改造常规医用加速器实现Flash-RT及物理性能测量

Transform routine medical accelerators to achieve Flash‐RT and physical performance measurements

目的探讨改造常规医用加速器实现超高剂量率放疗(Flash‐RT)的可行性,了解改造后Flash‐RT射线束的物理性能。方法改造Varian 23CX医用加速器,使设备在等中心处的电子线辐射平均剂量率不小于40 Gy/s。设计相关物理测量方案对不同源皮距条件下的实际辐射剂量率、改造后射线束的百分深度剂量(PDD)曲线和离轴剂量分布等参数进行测量。结果使用HD‐V2型胶片测量改造后9 MeV电子线的平均剂量率,出束设定时间为3、6 s一组的平均剂量率分别为97.9、99.27 Gy/s;在源皮距(SSD)为100、80、60 cm时,平均剂量率分别为99.3、168、297.5 Gy/s;改造后9 MeV射束PDD曲线的R_(100)、R_(50)分别为水下2.2、3.87 cm,电子射程Rp为4.58 cm,模体表面最大可几能量E_(p,0)为9.28 MeV,这些参数值均略高于常规9 MeV射束,表现为表面剂量略增加,高剂量坪区相对变宽;射野离轴剂量分布总体呈现中心轴最高,随离轴距离增加剂量逐渐下降的特点,在20 cm×20 cm,SSD为100 cm射野条件下,横向和径向离轴剂量分布曲线的半峰宽分别为16.6 cm和16.4 cm。结论改造后的常规医用加速器,射线束在等中心处的平均剂量率达到Flash‐RT要求,在SSD为60 cm条件下平均剂量率远高于开展Flash‐RT所需的至少40 Gy/s的要求。

Objective To investigate the feasibility of transforming conventional medical accelerator to achieve ultra-high dose rate required to achieve Flash radiotherapy(Flash-RT),and to understand the physical properties of the Flash-RT beam.Methods By transforming the Varian 23CX medical accelerator,the radiation average dose rate at the isocenter was not less than 40 Gy/s.The relevant physical measurement scheme was designed to accurately measure the actual radiation dose rate of different source skin distance(SSD)conditions,the percent depth dose(PDD)curve and the off-axis dose distribution of the beam.Results The average dose rate of 9 MeV electron beam after the transformation was measured using the HD-V2 type film,the average dose rate of 3 s was 97.9 Gy/s,and the average dose rate of 6 s was 99.27 Gy/s.When the SSD was 100 cm,80 cm and 60 cm,the average dose rate of 9 MeV electron beam after the transformation was 99.3 Gy/s,168 Gy/s and 297.5 Gy/s,respectively.After the transformation,the R_(100) of the 9 MeV beam was 2.2 cm underwater,R_(50) was 3.87 cm underwater,the electron range Rp was 4.58 cm,and the maximum possible energy E_(p,0) on the phantom surface was 9.28 MeV.These parameters were slightly higher than those of the conventional 9 MeV beam,manifested with slight increase in the surface dose and widening high dose flat area.The overall deposit dose distribution exhibited the highest central axis and the increase in dose declines from the axis distance.Under the condition that the field size was 20 cm×20 cm and the SSD was 100 cm,the FWHM of the vertical and horizontal off-axis dose distribution curves were 16.6 cm and 16.4 cm,respectively.Conclusion By transforming conventional medical accelerator,the average dose rate of the beam at the isocycle meets the requirement of Flash-RT,and the average dose rate under the condition of 60 cm SSD is much higher than the requirement of at least 40 Gy/s for Flash-RT.