In external beam radiation therapy the ability to measure a precisely absorbed dose under well-defined reference conditions (reference dosimetry) is an essential prerequisite for beam calibration. For MR guided radiotherapy (MRgRT), primary standards, which allow absorbed dose measurements under the influence of magnetic fields, are still lacking. Consequently, there is no consistent traceability route and there are no Codes of Practices that can be applied by hospitals for reference dosimetry in the presence of magnetic fields.

In the framework of the EMRP funded project JRP HLT06 MRI Safety, a water calorimeter was developed and its feasibility for performing dose measurements in the presence of a magnetic field was shown. The aim of work package 1 is to develop a metrological framework (primary and secondary standards) for traceable dosimetry under reference conditions for MR guided radiotherapy. For this purpose, the water calorimeter will be optimised and used for absorbed dose measurements with a target uncertainty of 1.0 % (k = 2) in the presence of magnetic fields at integrated radiotherapy MRI facilities. This will provide a metrological basis (primary standard) for subsequent investigations.

Furthermore, correction factors to account for the change of the response of ionisation chambers in high-energy photon beams in the presence of magnetic fields will be determined by calibrations traceable to the water calorimeter and other indirect methods based on secondary standards. To support a broad range of MRgRT modalities (currently commercially available or under development), these correction factors will be measured dependent upon

  1. the energy of the beam,
  2. the strength of the magnetic field,
  3. the orientation of the detector in the magnetic field.

Finally, a formalism will be developed and validated that allows absorbed dose measurements in high-energy photon beams in the presence of magnetic fields with a target uncertainty of 2.0 % (k = 2).