光子辐射场配置电磁场研究磁共振引导放疗(MRIgRT)参考剂量磁场因子的可行性

Feasibility study of integrating medical Linac with electromagnet to measure magnetic factor of reference dosimetry for MRIgRT

目的针对磁共振引导放疗(MRIgRT)射束参考剂量准确测量的需求,研究强磁场对辐射剂量测量的影响,验证医用加速器配置可变磁场测量磁场因子的可行性。方法利用常规医用电子直线加速器配置一对磁场强度高达1.5 T电磁铁所产生标称高压6 MV光子辐射场和可变磁场,使用PTW30013和PTW31010电离室,分别测试电离室轴与磁场方向夹角为0°、180°、90°和270°时0~1.5 T磁场强度范围内的电离室响应,对测量结果进行分析,计算磁场因子k_(B,M)。结果电离室响应读数随磁场强度增加而增加,并在1 T时达到峰值,当磁场强度继续增加,电离室读数随之降低。0.35 T磁场时,PTW31010电离室轴向与磁场方向垂直90°和平行0°下的电离室响应磁场因子k_(B,M)分别为0.9882±0.0003和0.9974±0.0004,后者与文献报道结果间差异为0.05%±0.04%。1.5 T磁场时,PTW30013电离室轴向与磁场垂直90°下k_(B,M)为0.9589±0.0005,与文献值相差0.60%±0.05%。结论在6 MV医用加速器配置电磁铁所测量的电离室响应磁场因子可用于进一步研究临床MRIgRT参考剂量磁场修正因子。

Objective To study the influence of intensive magnet fields on radiation dose measurement,and to demonstrate the feasibility of measuring magnet field correction factor by a combination of medical linac with variable magnet fields in view of needing for accurate measurement of the doses from reference beam arising in MR image-guided radiotherapy.Methods A photon radiation field and a variable field with 6 MV nominal high voltage were produced by using conventional medical electron linear accelerator equipped with a pair of electromagnets with magnetic field strength up to 1.5 T.Both PTW30013 and PTW31010 ionization chambers were used to test the responses of ionization chambers under different magnetic field strengths at four orientations in which the angles between ionization chamber axis and magnetic field direction were 0°,180°,90°and 270°,respectively.The magnetic factors,k_(B,M) was calculated and compared with the reported values in literature.Results The response of ionization chamber was proportional to the magnetic field strength before it reached to a peak around 1 T,and then fell down as the magnetic field continued to rise.When the magnetic field was 0.35 T,the magnetic factors of PTW31010 were 0.9882±0.0003 and 0.9974±0.0004 corresponding to 90°and 0°directions,the discrepancy between 0°scenario and literature was 0.05%±0.04%.When the magnetic field reached 1.5 T,the magnetic factor of PTW30013 was 0.9589±0.0005 at the situation of 90°,which was 0.60%±0.05%different from the literature value.Conclusions Conventional 6 MV medical accelerator equipped with electromagnet can be used to measure the magnetic field factor of reference dosimetry for MRIgRT.