1.5T磁共振直线加速器计划系统剂量学调试研究

Study on the dosimetry commissioning for the TPS of 1.5T MR-Linac

目的:对比研究1.5T磁共振直线加速器(MR-Linac)计划系统(TPS)计算与实测剂量学参数,开展磁场环境下的TPS调试,探讨磁场环境中的剂量学特征。方法:采用Unity型1.5TMR-Linac,基于MonacoTPS进行剂量计算。采用磁共振兼容三维水箱(BeamscanMR)扫描系统收集点剂量数据,设置27个基础野[机架角度(GA)=0°,源皮距(SSD)为133.5 cm、138.5 cm、143.5 cm,水下深度(d)=10 cm、5 cm、2 cm]及6类医学物理学实践指南(MPPG5.b)推荐测试例射野(GA=0°,SSD=133.5 cm,d=10 cm、5 cm),对相同条件下TPS计算与实测结果进行点剂量偏差分析,以及剂量分布γ通过率(2%/2 mm,3%/3 mm)分析。采用磁共振兼容(ArccheckMR)完成患者头部、胸部、腹部和盆腔调强放射治疗(IMRT)计划验证,根据美国物理师协会(AAPM)TG218推荐方法计算IMRT置信限值。结果:基础野中点剂量的小野剂量偏差为(-0.06±0.38)%,大野偏差为(-0.38±0.30)%,百分深度剂量(PDD)、x方向离轴比曲线(x-Profile)和y方向离轴比曲线(y-Profile)的2%/2 mm γ通过率分别为(96.1±1.9)%、(99.8±0.5)%和(99.9±0.2)%。测试例中水下5 cm、10 cm点剂量偏差分别为(-0.72±1.17)%、(-0.26±1.02)%;PDD、x-Profile和y-Profile 2%/2 mm γ通过率分别为(98.5±0.9)%、(99.4±0.8)%和(99.7±0.5)%。受磁场作用,x-Profile整体向+x方向偏移。IMRT计划点剂量偏差(0.24±2.02)%、3%/3 mmγ通过率为(99.5±0.6)%,置信限值分别为0.042,0.017(98.3%)。结论:MR-LinacTPS对1.5T磁场中的剂量分布变化进行准确建模,测试例及复杂临床计划验证符合度高,具有临床使用价值。

Objective:To compare and research the calculation and measured dosimetry parameters of treatment planning system(TPS)of 1.5T magnetic resonance linear accelerator(MR-Linac),and to conduct TPS commissioning for the magnetic environment,and to explore the dosimetry characteristics of that in magnetic field environment.Methods:The Unity type of 1.5T-Linac was adopted to conduct dosimetry calculation on the basis of Monaco TPS.The compatible three dimensional(3D)water tank with MR was adopted to collect data,and a total of 27 basis fields[gantry angle(GA)=0°,source-skin-distance(SSD)values were respectively133.5cm,138.5cm,143.5com,and the depth below water was 10cm,5cm and 2cm]and 6 kinds of fields(GA=0°,SSD=133.5cm,d=10cm,5cm)of tested cases that were recommended by practical guide of medical physics(MPPG5.b)were set.Under the same condition,the deviation analysis of point dose was conducted for the calculated TPS results and the actual measurement results,and theγpassing rate(2%/2mm,3%/3mm)of dose distribution were analyzed.The compatible Arccheck MR was used to complete the verifications of the intensity modulated radiation therapy(IMRT)plan for the head,chest,abdomen and pelvic of patients.The IMRT confidence limits were calculated according to the recommended method of American Association of Physicists in Medicine(AAPM)TG218.Results:In the basic fields,the dose deviation of the small field of point dose was(-0.06±0.38)%,and that of the large field of point dose was(-0.38±0.30)%.The 2%/2mmγpassing rates of percentage depth dose(PDD),the curve of off-axis ratio on x-direction(x-Profile)and the curve of off-axis ratio on y-direction(y-Profile)were respectively(96.1±1.9)%,(99.8±0.5)%and(99.9±0.2)%.In the tested cases,the dose deviations of 5cm and 10cm points below water were respectively(-0.72±1.17)%and(-0.26±1.02)%,and the 2%/2mmγpassing rates of PDD,x-Profile and y-Profile were respectively(98.5±0.9)%,(99.4±0.8)%and(99.7±0.5)%.Under the effect of magnetic field,the x-Profile whole skewed to x+direction.The deviation of point dose and 3%3 mmγpassing rate of IMRT plan were respectively(0.24±2.02)%and(99.5±0.6)%.The confidence limits of point dose and 3%/3mmγpassing rate were 0.042 and 0.017(98.3%),separately.Conclusion:MR-Linac TPS can accurately construct model for the changing of dose distribution in 1.5T magnetic field,and there is high consistence for the verification of the tested cases and the complexly clinical plans,which has clinical application value.