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Until this week, Mayo Clinic had tested its $180 million proton beam accelerator only on water, a cadaver and cuts of meat donated by Ye Olde Butcher Shoppe in Rochester.

But Monday, with a turn of a safety key and a press of a "beam on" button, radiation therapist Rebecca Keller sent positively charged protons through a series of powerful magnets, accelerating them to 60 percent of the speed of light, and focused them straight at the bottle cap-sized tumor in the brain of ­Ashley Sullivan.

Physicists and cancer specialists crowded over Keller's computer in the control room of Mayo's new proton beam center to watch protons pepper the tumor. "That's it," said Michael Herman, the center's chief physicist, once the tumor was coated.

So began one of the most ambitious, costly bets in Mayo's history — an investment in an emerging radiation therapy that Rochester doctors believe will revolutionize cancer care, but which critics believe is wasteful technology that will accelerate the high and rising cost of medicine in the United States.

The proton beam has been promoted as a more precise form of cancer treatment than conventional radiation: Because it can halt the cancer-killing particles at the site of a tumor, it is less likely to expose healthy ­tissue to harmful radiation.

But proton therapy can cost six times more, and there is insufficient research to ­determine which types of cancer it treats more effectively. Some insurers are reluctant to pay the bills, while economists have questioned its purpose.

"It is the Death Star of medical technologies," said Amitabh Chandra, a health economist at Harvard University. "Nothing so big and useless has ever been developed before."

In Ashley Sullivan, however, Mayo cancer specialists have found just the right first patient. Research supports its use for certain adult brain tumors and tumors in children, whose longer life spans increase their risk of second cancers and complications from radiation exposure.

Sullivan, 29, discovered her tumor when she had blurred vision while driving last fall. An imaging scan showed it pressing her optic nerve. Surgery in January removed some of the tumor, but doctors recommended proton therapy to target the rest because it would reduce radiation exposure to her brain and the chance of memory or cognitive side effects emerging years later.

"Her tumor is … 85 to 95 percent curable with radiation," said Dr. Nadia Laack, a Mayo radiation oncologist. "These patients have a long life ahead of them. They are the ones at risk for side effects from our cancer treatments."

Trouble is, brain tumors like Sullivan's aren't common, so treating them alone won't pay the huge costs for most proton centers. Researchers have found that cities big enough to support a single pro sports team can support a single proton beam center, said Dr. Peter Johnstone, who directed the proton beam facility at Indiana University until it closed due to financial and competitive pressures.

Some cities have too many proton beam centers, leaving them to fight for patients and to push pricey proton therapy, even in cases where its superiority hasn't been proven, so they can pay off their loans and investors, said Johnstone, who has published several studies on the economics of proton beam therapy. "If you're using venture capital money with a desire for a rapid return on investment, it's not going to be very sustainable."

Which is why Mayo's approach is intriguing. Instead of venture capital, Mayo used fundraising and a $100 million gift from Iowa philanthropist Richard Jacobson. To advance research on the benefits, Mayo plans to subsidize the portion that exceeds the cost of standard radiation.

Looking long-term

Resistance to proton therapy is similar to the resistance that occurred decades ago when radiation with the first linear accelerators emerged as an alternative to now-outdated cobalt treatments, said Dr. Sameer Keole, medical director of proton beam therapy at Mayo's Arizona campus, which will offer the technology next year.

"We have these [arguments] over and over throughout the course of time," he said. "Mayo isn't looking at it from a 5- or 10-year viewpoint. Mayo is looking at it as, 'What's the future of radiation oncology?' "

Sullivan's interest is more immediate. The discovery of a brain tumor was just the latest medical hardship for the Korean adoptee; she was diagnosed as a child with a rare condition, Sjögren's syndrome, which left her with rheumatoid arthritis.

Sullivan found her niche nonetheless, earning money by walking dogs and volunteering with groups that find homes for beagles. She also has volunteered in area nursing homes, even learning a bit of German and Polish to connect with her elderly friends.

But fatigue from the tumor and the surgery have forced her to curtail those activities. Monday's treatment was the first of about 40 that Sullivan hopes will "zap" the tumor and be a ticket back to her old life.

"I really just want to get it over with," she said, "and get back to dog-walking."

Open questions

Proton beams aren't the only recent advance in radiation therapy. Instead of large blunt beams of photon radiation, many cancer centers have upgraded to smaller "pencil beams" that focus radiation on the tumor sites. United Hospital in St. Paul purchased the latest device, known as a CyberKnife, that can direct precise beams of photon radiation from 1,200 different angles and automatically adjusts when patients breathe or shift their bodies.

"We are able to sculpt the beams into the form of the tumor or lesion so we are very exact with very minimal spillover," said Dr. Leslie Nussbaum, co-director of United's CyberKnife center.

Mayo has the latest photon radiation technology as well, but its radiation experts have been intrigued by proton beam therapy for more than a decade. They waited until it, too, could employ the pencil beam technology — and then, after their new center was completed, they spent months testing the accelerator and using proxies such as butcher meat to see if they could target tumors precisely.

"I want to get as much radiation in the tumor as possible to have the highest cure rate and the lowest doses possible to the normal organs to have the lowest complication rate," said Dr. Robert Foote, medical director of the Mayo Proton Beam Therapy Program.

Mayo doctors are participating in a study of the effectiveness of proton therapy for prostate cancer, and hope to study its use for other cancers, including those in the lungs or breasts. Standard radiation therapy for breast cancer has been shown to increase the risk of heart complications because of radiation spillover.

Foote said Mayo researchers already have presented data showing superior survival and complication rates using proton therapy for esophageal cancer. On the other hand, United's Nussbaum said some data shows more hearing loss when using protons to treat tumors near the nerves that link the inner ear to the brain.

Mayo also hopes the new technology means it won't have to send patients to proton centers in Massachusetts and Texas, but could draw patients nationally and conduct large-scale research on the lingering questions about its utility.

In the interim, insurers have been pushing back. One of Minnesota's largest health plans, Medica, requires prior authorization of any proton beam treatments, which are limited to three classes of cancer and only when tumors haven't spread to other organs in the body.

Mayo hopes insurers will approve more cases of proton beam therapy — even where its superiority hasn't been demonstrated — because the hospital will be picking up the extra cost.

"What they're pushing back on is the price," Foote said. "We'd like to take cost off the table because when [patients] come to see us, we'd like to be able to offer them what we see as the best treatment."

Chandra, the Harvard health economist, isn't impressed. The philanthropy to build the proton center, he said, could have gone to more established medical needs.

"Imagine if Mayo raised [$180 million] and spent it on buying insurance for 36,000 uninsured patients," he said. "They chose not to do this. They'd rather raise money for a dubious technology."

Sullivan and her parents have confidence in their choice. After the first proton treatment, they walked to the Rochester butcher shop to say thanks for the donated meat that was used to calibrate the beam for Ashley's procedure. Sullivan will be in Rochester for the next month and plans to participate in weekly zumba classes and visit local animal shelters.

Eventually, Sullivan's mother, Colleen, wants to thank Jacobson, the philanthropist, for helping bring proton beam therapy to Minnesota.

"How do you give a proper thanks," she asked, "to people who give your child that kind of gift?"

Jeremy Olson • 612-673-7744