ÌÇÐÄÊÓƵ

While radiation treatments designed to kill cancer cells have come a long way, scientists and doctors are always exploring new ways to zap tumors more effectively. Recent tests at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory show that a small array of magnets designed as an offshoot of the Lab's nuclear physics research could quite literally provide a path for such future cancer treatments.

The tests revealed that an arc of meticulously designed permanent magnets can transport beams of cancer-killing protons over a broad range of energies, from 50 to 250 million electron volts (MeV). "That's the highest energy ever for this sort of beamline," said Brookhaven Lab physicist Stephen Brooks, designer of the fixed-field magnets, and it's an energy range that could enable more effective cancer treatment.

Specifically, the project is a step toward a possible future accelerator built using this technology, where physicians could rapidly switch among beam energies to deliver very fast lethal proton doses throughout a tumor's depth.

"It's really like a flash, essentially an ultra-high dose-rate beam," said Samuel Ryu, chair of the Department of Radiation Oncology at Stony Brook Medicine, who partnered with the Brookhaven team on this project.

According to Ryu, "adjacent normal tissues appear to be better preserved" when radiation is delivered in very high doses very quickly, known as FLASH treatment. Building an accelerator that can achieve such "flash" doses with protons would give researchers a way to test the technology—and build on the advantages protons already offer for treating certain kinds of tumors.

"This work highlights important advances in accelerator science and technology gained through years of building accelerators for fundamental physics research—and how that research, conducted at the DOE national laboratories and universities using taxpayer dollars, can directly benefit society," said Brookhaven Lab Associate Laboratory Director for Nuclear and Particle ÌÇÐÄÊÓƵics Abhay Deshpande, who is also a professor of physics at Stony Brook University.

The power of protons

In more commonly used X-ray radiation therapy, "the X-rays propagate through the body without stopping," said Brookhaven Lab physicist Dejan Trbojevic, one of the architects of the new magnet array, who has been exploring medical applications for accelerator technologies for many years. He explained how X-ray beams deposit their energy along the way to a tumor and even after passing through the tumor. "The main advantage in proton or other particle radiation therapy is that the beam stops and deposits most of its energy in one place."

Since that place is determined by the particles' energy, tuning the proton beam energy gives doctors the ability to precisely target a tumor. And by depositing their energy mostly in one place, protons cause less collateral damage than X-rays do, particularly beyond the tumor.

But today's proton-therapy accelerators aren't agile enough for rapid switching among energies. They are typically made of electromagnets where the power provided to the magnets controls how much energy a single circulating beam has. Ramping up to higher energies takes time, and going to lower energies reduces beam intensity.

"Our design removes these limitations because the magnetic field is fixed using permanent magnets," Trbojevic said.

Permanent magnets for variable energy

Permanent magnets, like the ones that stick to a refrigerator or file cabinet, don't require power to produce a magnetic field. Instead, physicists like Trbojevic and Brooks control the strength and direction of the magnetic fields produced by powerful versions of these "always on" magnets by arranging them in unique ways.

Brooks used this approach, aided by computer codes he also built, to design a slightly curved array of nine permanent magnets. Each magnet is made of a series of wedges arranged roughly in an oval, with a horizontal opening at the center. That slot is the aperture through which the beams can travel when the magnets are lined up like beads on a string. Brooks designed the magnets so that the field strength varies from strongest at the outside edge of the curved array to weaker toward the center of the curve, which allows beams of different energies to traverse different paths.

"Stable orbits for each energy are arranged across the oblong slot," Brooks said. "In this design, all of the energies are possible all of the time. That's why we can deliver both high dose rates and rapid energy scaling."

As Ryu, the oncologist, noted, rapid energy switching would distribute proton energy more efficiently, "because different energies give you different depths of proton energy deposit. You can select these different energies instantaneously, so you can cover large tumors, especially for deep-seated tumors in the prostate, kidneys, pancreas, and brain."

