Magnetic resonance-guided radiotherapy

The just recently introduced magnetic resonance (MR)-guided radiotherapy (MRgRT) is a very promising treatment modality allowing for an optimized dose coverage of the tumor while sparing the surrounding normal tissue. This is realized by online MR imaging showing superior soft tissue contrast as compared to x-rays normally used for imaging in radiotherapy. However, MRgRT poses several challenges for clinical implementation originating from distortions of the dose distribution and the detector reading due to the Lorentz force acting on secondary electrons, "MR-only"-based treatment planning up to the development and verification of truly adaptive treatment workflows. To solve these issues, our group contributes to the metrological foundations of reference and small field dosimetry in presence of magnetic fields for photon (MRgRT-DOS project) and ion beams.

Figure 2. Anthorpomorphic quality assurance phantom to study interfractional uncertainties in MRgRT.
© Elter et al. 2019

Further activities involve the development of dosimetry protocols and their application in phantom measurements. For this, dosimeters ranging from 1D to 3D are combined to validate the geometric and dosimetric accuracy of new MRgRT-treatment workflows. This includes so-called end-to-end tests where the entire patient workflow is validated using in-house developed patient-equivalent phantoms with anthropomorphic imaging contrast and attenuation properties. Further activities address optimization of imaging protocols for MRI, image registration, generation of pseudo-CTs solely based on MR data, fractionated irradiation and verification of adaptive treatment strategies such as online plan adaption while the patient is on the table as well as gated treatments to account for organ motion. In many of these projects, gel dosimetry plays an important role.

Figure 3. Phantom to simultaneously measure isocenter accuracy and image distortions of a MR-linac device.
© Dorsch et al. 2019

The research activities in photon MRgRT are performed at the MR-Linac of the University Hospital Heidelberg in close cooperation with the responsible clinical medical physicists (Clinical Research Group Medical Physics). Research activities in ion beam MRgRT are performed together with medical physicists of the university hospital and the Heidelberg Ion Beam Therapy Center (HIT).

Selected publications

  • Spindeldreier C.K., Schrenk O., Bakenecker A., Kawrakow I., Burigo L., Karger C.P., Greilich S., Pfaffen¬berger, A.: Radiation dosimetry in magnetic fields with Farmer-type ionization chambers: determination of mag¬netic field correction factors for different magnetic field strengths and field orientations. Physics in Medicine and Biology 62, 6708-6728, 2017 https://iopscience.iop.org/article/10.1088/1361-6560/aa7ae4
     
  • Dorsch S., Mann P., Elter A., Runz A., Spindeldreier C.K., Klüter S., Karger C.P.: Measurement of isocenter alignment accuracy and image distortion of an 0.35 T MR-Linac system. Physics in Medicine and Biology 64, 205011, 2019 https://iopscience.iop.org/article/10.1088/1361-6560/ab4540
     
  • Elter A. Dorsch S. Mann P., Runz A., Johnen W., Spindeldreier C.K., Klüter S., Karger C.P.: End-to-end test of an online adaptive treatment procedure in MR-guided radiotherapy using a phantom with anthropomoric struc¬tres. Physics in Medicine and Biology 64, 225003, 2019 https://iopscience.iop.org/article/10.1088/1361-6560/ab4d8e
     
  • Schwahofer A., Mann P., Spindeldreier C.K., Karger C.P.: On the feasibility of absolute 3D dosimetry using LiF Thermoluminescence detectors and polymer gels on a 0.35T MR-LINAC. Physics in Medicine and Biology 65, 215002, 2020 https://dx.doi.org/10.1088/1361-6560/aba6d7
     
  • Elter A., Hellwich E., Dorsch S., Schäfer M., Runz A., Klüter S., Ackermann B., Brons S., Karger C.P. Mann P.: Development of phantom materials with independently adjustable CT- and MR-contrast at 0.35, 1.5 and 3T. Physics in Medicine and Biology (in press) https://iopscience.iop.org/article/10.1088/1361-6560/abd4b9

 

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