Characterising dose changes due to unplanned gas cavities in MR-guided radiotherapy

MR-guided radiotherapy (MRgRT) is an emerging technology that combines MR imaging with a radiotherapy linac. Due to the improved soft tissue visibility that MRI offers compared with cone beam CT, MRgRT promises more accurate treatments with improved normal tissue sparing for selected treatment sites. With improved visualisation, these systems also offer the potential for online adaptive radiotherapy, where a new plan is made at each fraction based on that day’s anatomy captured on the MRI acquired before treatment delivery. For treatment sites that display a large degree of motion there is an obvious benefit; for example, cervical or abdominal disease. Additionally, these systems offer the capability to image in real-time with MRI during the treatment delivery. This allows the anatomy to be monitored and the treatment stopped if significant intrafractional changes are observed.

Two commercial systems currently available are the MRIdian system from ViewRay and the Unity system from Elekta. The MRIdian system has been used in treatment clinically since 2016 and the Unity since 2018. The Christie NHS Foundation Trust in Manchester and The Royal Marsden in London are currently using MRgRT clinically.

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