The past few decades have seen MR-guided radiotherapy evolve from an idea on the medical physicists’ wish list to a clinical reality. At the recent AAPM Annual Meeting, experts in the field took a look at three MR-linac systems, the clinical impact of this advanced treatment technology and the potential future trajectory of MR-guided radiotherapy.

Millimetres matter

Maria Bellon from Cedars-Sinai (speaking on behalf of James Dempsey and ViewRay Systems) began the symposium with an update on the MRIdian, an MR-guided radiotherapy system that combines a 6 MV linac with a 0.35 T MRI scanner. She explained that ViewRay Systems was formed in early 2024 to save the MRIdian technology following the demise of ViewRay Technologies.

Bellon described ViewRay’s quest to minimize treatment margins – the region that’s deliberately destroyed outside of the tumour. In radiotherapy, geometric margins are necessarily added to account for microscopic disease or uncertainties. “But millimetres matter when it comes to improving outcomes for cancer patients,” she said.

The MRIdian A3i, the company’s latest platform, is designed to minimize margins and maximize accuracy using three key features: auto-align, auto-adapt and auto-target. Auto-align works by aligning a very sharp beam to high-resolution images of the soft tissues to be targeted or spared. The auto-adapt workflow begins with the acquisition of a high-resolution 3D MRI for localization. Within 30 s, it automatically performs image registration, contour mapping, predicted dose calculation, IMRT plan re-optimization, best plan selection and plan QA.

Once treatment begins, auto-targeting is employed to deal with organ motion. The treatment beam is controlled by the MR images and only turned on when the tumour lies within defined margins. Organ motion can also cause interplay effects, in which the dose distribution contains gaps or areas of overlap that result in hot and cold spots. Larger margins can worsen this effect – another reason to keep them as small as possible.

Bellon shared some clinical studies demonstrating how margins matter. The MIRAGE trial, for example, showed that 2 mm margins and MR-guided radiotherapy resulted in significantly lower toxicity for prostate cancer patients than 4 mm margins and CT guidance. Elsewhere, the multicentre SMART trial treated pancreatic cancer with a 3 mm margin, which improved two-year overall survival with few to no higher-grade GI toxicities.

“This is actual evidence that reducing margins, making them real, controlling them, will improve outcomes for patients,” she noted.

Looking to the future, could sub-millimetre margins be achievable? Bellon described how a new head coil and submillimetre-resolution imaging can enable frameless MRI-guided stereotactic radiosurgery (SRS) on the A3i platform. To date, the team has investigated phantoms and healthy volunteers. “I think that it would be a really great advantage of the system to step into the SRS space,” she said.

“Innovation remains at the forefront for ViewRay Systems as they continue to strive to image faster, image in more directions and planes, and use more automation and innovation to control margins and make them smaller than ever,” said Bellon.

Mitigating motion

The second speaker, Bas Raaymakers from UMC Utrecht, discussed the Elekta Unity, a MR-linac envisaged back in 1999 by Raaymakers and his colleague Jan Lagendijk, and designed and built in collaboration with industrial partners Elekta and Philips.

Unity comprises a ring-gantry mounted linac integrated with a 1.5 T MRI. Raaymakers described some of the clinical opportunities conferred by such MR guidance. For starters, high-precision dose delivery with small margins enables use of a lower number of treatment fractions. For adrenal gland and prostate treatments, the Utrecht team has moved from 20 to five fractions, and is studying ultra-hypofractionation to just one or two.

Precise dose delivery also protects organs-at-risk and could enable delivery of higher doses to hard-to-treat cancers, such as pancreatic or renal cell cancer, where surrounding tissues are highly radiosensitive. “This is the future of MR-guided radiotherapy, this gives all kinds of opportunities that we do not have now,” Raaymakers said.

The Unity can track all types of motion – breathing motion, drifts or sudden movements – in real time and in 3D. The system’s comprehensive motion management (CMM) system performs two orthogonal cine MR scans and then uses these scans to perform gating and intrafraction drift correction (in which the treatment centre is changed to correct for drifts). Treatments with CMM began last year and analysis of the first seven patients showed that the gating works and improves conformality.

Raaymakers described how CMM combined with high soft-tissue contrast enables prostate cancer treatments in five fractions with 2 mm margins. To minimize intrafraction motion, a necessity for such small margins, the Utrecht team developed a regime in which a new plan is created halfway through the treatment. This replanning reduced the residual motion at the end of the treatment enough to enable 2 mm margins.

The team also investigated the use of drift correction halfway through the fraction and found that, dosimetrically, it was same as the replanning approach. “The whole effort of replanning can also be done with drift correction,” said Raaymakers. “Now we can do prostate treatment in 30 minutes, with 2 mm margins. This will be used, and we will explore how we can use drift correction for all types of treatment.”

