电离室阵列在电子束旋转照射剂量验证中的应用

Application of 2-D Ionization Chamber Array For Validation of Electron Arc Therapy Plan

目的:考察一种二维电离室阵列对斜入射电子线剂量测量的特性与误差,探讨其用于电子束旋转照射计划剂量分布验证的可行性。材料与方法:(1)在±50°范围内比较电离室阵列与指形电离室测量的差别并校准电离室阵列;(2)设计6和10MeV电子束旋转照射体模计划各三个(0°机架角时的射线束中心轴对称夹角30°、60°和90°)。用二维电离室阵列分别测量验证各计划的剂量分布。结果:(1)电离室阵列中心探头与指形电离室在上述斜入射条件下对6MeV/10MeV电子线的测量差别小于2%。(2)各电子线旋转照射计划在中心轴上最大剂量深度处的剂量误差均小于3%。离轴剂量比误差在非旋转方向上旋转射野中间70%区域内小于2%;在旋转方向上最大不超过1.5%。冠状平面上的100%～20%各等剂量曲线符合性较好;6MeV和10MeV的电子线的30°、60°、90°旋转照射计划的Gamma指数通过率(=5%和=5mm)分别为99.98%、99.89%、99.74%、98.64%、99.16%和99.44%。结论:所测试的二维电离室阵列对斜入射电子线测量误差能满足±50°范围内的电子束旋转治疗的计划剂量验证要求。

Objectlve： To inspect the dosimetry characteristic of a 2-D ionization chamber array and its response to oblique in- cident electron beam. And to discuss feasibility to using it to validate the electron arc therapy plan. Materials and methods： （1） The 2-D ionization chamber array was calibrated referring to a themble chamber, comparing the response to electron beams with incidence angle ranged from -50° to 50° . （2） Phantom test plans of 6 and 10MeV electron beam arc therapy were de- signed, with the beam rotation arcs of 30 ° , 60° and 90° respectively, symmetric to the central axis of 0° gantry angle. Dose distribution of each plan was verified with the 2-D array. Results： （1） Measurement differences between the central detec- tor of the array and the thimble chamber were less than 2%, under the planned oblique incident 6/10MeV electron beam. （2） For the all test plans, dose error was less than 3% at dmax in the symmetric axis of the electron beam arcs. Off-axis dose error was less than 2% within the central 70% area of the profile in non-rotating direction, and smaller than 1.5% in the rotational di- rection. The measured dose distribution had good agreement with the TPS calculation in isodose curves between 100% and 20%. The gamma pass rates （△D=5%, Ad=5cm）were 99.98%,99.89%,99.74%, 98.64%,99.16% and 99.44% respectively for 6 and 10 MeV plans with electron beam arcs of 30° , 60° and 90° . Conclusion： it is practicable using the tested 2-D ionization chamber array to verify and validate the electron beam arc therapy plan.