 cGy, depending on the choice of imaging technique. Extra-target doses from MV-MV technique ranged from 0 to 2.54 cGy. The MV imaging dose was accurately calculated by Eclipse, with maximum differences less than 0.5% of a typical treatment dose (assuming a 60 Gy prescription). Therefore, the cumulative imaging and treatment plan dose distribution can be expected to accurately reflect the actual dose.
本研究的目的是测量并比较Halcyon医用直线加速器（瓦里安医疗系统公司）的兆伏级成像剂量与通过Eclipse治疗计划系统计算的剂量以及实测成像剂量。
使用Halcyon直线加速器可用的所有成像技术对一个仿真人体胸部模型进行成像——兆伏级锥形束计算机断层扫描（MV-CBCT）以及正交前后/侧位对（MV-MV），两种技术均采用高质量和低剂量模式。总共评估了54种成像技术、等中心位置和射野大小的组合。使用电离室测量模型中11个点（靶区内和靶区外）的成像剂量，并与使用Eclipse计算的成像剂量进行比较。
对于高质量MV-MV模式，每次分割给予心脏、左肺、右肺和脊柱的平均靶区外剂量分别为1.18、1.64、0.80和1.11 cGy。相应的平均靶区内剂量分别为每次分割3.36、3.72、2.61和2.69 cGy。对于MV-MV技术，靶区外成像剂量比靶区内剂量对成像射野大小的变化和依赖性更大。与MV-MV技术相比，MV-CBCT的成像剂量对器官相对于治疗射野的位置不太敏感。对于高质量MV-CBCT模式，每次分割给予心脏、左肺、右肺和脊柱的平均成像剂量分别为8.45、7.16、7.19和6.51 cGy。对于MV-MV和MV-CBCT技术，低剂量模式下的成像剂量约为高质量模式下的一半。
使用Halcyon进行兆伏级成像导致的靶区内剂量范围为0.59至9.75 cGy，具体取决于成像技术的选择。MV-MV技术的靶区外剂量范围为0至2.54 cGy。Eclipse能够准确计算兆伏级成像剂量，最大差异小于典型治疗剂量的0.5%（假设处方剂量为60 Gy）。因此，可以预期累积成像和治疗计划剂量分布能够准确反映实际剂量。