And if tests demonstrate that the ultra-high dose-rate flash effect truly minimizes peripheral damage, it could bring big progress to cancer treatment because minimizing damage to surrounding healthy tissue is paramount, he said.

Project origins

This work builds on an earlier research and development project using fixed-field magnets as part of an energy-recovery accelerator known as CBETA, built at Cornell University. Brookhaven scientists designed and built arrays of fixed-field permanent magnets that carried electron beams at four different energies in CBETA, where successful tests provided key insights for the new proton-carrying magnets.

"We used similar magnet technology but developed it to meet the needs of a medical machine," Brooks said.

For starters, protons are much more massive than electrons and the medical machine needs to steer these massive particles around a more compact design—both of which require significantly more powerful magnets.

"These magnets have about triple the field of CBETA because, for a hospital, you want it to be as small as possible, which means that we need the highest field we can get," Brooks said. "We really narrowed down the aperture so we could get more field concentrated into it. So, the design has kind of evolved and has become a bit more sophisticated," he said.

The nine-magnet array the team has already built would be just one section of a single curved arc in an accelerator made of two such arcs connected by straight sections to make a racetrack-shaped accelerator measuring approximately 30 feet by 10 feet. That's compact enough to fit within a typical hospital wing and small by comparison to existing football-field-sized proton-therapy accelerators.

The machine would also need to carry beams for up to 6,000 turns around the accelerator, compared to CBETA's seven turns. To accomplish that, Brooks designed the magnets to carry beams from 10 to 250 MeV with exceptional stability at each energy.

Industrial partners

To turn the designs into reality, the team partnered with SABR Enterprises, LLC, a Massachusetts-based engineering, designing, and manufacturing company specializing in advanced custom permanent magnet assemblies.

"When Stephen reached out about producing these magnet arrays, we immediately knew we wanted to be involved," said Robert Mercurio, president and technical director at SABR.

Mercurio collaborated closely with his engineering team—including Robert Lown, senior magnet design engineer, and Gregory Sencabaugh, mechanical engineer—to model and verify Brooks's designs, and to develop a method for translating them into a physical structure. George Mahler, the chief mechanical engineer in Brookhaven's Collider-Accelerator Department, acted as the project manager, organizing the transfer and delivery of parts and building and positioning the whole magnet assembly.

"Our team takes pride in solving complex fabrication challenges, and this project was a great opportunity for us," Sencabaugh said.

To ensure the final assemblies met all of Brookhaven Lab's stringent technical requirements, SABR created customized tooling to accurately and successfully position and secure the individual magnet blocks in precise alignment.

Critical tests and future plans

When the magnets arrived at Brookhaven, Katie Chen, a mechanical engineer, produced an architectural model of the assembly that Rob Karl, Adrian Timon, Travis Herbst, and Edward Dabrowski from the Mechanical Support Group used to properly align the magnets and bolt them to a supporting steel plate.

To test that the magnets would accommodate the planned beam trajectories, the team transported the assembled array to the NASA Space Radiation Laboratory (), a facility that draws particles from the collider-accelerator complex supplying beams to Brookhaven Lab's nuclear physics research facilities.

"This team tirelessly dedicated their time and expertise to completing the assembly and worked with exceptional dedication throughout Father's Day weekend to help with these tests," Mahler said.

Proton beams at each specified energy passed through the assembled magnet array as easily as planned, from 50 MeV—the lowest energy available at NSRL—to 250, the high end of Brooks's design range. Future tests of 10–50 MeV beams are next, using Brookhaven's Tandem Van de Graaff facility, Trbojevic said.

While the team is eager to build a full-scale facility, there are still many steps needed to test the potential of variable-energy FLASH proton treatment.

"An immediate goal is to do some cell culture research," said SBU's Ryu. "As a researcher and clinical investigator and a physician, I want to move this technology into patient care, hopefully in my time."