The ultimate aim, Raaymakers said, is to reach the position where we don’t worry about patient motion at all. For example, is it possible to treat a beating heart? As an example, he described the MEGASTAR study of MR-guided stereotactic arrythmia radioablation, in which MRI is used to follow the beating heart and MLC tracking employed to accurately hit the target.

Raaymakers concluded with a look at the future impact of MR-guided radiotherapy. He noted that radiotherapy is a low-cost technology used to treat 50% of all cancer patients and that MR guidance can improve it further, via hypofractionation, smarter workflows, smaller margins and reduced toxicity.

“I think this is an option to shift from invasive treatments towards radiotherapy; we can postpone surgery for certain patients or omit surgery for others,” he said. “This is something we should strive for in MR-guided radiotherapy, to make this message clear to the rest of the oncology world.”

The practical MR-linac

The final speaker in the symposium was Gino Fallone from the Cross Cancer Institute (CCI), the University of Alberta and MagnetTx Oncology Solutions. Fallone introduced the Aurora-RT, a rotating MR-linac that combines a 6 MV linac with a 0.5 T biplanar MRI with a beam stop. The system was first prototyped in 2008 and is now FDA approved and CE Marked.

The unique feature of the Aurora-RT is that it can be used in two configurations: with horizontal magnets and the beam perpendicular to the magnetic field, or vertical magnets with the beam parallel to B₀. Fallone noted that the parallel configuration is the clinical product as it significantly reduces dose perturbations and enables large 3D couch shifts

Fallone told the audience why MagnetTx chose to use 0.5 T MRI. Knowing that the system would require fast imaging techniques, such as bSSFP (balanced steady-state free precession), the team assessed the contrast-to-noise ratio for bSSFP at various field strengths, and found that it was greatest at 0.5 T. “While image quality is determined by the signal-to-noise ratio, which does go up with magnetic field, contrast-to-noise is also critical,” he explained.

The Aurora-RT has a wide bore of 110 x 60 cm, reducing patient claustrophobia and increasing throughput. This large opening also enables significant couch motion of ±23 cm in the vertical and lateral directions, allowing treatments to be performed in the same way as conventional radiotherapy and improving the clinical flow. “You can place the target at the planned location every time, you don’t have to do online replanning for every single patient,” said Fallone. “Such a large couch motion also allows isocentric treatment of peripheral targets.”

To track and treat moving organs, the CCI researchers developed a technique called NifteRT, or non-invasive intrafraction tumour tracked radiotherapy. The approach involves MR imaging at 4 frames/s, autocontouring, and tumour motion prediction for each patient. The predicted tumour position is then used to control the MLC to shape and position the beam to the target.

Fallone emphasized that the team employs a lot of AI and deep learning. “This allowed us to do faster imaging without creating distortions, it allowed us to do very accurate segmentation and it allowed us to do tumour tracking with prediction and irradiation,” he explained.

The Aurora-RT was designed with simplicity and cost reduction in mind. The system can be sited in any typically sized vault, installed through the vault door maze, and does not require a cryogen exhaust vent. Because the Aurora-RT has a beamstop, shielding is required only for scattered radiation, reducing site costs. Once installed, the system runs without needing liquid helium or any liquid cryogens, reducing operating costs. The magnet can be turned on or off in minutes, improving research and service operations. It also uses many existing radiotherapy techniques, for example, existing ion chambers, laser setup and table shifts.

Fallone concluded that the Aurora-RT offers increased throughput, decreased claustrophobia, no process changes, significantly reduced dose perturbations for safer delivery and improved MR guidance via use of the 0.5 T “sweet spot”. Simplified installation in any vault, without the need for an exhaust vent or shielding for the primary radiation beam, decreases installation and operating costs.

Prove its worth

Having discussed the advantages of and clinical evidence for MR-guided radiotherapy, the speakers were asked why MR-linacs still only comprise 2% of the market and why users appear slow to adopt this approach.

“Throughput is a constant conversation that we’re having, despite the fact that yearly throughput tends to be high because a lot of treatments can be hypofractionated,” said Bellon. “On-table adaptive is very intimidating for people, but I don’t know why it’s still considered a niche treatment.”

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Raaymakers believes that the conservatism of the medical field is working against them. “Right now, it’s a lot of hassle and there’s no proof…We have to prove that it’s really worth it and hopefully then adoption will get faster.”

Fallone suggests that medical physicists are too scared of MR and that MR-linacs are still too expensive. “We know MRI is better than CT, now we have to convince the bosses,” he said. “If you get a better image you will treat better; there’s nothing to prove.”