Magnetic resonance (MR) guided radiotherapy combines real-time MR images with radiotherapy. This provides more detailed images of a patient during treatment than conventional techniques, which use a limited set of CT images made prior to treatment.
The magnetic field of the MR scanner influences both the detector response and the beam dose. At this moment, there is not sufficient knowledge to correct for these deviations. This means there are no protocols for dosimetry for beam calibration and measurement of dose distributions available. Also, there are no quality assurance methods for dose delivery for adaptive treatment.
Dosimetry is the measurement of the absorbed dose delivered by ionizing radiation. In health care, this is crucial for correct treatment of patients. In radiotherapy, we talk about the dose of radiation targeted on a patient.
Measurement traceability means that there is an unbroken chain of comparisons relating an instrument’s measurements to a known standard. Calibration to a traceable standard can be used to determine an instrument’s bias, precision and accuracy.
The metrological infrastructure consists of the equipment, simulation tools, procedures and qualified personnel to carry out the necessary calibrations. In this case, this includes a primary standard (water calorimeter) that is optimised and used for absorbed dose measurements in the presence of magnetic fields, and secondary standards (ionisation chambers) that can be made traceable to a primary standard for their field of application.
A primary standard is a metrological instrument to measure a quantity without the need for a calibration in terms of the same quantity. For dosimetry, most primary standards are either a water calorimeter or a graphite calorimeter. A special feature of VSLs primary standard for dosimetry is its transportability, allowing on-site application, for example in radiotherapy clinics.
It is expected that MR guidance will not remain restricted to high-energy photons beams. Currently, many initiatives exist to introduce MR guidance for brachytherapy and, for the more distant future, researchers are working on MR guidance for proton beams.