"Several things have changed how we treat patients. For one, we are able to see things that we haven't seen before. Also, during treatment we can see where the tumour is," Mutic told medicalphysicsweb. "We can deliver radiation with much more certainty now, so we can treat with smaller margins or maybe treat a patient who we wouldn't have treated previously."

MR guidance also enables the use of different types of radiation treatments. One such example is stereotactic body radiotherapy (SBRT), which utilizes high doses of radiation in a short course of therapy, and relies on precision target localization. The Siteman team has used SBRT to perform a number of single-fraction breast treatments, which would not previously have been possible. "Because we have the MR, we can see the targets much more clearly and have a greater confidence that we're treating what we're supposed to," Mutic said.

Mutic points out that the Siteman Cancer Center is also now performing on-couch adaptive treatments – as routine. For each fraction, the patient is scanned on the treatment couch, a new plan is created and quality assurance is performed prior to treatment. The adaptive treatment planning process takes roughly 10–20 minutes, and almost 200 adaptive treatment fractions have been delivered in this way.

In addition, patients are imaged throughout radiation delivery. The MRIdian currently records images at 4 frames/s, with 8 frames/s capability in the pipeline for later this year. For treatment of moving targets, this functionality enables gating to be performed directly on the target or the normal anatomy, without the need for surrogates.

To date, the Siteman team has treated 26 different disease sites using MRI-guided radiotherapy, with breast, lung and gastrointestinal tumours the most common cases. "The system was designed to function as a regular treatment machine to fit with the rest of our practice, so we are treating a bit of everything on it," explained Mutic. "The same people that are treated on a linac can also use this system."

Work in progress

Mutic and colleagues are currently running three clinical trials using the MRIdian. The first of these involves treatment of oligometastases with as high a dose as possible, based on isotoxicity to normal structures. "All of these patients are treated adaptively, where the target dose is driven by keeping the dose to critical organs at the normal level," he explained.

The second study, which is still in its early stages, investigates the ability of using improved image guidance to escalate dose to stage III lung tumours, while treating less normal lung tissue. The team is also using the MR imaging capability to track how the tumours change each day, and then adapt the treatment accordingly.

The third trial exploits MR guidance to reduce the volume of irradiated breast tissue in partial-breast treatments. "The idea is that if we can see the treatment volume better on the MR image, then we can reduce the margins," said Mutic. Early results, which have been submitted for publication, have demonstrated the ability to shrink the irradiated volume by 55%.

The researchers are planning additional future trials, including studies across multiple facilities. Mutic notes that there are now three other MRIdian systems in clinical use – at UCLA's Jonsson Comprehensive Cancer Center, the University of Wisconsin Carbone Cancer Center and Seoul National University Hospital in Korea. Two other systems are currently in the commissioning phase, including one in Rome, Italy.

"We have succeeded in what we wanted to do – to make a viable clinical device that anybody can use routinely every day," Mutic concluded. "From showing the practicality of MR-guided radiotherapy, we've realised the goal of online adaptive radiotherapy. I can really say it is now 100% routine."

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