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

• Applicant: Dr. Hui Khee Looe
• Subject Area: Medical Physics, Biomedical Technology
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 286652001

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.

• DFG Programme: Research Grants