移动式术中放疗Mobetron加速器的测试

Commissioning of Mobetron mobile intra-operative radiotherapy accelerator

目的测试移动式术中放疗Mobetron加速器，分析它的电子束剂量学特点。方法测量移动式术中放疗Mobetron加速器电子束的剂量学特点，并与西门子常规加速器电子束进行比较。Mobetron加速器配置有4、6、9、12MeV电子束。测量项目包括垂直于水模体表面的中心轴百分深度剂量和平行于水模体表面的射野离轴比、输出因子、限光筒外漏射剂量、铅挡块对电子束的衰减、输出量校准。使用的测量仪器包括三维水箱、静电计、0．6cm。Farmer电离室、平行板电离室和固体水。测量时将不同端面和直径限光筒依次与加速器机头连接，并使端面与模体表面相切。结果除12MeV外其他能量的表面剂量均低于90％，相同能量下术中加速器表面剂量明显高于常规加速器剂量。对10cm直径、0。倾斜角的限光筒四档能量的最大剂量深度依次为0．7、1．3、2．0、2．2cm，治疗深度依次为1．0、1．8、2．7、3．6am；对0°限光简治疗时只需选直径比瘤床大1cm的筒即可。由于斜端面的限光筒照射野平坦度和对称性明显变差，限光筒尺寸的选择要依据等剂量分布图。四档能量的限光筒外1cm处漏射线分别为1．2％、5．1％、10．0％、9．1％，全挡时铅挡厚度分别为1．5、3．0、4．5、6．0mm。结论通过测试了解了Mobetron加速器性能特点并获得了临床应用和日常质量保证所需数据。

Objective To commission a Mobetron intra-operative mobile accelerator and analyze the characteristics of its electron beams. Methods The dosimetric characteristics of the electron beams generated by Mobetron accelerator were measured and compared with those generated by conventional accelerator （Primus, Siemens）. Moberton accelerator can generate electron beams of nominal energies of 4,6,9 and 12 MeV. The measurement items were as followings ： percentage depth dose perpendicular to water phantom surface and beam profiles parallel to water phantom surface, output factors, applicator leakage, electron beam attenuation made by lead blocks, and machine output calibration. The measurement devices included a threedimensional（3D） water scanning phantom,an eleetrbmeter,a 0.6 cm3 Farmer ionization chamber,a parallelplate ionization chamber and solid water slabs. During measurement, all applicators of different tilt angles and diameters were attached to the machine head, and their ends were adjusted to be tangent to the phantom surface. Results Except for the 12 MeV,skin dose for all energies was no more than 90%. The skin dose was higher for Mobetron accelerator electron beams than for regular electron beams. The Dmax depth in water for a 10 cm flat applicator were 0.7,1.3,2.0 and 2.2 cm for the 4 energies, respectively. The depths of 90% dose were 1.0,1.8,2.7 and 3.6 cm, respectively. The selected flat applicator was just 1 cm larger than the tumor bed. But for the beveled applicators, the field flatness and symmetry became worse, and consequently,the applicator size had to be selected based on the isodose distribution. The leakage dose at 1 cm outside the applicator was 1.2% ,5.1% , 10.0% and 9.1% , respectively. The lead thickness for full block was 1.5,3.0,4.5 and 6.0 mm,respectively. Conclusions Through the commissioning of Mobetron accelerator, the machine characteristics are understood, and the data for clinical implementation and routine quality assurance are acquired.