Chiseling away at tumors with protons.

Chiseling away at tumors with protons

In 1946, physicist Robert R. Wilson proposed that accelerated beams of protons -- electron-less hydrogen atoms -- might make good cancer stoppers. Now proton therapy is far more than a concept. Doctors around the world have used it to treat more than 6,000 patients with cancers and other abnormal growths. At the Loma Linda (Calif.) University Medical Center, doctors are preparing to install a newly designed accelerator that will open up proton therapy to many more patients.

The circular, 20-foot-diameter accelerator passed a crucial test on Dec. 29 when its designers -- at the Fermi National Accelerator Laboratory in Batavia, Ill. -- accelerated some protons in it for the first time. This summer, Fermilab technicians plan to dismantle the accelerator so that it can be shipped to Loma Linda and reassembled there.

Proton therapy is especially suited for treating tumors that don't respond well to traditional surgical or radiation therapies, says James M. Slater, a radiation oncologist at Loma Linda and head of the new facility. But for a number of technical reasons, applications have been largely limited to patients with head and neck tumors. For one, these tumors are more accessible to the fixed beams emanating from existing accelerators. Also, most beams of therapeutic protons are too weak to penetrate more than 6 inches into the body, doctors say.

Using the new, more powerful accelerator, doctors at Loma Linda expect to be able to treat virtually any tumor no matter where it is in the body. Slater says. "It's the first proton accelerator that ever has been designed specifically for treating patients," he says. In contrast, the aging accelerators now used for proton therapy were designed for basic physics research and are located in nonhospital settings. Presently under construction at Loma Linda is a building dedicated to housing the accelerator and treating patients.

The new accelerator will enable doctors to both scan the proton beam over a target area of tissue and quickly change the energy of the speeding protons, says principal designer Lee Teng, a senior physicist at Fermilab. It is the energy of the beam that determines how far each pulse of protons will penetrate. By combining area scanning with protons of varying energies, doctors should be able to bombard nearly any three-dimensional tumor while minimizing exposure of nearby and overlying healthy tissue, Slater says.

Perhaps more important, he adds, will be the unique ability to ferry the beam around the stationary patient using a movable structure that girdles the treatment table. "It's a great thing," says Wilson, professor emeritus of nuclear studies at Cornell University. No other therapeutic proton accelerator has this feature, he adds.