MRI-guided radiotherapy improves on conventional image-guided radiotherapy (IGRT) by providing enhanced soft tissue definition without additional radiation exposure through high-quality MR images.

By capturing MR images of the target many times a second, the MRIdian provides real-time moving images that capture the anatomical positional changes that occur naturally within the body. Uniquely, the MRIdian allows tracking of the target position in real time, coupled with a gating facility that prevents beam delivery when the target is outside the treatment boundaries. This level of accuracy means that uncertainty is reduced, eliminating the need for large margins around the target.

The MRIdian has opened up new avenues of treatment, particularly tumours that move and are within close proximity of organs at risk (OAR) such as tumours of the liver and pancreas, where high doses are achievable while keeping the doses to OARs within tolerance.

The MRIdian allows clinicians to provide true online adaptive radiotherapy. The treatment planning system is clear and intuitive, making daily re-contouring of tumour and normal tissues straightforward. The updated plans are rapidly re-optimised providing a personalised radiation treatment on every patient, every fraction.

• The MRIdian uses a patented split-magnet MR design, so that the radiation beam is not distorted by the magnetic field and is at the optimum distance for exceptionally sharp, high dose-rate SABR
• The MR set-up captures multiple high-definition, high-quality images many times a second, seeing even the tiniest movement of the tumour and surrounding organs
• The MRIdian performs real-time on-table plan adjustments to reflect both inter- and intra-fraction anatomical changes or movements
• When the gating target moves outside the pre-specified gating boundary, the radiation beam automatically stops until the target returns back inside the gating boundary
• The gating target and gating boundary are visualised on the display and some patients benefit from being able to view this while inside the MRIdian – enabling them to take an active role in gated delivery with adjustments to their breathing
• Coupled with the unique target tracking and gating system is a precise stereotactic radiation beam, giving the clinician greater control and confidence in delivering maximal dose to the tumour target while preserving critical structures

The evidence base suggests that the MRIdian makes it possible to achieve ablative doses without reaching prohibitive levels of toxicity, particularly in the abdomen. The offers the prospect of improving clinical outcomes for previously hard-to-treat cancers such as locally advanced or recurrent pancreatic cancer and inoperable liver tumours.

We have defined SABR referral criteria based on the experience of our SABR clinical expert team, published clinical studies and international best practice. All SABR referrals will be assessed for a possible benefit of treatment on MRIdian. For example:

• Tumours where there is significant movement, e.g. in the upper abdomen and lung
• Tumours that are in close proximity to organs at risk (OAR), e.g. in the prostate, upper abdomen and central lung