March 14, 2018
Merging an MRI with a linear accelerator allows greater precision in cancer treatment
BY Ronda Wendler
When people breathe, there’s movement inside their bodies.
Organs, tissues and even cancer patients’ tumors move around all the time,” says Geoffrey Ibbott, Ph.D., professor of Radiation Physics at MD Anderson. “It’s not unusual for a tumor to shift an inch or slightly more while a patient breathes in and out.”
In the past, this movement has confounded doctors’ ability to pinpoint a tumor’s exact location before zapping it with high-energy beams of cancer-killing radiation.
But a new device being tested at MD Anderson promises to overcome that obstacle.
The MR-linac, as it is called, merges a magnetic resonance imaging (MRI) machine and a linear accelerator into a single device. The MRI machine relays high-quality, real-time images of tumors as they’re blasted with radiation beams from the linear accelerator.
MRI machines and linear accelerators have been used in cancer therapy for years. What’s new is the two are now one.
“For the first time, this new technology lets us see where the radiation dose is being delivered, as it’s being delivered,” Ibbott explains. “The live images keep the radiation directly on target during the treatment.”
If a tumor moves out of the targeted radiation area when a patient inhales, the MR-linac automatically turns off the radiation beam. When the patient exhales and the tumor returns to its original position, radiation automatically resumes. This on-off treatment spares healthy tissue and allows doctors to deliver a more powerful dose of radiation.
“We used to treat the entire area where the tumor might travel during a normal breathing cycle,” he explains. “But with this new technology, radiation is delivered only when the tumor is located where it should be.”
Doctors watch treatment sessions on a video monitor. If what they see causes them to tweak the patient’s treatment plan, the MR-linac can refine the target and reconfigure the radiation dose in less than two minutes, all while the patient is on the table. Ibbott calls this “radiation on the fly.”
“Tailoring radiation to patients’ tumors in real time – it’s the ultimate in personalized medicine,” he says.
Magnets and metal
Before the MR-linac’s debut, scientists wouldn’t dare place an MRI machine near a linear accelerator.
“That’s because MRI technology uses a powerful magnet to produce high-quality images,” Ibbott explains. “The strong magnetic field would pose an obvious safety hazard in the presence of an all-metal linear accelerator, with parts that could fly across the room at a dangerous speed toward the magnet, harming everything in their path. Also, the magnetic field would disturb the operation of the linear accelerator and degrade tumor images.”
The MR-linac’s developers created a simple but elegant workaround to bypass this obstacle.
Here’s how Ibbott explains it:
“Think of the MRI machine as a hollow paper-towel tube, with a magnetic metal coil wrapping around it. The patient slides inside the tube where imaging takes place.”
The MR-linac splits the metal coil in half, creating a gap in the middle where the patient is positioned. Radiation passes through this de-magnetized “safety zone” and images are created without distortion.
Leading the way
MD Anderson is the first clinical site in the world to install the MR-linac, other than the University Medical Center in Utrecht, The Netherlands, where it was designed with input from MD Anderson and several other institutions. Today, seven cancer centers worldwide have MR-linacs in place.
The technology has not yet been approved by the Food and Drug Administration. It’s too new, says Clifton David Fuller, M.D., Ph.D., who is teaming with Ibbott to conduct a clinical trial of the MR-linac at MD Anderson.
Data gathered at MD Anderson and other trial sites will provide the machine’s manufacturer, Elekta, with information needed to pursue FDA approval.
Read more about MD Anderson’s role in pursuing FDA approval for the MR-linac in the Annual Report.