无均整器型加速器治疗室中子剂量分布

Neutron dose distribution in treatment room for an accelerator with flattening filter free

目的调强放疗技术极大改善了放疗计划质量,但相对适形放疗也随之增加了治疗出束剂量及照射时间,移除均整器(flattening filter free,FFF)技术能在保持计划质量的前提下减少照射时间。本研究测量和分析瓦里安TrueBeam型加速器常规剂量率有均整器(flattening filter,FF)模式和超高剂量率FFF模式下中子累积剂量和持续时间,及其在机房的分布规律,为临床使用提供参考。方法机房内选取加速器治疗高度等中心技师摆位处、治疗床远端、迷路近端和防护门内侧4个位置,设置5cm×5cm、10cm×10cm和30cm×30cm 3种射野面积,出束剂量分别为500、1 000和2 000 MU,等中心位置放置30cm厚度固体水模体模拟患者身体,在10 MV X射线600 MU/min常规剂量率FF模式和2 400 MU/min超高剂量率FFF模式下,测量中子的累积剂量及消退时间,同时在机房外选取防护门外、操作室和主防护墙中心3个位置进行测量。结果中子累积剂量与射野面积大小呈反比;距离靶中心越近,中子累积剂量越大,机房内中子累积最大位置为治疗床远端;中子累积剂量FF模式高于FFF模式,差异有统计学意义,P<0.05。中子消退时间与出束剂量成正比;相同条件FFF模式下的中子消退时间小于FF模式,距离中心靶位点越远中子消退时间越快。结论加速器移除均整器后中子累积剂量及持续时间明显下降,有益于患者及工作人员的辐射防护;现有加速器机房防中子设计同样适用于FFF模式加速器。

OBJECTIVE The objective of this study was to provide specific data and make recommendation for radiation protection for clinical application, neutron cumulative dose and existing time in the treatment room around an acceler ator operating at 10 MV both with the condition of conventional dose rate （with the flattening filter,FF mode） and ultrahigh dose rate （with the flattening filter free,FFF） were measured and analyzed. METHODS In the accelerator treatment room, the data was collected in four representative sites：therapists operating location, the distal end of the couch in the pa tient plane,maze proximal and protective door. The field size were 5 cm×5 cm, 10 cm× 10 cm and 30 cm× 30 cm respectively,output dose were 500 MU, 1 000 MU and 2 000 MU respectively, the solid water phantom simulating the patient＇s body was put on the isocentre,dose rate of FF mode was 10 MV Xray 600 MU/min and dose rate of FFF was 2 400 MU/min. In the same condition the data was collected outside the selected measuring three positions：protective door, operating room and center of main protective wall position. RESULTS The neutron cumulative dose was negatively correlated with field size. The closer to the target, the larger the neutron cumulative dose was. The neutron cumulative dose was the largest in the distal end of the couch. The neutron cumulative dose of FF mode was almost three times as much as thai of FFF mode. Neutron existing time was positive correlated with output dose. FF mode took slightly longer than the FFF mode. No neutron dose was read outside the accelerator room. CONCLUSIONS Operating the accelerator of FFF mode will benefit both patients and radiation therapists by reducing the neutron cumulative dose and existing time. Existing anti neutron accelerator room design is equally applicable to ultra-high-dose rate （FFF） mode accelerator.