The cancer has long been controversial because many men die with it, not of it. But doctors have trouble identifying the most dangerous cases, and patients are reluctant to leave well enough alone. Compared with many other cancer patients, men with prostate cancer can choose from a cornucopia of effective treatments, but they also risk side effects such as urinary and bowel problems and impotence.

Medicare cannot consider cost in coverage decisions; a spokeswoman said she could not supply payment data. In her studies, Perlroth found that, for men with cancer confined to the prostate, the cost of treatment over two years was $23,000 for surgery, $50,000 for IMRT, and almost $29,000 for brachytherapy, in which tiny radioactive seeds are implanted in the prostate.

Especially for men older than 65, she is partial to watchful waiting, which costs $2,436. "Doing nothing is pretty good," she said.

Because SBRT is so new, Perlroth could not get data for it or an even more expensive and controversial emerging technology, proton-beam therapy. (Highmark also has a proposed policy on protons that would allow the treatment for some men with prostate cancer when "reasonable and necessary." The University of Pennsylvania plans to open its new Roberts Proton Therapy Center in late fall.)

Proponents of SBRT say it is cheaper than IMRT by thousands of dollars. LaCouture said that, according to her analysis of Medicare payment codes, the government pays about $21,700 for hospital and physician fees for CyberKnife treatment. Regular IMRT is about $25,100, and a variation, image-guided radiation therapy (IGRT), gives doctors another $3,500.

Pro-SBRT forces also point to a recent Agency for Healthcare Research and Quality study, which found that no prostate-cancer treatment was superior to the others. The report also noted the lack of good comparative studies. "My societal hat tells me that we should be using in health care what works and not simply what patients want and not simply the latest technology, so I'm a believer in comparative-effectiveness research," said Wallner, who is vice president for medical affairs at 21st Century Oncology, which develops and operates radiation-treatment centers. Patients treated with SBRT by his company are told it is experimental.

Wallner has served on the board of a company that sought to develop proton-beam centers. He said he quit earlier this year and thinks that approach also needs further study.

The idea of giving bigger doses of radiation over fewer days is attractive, but the important thing is giving the right dose, Wallner said.

Think about having a headache and a bottle of aspirin, he said: If you take the whole bottle, "your headache will clearly go away, but you'll end up in the hospital getting your stomach pumped."

Philadelphia CyberKnife's Brady said that his center had treated about 150 prostate-cancer patients and that his results were similar to those of patients treated with IMRT.

Sinai Hospital's Brenner has treated 70 to 80 patients over the last 18 months, with few side effects and not "even a hint of a failure." He said he did not have an ownership interest in the machine, but he owns some Accuray stock.

Richard Vanderveer, a 61-year-old Gwynedd Valley psychologist who runs a company that does medical-marketing research, was diagnosed with prostate cancer earlier this year. He rejected surgery because he had "no great interest in being filleted." And, he said, getting radiation therapy every day for weeks "does not fit in smoothly with my lifestyle." He saw an ad for CyberKnife, recognized Brady's name, and started researching. Soon, he was sold. "It looked like the future," Vanderveer said. He knew he was trying something with a short track record. "You can't have long-term data on leading therapy," he said.

Vanderveer, who said he had never worked for Accuray, said he thought he made the right choice. "As I sit here today, I could not be more satisfied."