Establishing general correction strategies for photon detectors in magnetic fields based on convolution analysis

目前MR综合远程治疗系统的发展为高精度图像引导放射治疗开辟了新的可能性，包括基于日常解剖信息的实时跟踪和在线计划重新优化。然而，永久强磁场的存在对辐射场的精确剂量学表征提出了重大挑战，因为辐射探测器的响应将被磁场改变。最初的项目是调查磁场对探测器横向剂量响应函数的潜在影响，并在此基础上提出校正策略。最初项目的结果已经发表在三篇同行评议的论文中，还有一篇正在审查中。用蒙特卡罗方法和实验研究证明，由于磁场的存在对探测器响应的附加扰动可以用修正的横向剂量响应函数来表征。阐明了剂量学影响，包括在输出和剖面测量过程中剂量响应的变化。基于横向剂量响应函数的卷积分析校正策略在两个电离室中得到了成功的验证。此外，对EBT3辐射变色薄膜在磁场中作为潜在参考探测器的修正剂量响应进行了全面的表征，为进一步建立磁场下的剂量测量技术和校正策略奠定了坚实的基础。在最初项目中开始的调查将在这一后续项目期间扩展到其他商业探测器：不同设计的电离室、半导体、EBT3薄膜和塑料闪烁体。到目前为止尚未解决的问题，如在我们的测量中观察到的半导体探测器的绝对剂量响应的强烈相关性，将被探索以澄清潜在的物理机制。对EBT3薄膜作为参比探测器的应用进行了进一步的评价。在认识到水当量探测器在初始项目中的优势后，将从磁场相关闪烁体响应和切伦科夫信号微扰两个方面来研究商用塑料闪烁体的响应。一项涉及不同探测器类型的综合研究将有助于对磁场对探测器响应的影响有更好的总体了解，以帮助选择适合于不同磁场剂量测量任务的探测器。这项建议的最终目的是为不同类型的探测器推导出绝对和相对剂量学的一般校正策略，特别是在非参考条件下。其中之一是卷积/反卷积方法，已在初始项目中进行了评估。

The current development of MR-integrated teletherapy systems has opened new possibilities for high precision image-guided radiotherapy, including real-time tracking and online plan re-optimization based on the daily anatomical information. However, the presence of a permanent strong magnetic field poses a major challenge to the accurate dosimetric characterization of radiation fields, since the responses of radiation detectors will be altered by the magnetic field. The initial project was started to investigate the potential influence of magnetic fields on the detector lateral dose response functions, based upon which correction strategies can be proposed. The results from the initial project have been published in three peer-reviewed papers and a forth is in review. Promising results have been obtained, where it has been demonstrated both using Monte-Carlo and experimental study that the additional perturbations on detector’s response due to the presence of magnetic field can be characterized in terms of the modified lateral dose response functions. The dosimetrical impacts, including the alteration of dose response during output and profile measurements, were elucidated. Correction strategies using convolution analysis based on the lateral dose response functions were successfully validated for two ionization chambers. Furthermore, the modified dose response of EBT3 radiochromic films as potential reference detector in magnetic fields have been characterized comprehensively.The results obtained so far serve as a solid foundation, on which dosimetry techniques and correction strategies in the presence of magnetic fields can be further established. The investigations started in the initial project will be extended during this follow-up project to other commercial detectors: ionization chambers of different designs, semiconductors, EBT3 films and plastic scintillators. Unaddressed issues so far, such as the strong dependence of absolute dose response of semiconductor detectors observed in our measurements, will be explored to clarify the underlying physical mechanisms. The applications of EBT3 films as reference detector will be further evaluated. After recognizing the advantage of water equivalent detector during the initial project, the response of a commercial plastic scintillator will be investigated in terms of its magnetic field dependent scintillator response and Cerenkov signal perturbation. A comprehensive study involving different detector types will allow for a better general understanding on the influence of magnetic field on detector’s response to aid the choice of suitable detectors for different dosimetrical tasks in magnetic fields. The final aim of this proposal is to derive general correction strategies for absolute and relative dosimetry, especially under non-reference conditions, for different detector types. One of these being the convolution/deconvolution approach, as have been evaluated in the initial project.