Okamoto Hiroyuki, Nishioka Shie, Iijima Kotaro, Nakamura Satoshi, Sakasai Tatsuya, Miura Yuki, Takemori Mihiro, Nakayama Hiroki, Morishita Yuichiro, Shimizu Morihito, Abe Yoshihisa, Igaki Hiroshi, Nakayama Yuko, Itami Jun
Department of Radiation Oncology, National Cancer Center Hospital, 5-1-1 Tsukiji, Chuo-ku, Tokyo, Japan.
Department of Radiological Sciences, Graduate School of Human Health Sciences, 7-2-10 Higashi-Ogu, Arakawa-ku, Tokyo, Japan.
J Radiat Res. 2019 Jan 1;60(1):116-123. doi: 10.1093/jrr/rry087.
Our purpose was to establish the commissioning procedure of Monte Carlo modeling on a magnetic resonance imaging-guided radiotherapy system (MRIdian, Viewray Inc.) under a magnetic field of 0.345 T through experimental measurements. To do this, we sought (i) to assess the depth-dose and lateral profiles generated by the Geant4 using either EBT3 film or the BJR-25 data; (ii) to assess the calculation accuracy under a magnetic field of 0.345 T. The radius of the electron trajectory caused by the electron return effect (ERE) in a vacuum was obtained both by the Geant4 and the theoretical methods. The surface dose on the phantom was calculated and compared with that obtained from the film measurements. The dose distribution in a phantom having two air gaps was calculated and measured with EBT 3 film. (i) The difference of depth-dose profile generated by the Geant4 from the BJR-25 data was 0.0 ± 0.8% and 0.3 ± 1.5% for field sizes of 4.5 and 27.3 cm2, respectively. Lateral dose profiles generated by Geant4 agreed well with those generated from the EBT3 film data. (ii) The radius of the electron trajectory generated by Geant4 agreed well with the theoretical values. A maximum of ~50% reduction of the surface dose under a magnetic field of 0.345 T was observed due to elimination of the electron contamination caused by the magnetic field, as determined by both the film measurements and the Geant4. Changes in the dose distributions in the air gaps caused by the ERE were observed on the Geant4 and in the film measurements. Gamma analysis (3%/3 mm) showed a pass rate of 95.1%. Commissioning procedures for the MRI-guided radiotherapy system on the Geant4 were established, and we concluded that the Geant4 had provided high calculation accuracy under a magnetic field of 0.345 T.
我们的目的是通过实验测量，建立在0.345 T磁场下的磁共振成像引导放射治疗系统（MRIdian，Viewray公司）上进行蒙特卡罗建模的调试程序。为此，我们试图：（i）使用EBT3薄膜或BJR - 25数据评估由Geant4生成的深度剂量和横向剂量分布；（ii）评估在0.345 T磁场下的计算精度。通过Geant4和理论方法均获得了真空中由电子返回效应（ERE）引起的电子轨迹半径。计算了模体上的表面剂量，并与薄膜测量得到的表面剂量进行比较。计算并使用EBT 3薄膜测量了具有两个气隙的模体内的剂量分布。（i）对于4.5和27.3 cm²的射野尺寸，Geant4生成的深度剂量分布与BJR - 25数据的差异分别为0.0±0.8%和0.3±1.5%。Geant4生成的横向剂量分布与EBT3薄膜数据生成的横向剂量分布吻合良好。（ii）Geant4生成的电子轨迹半径与理论值吻合良好。通过薄膜测量和Geant4均确定，由于消除了磁场引起的电子污染，在0.345 T磁场下观察到表面剂量最多降低了约50%。在Geant4模拟和薄膜测量中均观察到ERE导致的气隙中剂量分布的变化。伽马分析（3%/3 mm）显示通过率为95.1%。建立了基于Geant4的MRI引导放射治疗系统的调试程序，并且我们得出结论，Geant4在0.345 T磁场下提供了较高的计算精度。
