Fisk decided to hold off on the operation until the tumor's growth forced the issue. When that point arrived in August 2005, Fisk's Harvard University neurosurgeon gave her a new option - proton beam therapy, a type of radiation offered at only a few hospitals in the country.

So she traveled to Boston for six weeks of treatment that she says was painless and left her feeling like her old self.

"What is amazing about it is they measure the tumor so closely that only the tumor itself receives radiation," says Fisk, now 62.

In a couple of years, people like Fisk won't have to travel so far. The University of Pennsylvania is building a $144 million proton therapy center on the old Philadelphia Civic Center site, scheduled to open in 2009. It would be one of only seven in the nation that use the tiny particles to destroy tumors.

Believers in the novel approach say the proton beam delivers the radiation so precisely that the therapy kills tumor cells while largely sparing nearby tissue. They say that's especially important with children - to avoid long-term side effects - and in adults with tumors near vital organs such as the brain.

"We feel we are going to be able to treat tumors in very, very difficult-to-reach positions," says Stephen M. Hahn, head of radiation oncology at the Penn health system.

The precision of proton therapy comes from the physical properties of the particles. The radiation dose deposited by a proton beam peaks when it reaches the tumor, increasing three to four times in intensity. After that point, known as the Bragg peak, the energy of the particles quickly drops to zero, limiting collateral damage.

In Penn's proton program, a beam will be created using a 220-ton machine called a cyclotron. The massive machine, which consists largely of steel and electrical coils, generates a magnetic field that causes the particles to move in a circle. During each cycle, the protons pass by a radiofrequency system that boosts their speed.

The charged particles are then sent down a long pipe, or tube, with branches leading to each treatment area. There, it is fed through a mechanism that varies the beam's energy, extending the Bragg peak to conform to the three-dimensional shape of a tumor.

By comparison, standard radiation treatments use photons - electromagnetic energy - to deposit radiation in tumors.

While both types of radiation therapy attack cancer cells by disrupting their DNA, photons do not behave in the same way as protons.

The energy deposited by photon beams declines gradually, so healthy cells surrounding a tumor are potentially damaged as the beam passes through a patient's body.

Sophisticated technology now enables doctors to break up the photon beams and target tumors from many directions to decrease the amount of energy sent through normal tissues. Still, healthy cells face more potentially damaging radiation from photons, proponents of proton therapy argue.

Yet few studies compare the two.

"While proton facilities are truly impressive in terms of their technology, there is really . . . no definitive study" proving the superiority of the treatment, says Walter J. Curran Jr., chairman of radiation oncology at Jefferson Medical College.

Don't tell that to Bill Doerler, 72, of Princeton; Sal Salamone, 64, of Norristown; or John Gorman, 62, of Media. All three men traveled to the proton treatment center at Loma Linda University Medical Center in Southern California after being diagnosed with prostate cancer.

They'd all known men with prostate cancer who'd had surgery or standard radiation therapy, and they were eager to avoid the potential consequences - impotence and incontinence.

"The protons seemed to have just as good a cure rate as any other treatment with few of the side effects," says Salamone, a retired heating-and-air-conditioning contractor. "Another thing that stood out in my mind was that an awful lot of physicians were out there getting treatment. "

Cancer doctors at Children's Hospital of Philadelphia also think proton therapy is the way to go. So they've partnered with Penn to create a specialized pediatric care area in the proton therapy center.

"When treating prostate cancer in a 70- or 80-year-old man, you are not really worried about 20 years from now," says Garrett M. Brodeur, chief of oncology at Children's Hospital. "But if you are treating a 2-year-old, you need to think about impact over the next 70 or 80 years. "

Penn administrators expect the facility to pay for itself. Ralph W. Muller, chief executive of the Penn health system, figures most patients will come from within 100 miles of Philadelphia, but as the only proton facility between Boston and Jacksonville, Fla., some patients are likely to come from much farther.

Penn should be able to cover its costs by treating about 80 patients a day, since it's covered by Medicare and private insurance, says Jay Loeffler, director of the Northeast Proton Therapy Center at Massachusetts General Hospital.

John H. Glick, the longtime director of Penn's Abramson Cancer Center, has pushed Penn administrators for more than a decade to build a proton center.

"For me this is a fulfillment of a dream," says Glick, who stepped down as head of the cancer center earlier this year, but remains at Penn as associate dean of the medical school.

Even as Penn and Children's Hospital pump huge sums into the proton therapy center, a small group of scientists at the Fox Chase Cancer Center in Northeast Philadelphia is trying to develop a less expensive approach.

Physicist Charlie Ma left Stanford University in 2001 to pursue research into creating proton beams for therapy using lasers instead of the huge cyclotrons like the one Penn is buying.

Although the therapy is still experimental, Ma said, he hopes to have a laser-generated proton beam available at Fox Chase in the next three to five years. Just last week, Fox Chase won a patent for the process Ma's team is developing.

Not to be outdone, many community hospitals in the region are spending millions on equipment to provide the latest in traditional photon treatments for cancer patients close to home.

"Since proton therapy is literally 20 to 30 times more expensive, I would be hard-pressed, outside a research setting, to say that protons are the way to go," says Scot Fisher, director of radiation oncology at Frankford Hospitals in Philadelphia.

Penn's proton facility will cover an area the size of a football field. It will house four versatile treatment areas, known as gantries, where the direction of the proton beams can be adjusted to provide a patient with 360 degrees of coverage. A fifth treatment room with a fixed beam is also planned.

The 75,000-square-foot building will be mostly underground, although Penn planners say it will be able to support 14 additional stories for future expansion. In addition to the gantries and concrete shielding, the center will house the 6 1/2-foot cyclotron on a floor 17 feet underground.

Hahn says the addition of proton therapy should allow doctors to give patients new combinations of therapies spanning chemotherapy, surgery, standard radiation, and protons.

Hahn acknowledges that there are real risks to Penn's decision to invest so much in proton therapy. Still, he says, "we have enough confidence in the science and the studies that are out there today that this has the real potential to help folks. "

Contact staff writer Josh Goldstein at 215-854-4733 or jgoldstein@phillynews.com.