National Physical Laboratory, Teddington, United Kingdom.
Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom.
Phys Med Biol. 2021 Aug 6;66(16). doi: 10.1088/1361-6560/ac0680.
Magnetic resonance imaging (MRI)-guided radiotherapy (RT) (MRIgRT) falls outside the scope of existing high energy photon therapy dosimetry protocols, because those protocols do not consider the effects of the magnetic field on detector response and on absorbed dose to water. The aim of this study is to evaluate and demonstrate the traceable measurement of absorbed dose in MRIgRT systems using alanine, made possible by the characterisation of alanine sensitivity to magnetic fields reported previously by Billas(2020115001), in a way which is compatible with existing standards and calibrations available for conventional RT. In this study, alanine is used to transfer absorbed dose to water to MRIgRT systems from a conventional linac. This offers an alternative route for the traceable measurement of absorbed dose to water, one which is independent of the transfer using ionisation chambers. The alanine dosimetry is analysed in combination with measurements with several Farmer-type chambers, PTW 30013 and IBA FC65-G, at six different centres and two different MRIgRT systems (Elekta Unity™ and ViewRay MRIdian™). The results are analysed in terms of the magnetic field correction factors, and in terms of the absorbed dose calibration coefficients for the chambers, determined at each centre. This approach to reference dosimetry in MRIgRT produces good consistency in the results, across the centres visited, at the level of 0.4% (standard deviation). Farmer-type ionisation chamber magnetic field correction factors were determined directly, by comparing calibrations in some MRIgRT systems with and without the magnetic field ramped up, and indirectly, by comparing calibrations in all the MRIgRT systems with calibrations in a conventional linac. Calibration coefficients in the MRIgRT systems were obtained with a standard uncertainty of 1.1% (Elekta Unity™) and 0.9% (ViewRay MRIdian™), for three different chamber orientations with respect to the magnetic field. The values obtained for the magnetic field correction factor in this investigation are consistent with those presented in the summary by de Pooter(202105TR02), and would tend to support the adoption of a magnetic field correction factor which depends on the chamber type, PTW 30013 or IBA FC65-G.
磁共振成像引导放射治疗（MRIgRT）超出了现有高能光子治疗剂量学协议的范围，因为这些协议没有考虑磁场对探测器响应和水吸收剂量的影响。本研究的目的是评估和证明使用先前由 Billas（2020115001）报告的丙氨酸对磁场的敏感性特征，通过使用丙氨酸在 MRIgRT 系统中进行可追溯的水吸收剂量测量，这种方法与传统 RT 可用的现有标准和校准兼容。在这项研究中，丙氨酸用于将水吸收剂量从常规直线加速器转移到 MRIgRT 系统。这为水吸收剂量的可追溯测量提供了一种替代途径，该途径独立于使用电离室进行的转移。对来自六个不同中心和两个不同 MRIgRT 系统（Elekta Unity™和 ViewRay MRIdian™）的丙氨酸剂量计进行了分析，并结合使用了几个 Farmer 型剂量计，PTW 30013 和 IBA FC65-G 进行了测量。结果根据磁场校正因子进行分析，并根据每个中心确定的室吸收剂量校准系数进行分析。这种在 MRIgRT 中进行参考剂量学的方法在访问的中心之间产生了很好的结果一致性，在 0.4%（标准偏差）的水平。 Farmer 型电离室磁场校正因子是通过比较一些 MRIgRT 系统在有磁场和无磁场情况下的校准直接确定的，也可以通过比较所有 MRIgRT 系统的校准与常规直线加速器的校准间接确定。在 MRIgRT 系统中，通过标准不确定性为 1.1%（Elekta Unity™）和 0.9%（ViewRay MRIdian™），对于相对于磁场的三个不同的室取向，获得了校准系数。本研究中获得的磁场校正因子值与 de Pooter（202105TR02）综述中提出的值一致，并且倾向于支持采用依赖于室类型（PTW 30013 或 IBA FC65-G）的磁场校正因子。
