Stanford Cancer Institute News

Fall 2017

PHASER is a more effective form of radiation therapy, with fewer side effects.

The Fastest-Ever Medical Linear Accelerator Could Revolutionize Cancer Treatment

PHASER: Next-generation Radiation Therapy

Today, the quickest radiation treatments for cancer take several minutes per session. A large and clunky linear accelerator, which produces radiation, must mechanically rotate around a patient’s body, hitting a tumor from many angles as it bombards the cells in its path with high-energy beams. During that time—which can be as long as an hour and a half in somecases—it’s a given that the tumor will move at least slightly.

“We’re always trying to hit moving targets,” said Billy W. Loo Jr., MD, PhD, an Associate Professor in the Department of Radiation Oncology and SCI member. “Every part of the body moves all the time. Things move as you breathe in and out, the heart pumps, organs like the stomach squeeze, and people move around and wiggle.”

That movement means that non- cancerous cells invariably end up in the path of the radiation. And it’s why Loo is trying to make radiation therapy last less than one second instead of many minutes.

“What if we could give radiation nearly instantaneously? What if treatment could be given before any part of the body moves?” asked Loo. “That was the seed of the idea we came up with.” If radiation could be given quickly enough to “freeze” motion, Loo reasoned, then stronger, more focused beams could be given to the sites of tumors without having to worry about hitting the surrounding tissues when a patient moved.

Loo’s seed of an idea is now a large, interdisciplinary project that’s pushing the boundaries of physics, engineering, and radiation oncology at Stanford. It’s called PHASER—for Pluridirectional High-energy Agile Scanning Electronic Radiotherapy—and will be the fastest, most powerful radiation delivery system when it’s complete.

Loo is a physician-scientist who has a bioengineering background in addition to his radiation oncology training. He was familiar with ultra-fast CT scanners designed with fully electronic controls in place of mechanical components that physically slow down the imaging. Believing that the same thing could be done with the linear accelerators that produce radiation for cancer therapy, he reached out to colleagues at the SLAC National Accelerator Laboratory. As it turned out, Sami Tantawi, PhD, Professor, Particle Physics and Astrophysics, had just discovered new principles that would enable him to build a compact and far more efficient linear accelerator than ever before with the characteristics needed for this novel medical concept.

Loo launched a collaboration with Tantawi—and other colleagues with physics, engineering, imaging, and radiation expertise—to use the new linear accelerator principles in radiation oncology. Today, a prototype linear accelerator has been built— and is demonstrating world-record performance at SLAC—based on the collaboration.

But that advance alone isn’t enough to make Loo’s vision come true.  He and colleagues have also been working on a design that eliminates the slow mechanical movement that other radiation systems relied on to direct the energy emitted by the linear accelerators. Rather than move a single linear accelerator in a circle, PHASER has a set of accelerators that can be quickly switched between to hit a tumor from many directions. And the controls that shape the beams of energy—previously in a mechanical fashion—have been redesigned to be fully electronic. Each piece of the puzzle shaves seconds, or minutes, off a standard radiation therapy session.

“The next major technological step is the integration of all these pieces to work together,” said Loo.

It will take about five years, he estimates, for PHASER to be ready to treat patients. But it will be worth the wait; when PHASER is deployed, it could upend radiation therapy. Not only would the fast, more targeted beams of radiation lead to fewer side effects because the radiation would have less off-target hits, but Loo’s basic research suggests that the radiation itself could be even more effective. In initial studies on mice, he and his collaborators have shown that the extremely fast form of radiation—even when it’s the same total dose—shrinks tumors with less damage to normal tissues. These experiments are possible to do in mice with existing customized accelerators, but requires PHASER technology to scale up to humans.

Because PHASER will be so fast, it also could allow hospital systems to offer treatment to many more patients in a day. In parts of the developing world, where there are limited places for patients to go for radiation therapy, this could be a huge advancement in cancer treatment.

“Once the machine is developed, the goal is to be able to have it accessible to anyone for the same cost as a standard accelerator,” said Quynh-Thu Le, MD, Professor and Chair of the Department of Radiation Oncology, and co-director of the SCI Radiation Biology Program. During each step of engineering, Le said, keeping costs low has been key.

The project, Le added, is showing where Stanford shines—cutting-edge, interdisciplinary work.

“We’ve always had this sense of innovation,” said Le. “Between the people in this department, at SLAC, and in the School of Engineering, we’re really leaders in this area.”