The integration of magnetic resonance imaging with linear accelerator technology, known as MR-LINAC, is revolutionizing radiation oncology by offering unprecedented precision in cancer treatment. This advanced system allows radiation oncologists to adapt treatment plans in real-time based on daily changes in patient anatomy, potentially improving outcomes while reducing side effects.
"MR-LINAC can be used across a broad range of cancers and is not limited to any one [cancer type], although it shines [when used] with stereotactic body radiation therapy [SBRT]," explains Dr. Paul B. Renz, radiation oncologist at Allegheny General Hospital, Allegheny Health Network (AHN) in Pittsburgh, Pennsylvania.
Adaptive Treatment Planning: A New Paradigm
Unlike conventional radiation therapy, which relies on an initial planning session and attempts to replicate that plan throughout treatment, MR-LINAC captures fresh, detailed images before each treatment session. This allows clinicians to reshape and redirect radiation based on the patient's daily anatomy.
"A patient's anatomy may change throughout the course of the treatment. They may gain or lose weight; their tumor may move or change its motion based on their respirations," Dr. Renz notes. "On a regular LINAC, these changes are not accounted for, and instead, we try to mimic the plan [that is established] at their initial planning session."
The adaptive capability becomes particularly valuable when treating tumors near sensitive organs. For instance, the prostate has the rectum within millimeters, while the pancreas is adjacent to the stomach and duodenum—all organs highly sensitive to radiation. MR-LINAC's precision allows for maximizing the dose to the tumor while minimizing exposure to surrounding healthy tissues.
Real-Time Monitoring During Treatment
One of MR-LINAC's most significant advantages is the ability to monitor treatment delivery in real-time. Conventional linear accelerators use cone beam CT for initial positioning but cannot track movement during the actual treatment.
"What is different with MR-LINAC–guided radiation is that we can monitor the patient's movements in real time during treatment," Dr. Renz explains. "We can watch a movie of the patient's body and tumor while delivering the treatment."
This continuous monitoring allows the team to pause the radiation beam if a patient coughs, moves, or if internal organs shift position. The technology can also track breathing patterns and adjust accordingly, ensuring the radiation remains precisely targeted throughout the entire treatment session.
Clinical Benefits and Early Results
While MR-LINAC is relatively new technology with mature outcome data still being collected, early studies show promising results. "Randomized trials in both prostate and pancreatic [cancers] have shown a decrease in toxicity rates throughout the course of the treatment due to the accuracy of treatment delivery," Dr. Renz reports.
Dr. Timothy Dougherty, a radiation oncologist specializing in genitourinary cancers at AHN Cancer Institute, has observed fewer adverse events in patients treated with MR-LINAC compared to conventional treatments since implementing the technology in 2018.
The MOMENTUM study (NCT04075305), an international consortium analyzing MR-LINAC data from 30 centers across the US, Canada, and Europe, aims to provide further insights as more data is accrued over the next few years.
Patient Experience Considerations
Despite its advantages, MR-LINAC treatment presents some challenges for patients. Sessions typically last 40-45 minutes compared to 10 minutes with conventional radiation therapy. The MRI environment can trigger claustrophobia in some patients, and the treatment requires patients to remain still for extended periods.
"It is not the most comfortable of treatments from [the patient's] standpoint. However, many patients prefer the 5 claustrophobic, longer treatments to 30 quicker treatments," Dr. Renz observes.
To address these challenges, the team employs various techniques including goggles that provide a view outside the magnet tube, anxiety medications, or mild sedation when necessary. Dr. Renz also mentions potential future initiatives like playing relaxing music or conducting guided meditations during treatment to improve the patient experience.
Innovative Research and Future Directions
Exciting research is underway to further enhance MR-LINAC's capabilities. Dr. Alexander V. Kirichenko, clinical director of the gastrointestinal and SBRT programs at AHN Cancer Institute, is pioneering novel MR contrast agents that improve tumor visualization.
"Patients were referred to him for a single tumor, and when he enrolled them in the clinical trial, evaluating the novel MR contrast agent, he found 1 or 2 more [tumors] and was able to treat all of them at the same time and clear all their disease early on," Dr. Renz explains.
Another promising area is biology adaptive radiation, which analyzes MRI scan dynamics during treatment to predict tumor response. This information could help clinicians customize radiation and chemotherapy plans based on how individual tumors are responding.
Accessibility and Insurance Coverage
Currently, MR-LINAC treatments use the same billing codes as conventional linear accelerator treatments, making them covered by insurance. However, the adaptive planning aspect—which requires a physicist, physician, and two radiation therapists throughout each treatment—lacks specific Medicare codes.
"There are many large institutions lobbying Congress to plan how to accommodate those adaptive planning charges because [the process] is very labor intensive from a staffing standpoint," Dr. Renz notes.
This billing limitation has slowed the expansion of MR-LINAC to more patients. Once appropriate billing codes are established, hospitals will be able to broaden access beyond the current focus on patients who would benefit most, such as those with isolated cancers near sensitive organs.
Future Outlook
Dr. Renz anticipates MR-LINAC becoming the standard for treating cancers of the prostate, pancreas, and liver, with applications in other areas like head and neck cancers, lung cancer, and sarcomas continuing to develop.
Competing technologies are also emerging, including CT and artificial intelligence-based adaptive treatment planning solutions that aim to provide daily customization without the time constraints of MRI. However, MR-LINAC's real-time motion monitoring capabilities offer a level of precision these alternatives currently cannot match.
For community oncologists, Dr. Renz emphasizes that this technology is available for appropriate patients. "As this becomes more mainstream and the data mature, when evaluating patients, consider whether they should be treated locally with a more conventional treatment, or whether the advantages of MR-LINAC make it a good fit for the patient."
