Metrology for MR guided radiotherapy

Cancer is responsible for roughly one in five deaths in Europe, with radiotherapy, which uses beams of ionising radiation to kill cancerous cells, used to treat around half of patients. Image-guided radiation therapies have become a common diagnosis and treatment tool, combining therapy and imaging in one device. MR-guided radiotherapy (MRgRT) combines Magnetic Resonance Imaging (MRI) with high-energy photon or X-ray beams for therapy, offering zero additional radiation exposure for unlimited imaging, as well as clearer differentiation between healthy and cancer cells.

Critical for success is the delivery of accurate radiation doses. For existing therapies, this is achieved by calibration chains. However, existing methods failed for the MR-linacs that deliver MRgRT, as MRI magnetic fields interact with treatment beams and ionisation chambers. The primary standards used to calibrate reference dosimeters were too large to use within some MR-linacs designs and the effects of magnetic fields on measurement devices used for calibrating and characterising radiation fields were unknown.

This project developed absolute and relative dosimetry and imaging metrology to support safe clinical implementation and innovation in MRgRT.

A water calorimeter, developed by the project MRI Safety as a primary standard for MR-linacs, was characterised and applied to measure radiation doses in a prototype ‘Elekta Unity’ MR-linac. The calibration coefficients of several ion chamber secondary standards were also determined. A method to deliver reference dosimetry in MRgRT clinics was devised, applying device-specific correction factors to existing methods, as well as three other ways to measure correction factors for ionisation chambers.

A film-based approach was developed to replace water tank measurements for MRgRT treatment planning systems and correction factors for detectors used to calibrate reference fields in MRgRT facilities were also established. Quality assurance procedures for MRgRT facilities were developed, as well as methods to track organ motion during treatment.

Ionisation chambers calibrated to the optimised primary standard were used to prepare the first clinical treatments with an MR-linac, supported by the Dutch Cancer Society. Four spine cancer patients were treated, with palliative effects achieved for all four, showing the clinical feasibility of MRgRT.

Approval for European commercial sales and clinical use was granted for the Elekta Unity in June 2018. It was used at UMCU for a first clinical treatment, cutting the time for personalised treatment planning from days to minutes.

Providing trust in absolute dosimetry and treatment planning advanced the development and clinical acceptance of MRgRT, heralding more precise and personalised treatments, shorter treatment times and reduced cost of care for healthcare providers.