They are big. They are expensive. They are controversial. And they are viewed by some as the newest form of a medical arms race: proton cancer centers.

Nationwide, nine centers currently are operating—up from five in 2008—and another 14 centers have been proposed to open within the next five years.

That includes two centers to be built by the Mayo Clinic and two centers—one under construction and one announced—within 10 miles of each other in San Diego.

"We're on the cusp of a massive potential proliferation," says Matthew Garabrant, senior consultant at the Advisory Board Company, a health care professional services company in Washington, D.C. "But I want to stress the word 'potential.' "

That is because large-scale proton centers cost at least $125 million and upward of $200 million in some cities. They provide advanced radiation therapy that is considerably more expensive than older modalities and take up a large chunk of space—anywhere between 75,000 and 100,000 square feet.

Because of high reimbursement rates, investors and development companies are willing to bet on the centers, even when capital is tight for other health care projects.

Medicare pays for proton therapy for all tumor types, and private payers pay for its use on pediatric patients and for ocular and some central nervous system tumors in adults. Proponents say it holds promise, especially for treating tumors near vital organs. By concentrating subatomic particles directly on tumors, proton therapy allows for higher doses of radiation without damaging the surrounding tissue.

But the technology has not yet been proven to be clinically superior for prostate cancer, the diagnosis for which it is most often used. As comparative effectiveness and value-based medicine move to the fore, health care institutions that have announced plans to build proton therapy centers "face a great deal of business risk," Garabrant says.

And a great deal of potential reward. The nation's newest center, CDH Proton Center in Winfield, Ill., puts Central DuPage Hospital on the health care map in a new way. Jim Spear, Central DuPage's executive vice president and chief financial officer, says the decision to build the state's first center was relatively easy, considering the attractive reimbursement rates, growing patient demand and opportunity to complement the hospital's new cancer center.

When Illinois' largest radiation oncology practice learned of Central DuPage's plans, it came in as an investor—and the physicians joined the hospital's staff. The physicians and hospital partnered with a development company to form a joint venture that built the $135 million center. Spear expects CDH to match the experience of other centers, which attract 50 percent of their patients from outside the market. Based on his analysis, Spears thinks the Chicago area could support two other proton centers in the foreseeable future.

Paul Levy, the former CEO of Beth Israel Deaconess Medical Center in Boston, irked supporters of proton cancer centers when he identified the proliferation as "the medical arms race at work," saying it raises health care costs unnecessarily. "Once they are built, they are used for diseases that don't have to be treated with such expensive machines, because … once you have them, you try to keep them busy," he wrote in an entry on his blog site, "Running a Hospital,"?late last year.