Current proton therapy uses cyclotrons or synchrotrons nearly 10 ft in diameter and weighing up to several hundred tons. An enormous gantry and bending magnets focus and direct the beams.

Now, thanks to scientists at Lawrence Livermore National Laboratory, the first compact proton-therapy machine — one that would fit in any major cancer center and cost a fifth as much as current devices — is one step closer to reality. The new machine overcomes the size obstacle by using dielectric wall acceleration. Scientists have demonstrated in principle that this lets proton particles accelerate to an energy of at least 200 million eV within a lightweight, insulated structure about 6.5-ft long. The machine won’t use bending magnets and will change the protons’ energy and intensity between bursts that occur many times per second. The new device should also vary the energy, intensity, and “spot” size of the proton beam.

Charged protons were first used to treat human cancer at the Berkeley Radiation Laboratory more than 50 years ago. But early machines cost more than $100 million and required 90,000 sq ft to house. The new device could fit in standard radiation-treatment suites and cost less than $20 million.

“This technology has grown out of work to develop compact, high-current accelerators as flash X-ray radiography sources for managing nuclear-weapons stockpiles,” says George Caporaso, lead scientist on the project. Conventional radiation therapy uses high-energy X-rays that deliver energy to all the tissues they pass through. Doctors therefore limit doses delivered to tumors to minimize damage to surrounding tissue.

Unlike high-energy X-rays, proton beams deposit almost all of their energy on the target, with little ending up in healthy tissue. This lets doctors use higher, potentially more effective doses than is possible with gamma radiation. The first clinical prototype will be tested at the UC Davis Cancer Center, which shared funding of the project with Livermore.

Proton therapy, considered the most-advanced form of radiation therapy, could become more widespread thanks to defense-related research. The gantrymounted device rotates around the patient.