Zhang Yan, Yan Shaojie, Cui Zhen, Wang Yungang, Li Zhenjiang, Yin Yong, Li Baosheng, Quan Hong, Zhu Jian
School of Physics and Technology, Wuhan University, Wuhan, P.R. China.
Department of Radiation Oncology Physics and Technology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, P.R. China.
Med Phys. 2021 Jul;48(7):4027-4037. doi: 10.1002/mp.14839. Epub 2021 May 10.
To assess the out-of-field surface and internal dose of the 1.5 T MR-Linac compared to the conventional external beam linac using optically stimulated luminescence dosimeters (OSLDs), and evaluate the out-of-field dose calculation accuracy of the Monaco treatment planning system (TPS) of the 1.5T MR-Linac.
A cubic solid water phantom, with OSLDs on the surface, was vertically irradiated by MR-Linac square fields with different sizes. In addition, OSLDs were arranged out of the beam edges in four directions. An anthropomorphic adult phantom, with 125 cm simulated volume, was irradiated in four orthogonal directions by both MR-Linac and conventional linac at the head, thoracic, and pelvic sites. Out-of-field doses were measured by OSLDs on both the surface and internal emulational organs at risk (OARs). The results were compared to the simulated dose from Monaco TPS.
At different field sizes (5 × 5 to 20 × 20 cm ) and distances (1 to 10 cm) to beam edge, the out-of-field surface dose measured on MR-Linac varied from 0.16 % (10 cm to 5 × 5 cm edge) to 7.02 % (1 cm to 20 × 20 cm edge) of the maximum dose laterally and from 0.14 % (10 cm to 5 × 5 cm edge) to 8.56 % (1 cm to 20 × 20 cm edge) of the maximum dose longitudinally. Compared to the OSLDs measured data, the Monaco TPS presented an overestimate of the out-of-field dose of OARs at 0-2 % isodose area on both surface and internal check points, and the overestimation gets greater as the distance increases. The underestimation was found to be 0-35% at 2-5% isodose area on both surface and internal check points. Compared to the conventional linac, MR-Linac delivered higher average values of out-of-field dose on surface check points (20%, 19%, 21%) and internal simulated OARs (42%, 37%, 9%) of the anthropomorphic phantom at head, thoracic, and pelvic irradiations, respectively.
Compared to the conventional linac, MR-Linac has the same out-of-field dose distribution. However, considering the absolute dose values, MR-Linac delivered relatively higher out-of-field doses on both surface and internal OARs. Additional radiation shielding to patients undergoing MR-Linac may provide protection from out-of-field exposure.
使用光激励发光剂量计（OSLD）评估1.5T MR直线加速器与传统外照射直线加速器相比的射野外表面剂量和内部剂量，并评估1.5T MR直线加速器的Monaco治疗计划系统（TPS）的射野外剂量计算准确性。
一个表面带有OSLD的立方固体水模体，被不同大小的MR直线加速器方形野垂直照射。此外，OSLD在四个方向上布置在射束边缘之外。一个模拟体积为125cm的人体成人模体，在头部、胸部和骨盆部位由MR直线加速器和传统直线加速器在四个正交方向上进行照射。射野外剂量通过表面和内部模拟危及器官（OAR）上的OSLD进行测量。将结果与Monaco TPS模拟剂量进行比较。
在不同野大小（5×5至20×20cm）和距射束边缘的距离（1至10cm）下，MR直线加速器上测量的射野外表面剂量在横向为最大剂量的0.16%（距5×5cm边缘10cm处）至7.02%（距20×20cm边缘1cm处），纵向为最大剂量的0.14%（距5×5cm边缘10cm处）至8.56%（距20×20cm边缘1cm处）。与OSLD测量数据相比，Monaco TPS在表面和内部检查点的0-2%等剂量区域对OAR的射野外剂量高估，且随着距离增加高估程度更大。在表面和内部检查点的2-5%等剂量区域发现低估为0-35%。与传统直线加速器相比，MR直线加速器在人体模体的头部、胸部和骨盆照射中，在表面检查点（分别为20%、19%、21%）和内部模拟OAR（分别为42%、37%、9%）上的射野外剂量平均值更高。
与传统直线加速器相比，MR直线加速器具有相同的射野外剂量分布。然而，考虑到绝对剂量值，MR直线加速器在表面和内部OAR上的射野外剂量相对较高。对接受MR直线加速器治疗的患者进行额外的辐射防护可能有助于防止射野外照射。
