背景:磁共振(MR)引导的放射治疗提供了在治疗之前和治疗期间利用高分辨率和实时MR成像的能力,这对适应性放疗至关重要。在适应参考剂量测定形式主义之前,这种新兴的方式已在临床环境中得到迅速采用,而参考剂量测定形式主义是考虑到强磁场的存在所必需的。特别是,需要确定磁场对参考剂量测定方程中参数不确定性的影响,以便充分表征MR引导放射治疗系统中参考剂量测定的不确定性预算。
目的:在强磁场存在的情况下,识别和量化外部高能放射治疗束的参考剂量测定中不确定性的关键来源。
方法:在没有关于MR集成直线加速器参考剂量测定的正式任务组报告的情况下,目前建议的形式主义遵循TG-51协议,增加了质量转换因子kBQ,考虑了磁场对电离室响应的影响.在这项工作中,我们量化了影响形式主义中每个参数的各种不确定性来源,并评估它们对最终剂量的总体贡献。测量在1.5TMR直线加速器中完成(Unity,ElektaAB,斯德哥尔摩,瑞典)集成了1.5T飞利浦MR扫描仪和7MVFFF直线加速器。几个参考类小体积电离室的响应(Exradin:A1SL,在整个过程中评估了IBA:CC13,PTW:Semiflex-3D)和Farmer型电离室(Exradin:A19,IBA:FC65-G)。光束质量的长期再现性和稳定性,TPR1020${\\mathrm{TPR}}_{10}^{20}$,也是用内部建造的幻影测量的。
结果:相对于传统的外部高能直线加速器,MR直线加速器中总参考剂量的不确定性受腔室设置的影响更大:沿y轴±3mm的平移位移导致剂量变化<|0.20|±0.02%(k=1),在相对于磁场方向的水平和垂直平行平面中旋转±5°时,所有5个电离室的变化不超过<|0.44|±0.02%。我们测量到xy平面(水平)旋转(<|0.44|±0.02%(k=1))比yz平面(垂直)旋转(<||0.28|±0.02%(k=1))更大的剂量变化,我们将其与kB的梯度相关联,Q作为相对于B0场方向的腔室取向的函数。Pion的不确定性(对于两个深度),Ppol(具有各种子研究,包括电缆长度的影响,MRgRT孔中的电缆循环,磁环境中的连接器类型),并确定了Prp。综合换算系数kQ×kB,在四个基本角度方向上为两个参考深度提供了Q。在两年的时间里,光束质量相当稳定,TPR1020${\\mathrm{TPR}}_{10}^{20}$为0.669±0.01%。使用相同的设备测量TPR1020${\\mathrm{TPR}}_{10}^{20}$的实际大小,并在两个不同的ElektaUnityMR-Linacs之间进行比较,结果一致在0.21%以内。
结论:在这项工作中,对影响参考剂量测定的多个参数的不确定性进行了量化.这项工作的结果可用于确定在存在磁场的情况下参考剂量测定的最佳实践指南,并评估MR-直线加速器未来参考剂量测定协议的不确定性预算。
BACKGROUND: Magnetic resonance (MR)-guided radiation therapy provides capabilities to utilize high-resolution and real-time MR imaging before and during treatment, which is critical for adaptive radiotherapy. This emerging modality has been promptly adopted in the clinic settings in advance of adaptations to reference dosimetry formalism that are needed to account for the presence of strong magnetic fields. In particular, the influence of magnetic field on the uncertainty of parameters in the reference dosimetry equation needs to be determined in order to fully characterize the uncertainty budget for reference dosimetry in MR-guided radiation therapy systems.
OBJECTIVE: To identify and quantify key sources of uncertainty in the reference dosimetry of external high energy radiotherapy beams in the presence of a strong magnetic field.
METHODS: In the absence of a formalized Task Group report for reference dosimetry in MR-integrated linacs, the currently suggested formalism follows the TG-51 protocol with the addition of a quality conversion factor kBQ accounting for the effects of the magnetic field on ionization chamber response. In this work, we quantify various sources of uncertainty that impact each of the parameters in the formalism, and evaluate their overall contribution to the final dose. Measurements are done in a 1.5 T MR-Linac (Unity, Elekta AB, Stockholm, Sweden) which integrates a 1.5 T Philips MR scanner and a 7 MVFFF linac. The responses of several reference-class small volume ionization chambers (Exradin:A1SL, IBA:CC13, PTW:Semiflex-3D) and Farmer type ionization chambers (Exradin:A19, IBA:FC65-G) were evaluated throughout this process. Long-term reproducibility and stability of beam quality, TPR 10 20 ${\\mathrm{TPR}}_{10}^{20}$ , was also measured with an in-house built phantom.
RESULTS: Relative to the conventional external high energy linacs, the uncertainty on overall reference dose in MR-linac is more significantly affected by the chamber setup: A translational displacement along y-axis of ± 3 mm results in dose variation of < |0.20| ± 0.02% (k = 1), while rotation of ± 5° in horizontal and vertical parallel planes relative to relative to the direction of magnetic field, did not exceed variation of < |0.44| ± 0.02% for all 5 ionization chambers. We measured a larger dose variation for xy-plane (horizontal) rotations (< |0.44| ± 0.02% (k = 1)) than for yz-plane (vertical) rotations (< ||0.28| ± 0.02% (k = 1)), which we associate with the gradient of kB,Q as a function of chamber orientation with respect to direction of the B0 -field. Uncertainty in Pion (for two depths), Ppol (with various sub-studies including effects of cable length, cable looping in the MRgRT bore, connector type in magnetic environment), and Prp were determined. Combined conversion factor kQ × kB,Q was provided for two reference depths at four cardinal angle orientations. Over a two-year period, beam quality was quite stable with TPR 10 20 ${\\mathrm{TPR}}_{10}^{20}$ being 0.669 ± 0.01%. The actual magnitude of TPR 10 20 ${\\mathrm{TPR}}_{10}^{20}$ was measured using identical equipment and compared between two different Elekta Unity MR-Linacs with results agreeing to within 0.21%.
CONCLUSIONS: In this work, the uncertainty of a number of parameters influencing reference dosimetry was quantified. The results of this work can be used to identify best practice guidelines for reference dosimetry in the presence of magnetic fields, and to evaluate an uncertainty budget for future reference dosimetry protocols for MR-linac.