CyberKnife radiosurgery -- which uses narrow beams of radiation to kill several types of cancer - is marketed as a less invasive, more convenient way to treat prostate cancer, a pitch that has proved convincing for about 3,000 men over the last six years.

But some prostate-cancer experts have reservations. Because prostate cancer grows slowly and because radiation side effects can emerge after many years, they say it is too soon to call the treatment a success.

And those concerns have unleashed a battle over insurance payments that may soon leave thousands of men unable to afford this increasingly popular option.

Highmark Medicare Services Inc., which administers payments for 4.2 million Medicare subscribers in Pennsylvania, New Jersey, Delaware, Maryland, and Washington, D.C., is considering dropping most coverage for the treatment.

Medicare has not set a national standard. The treatment is covered in 36 states.

Private insurers often follow Medicare's lead. Locally, Independence Blue Cross covers the radiation treatment - known generically as stereotactic body radiotherapy (SBRT) - for prostate cancer when "medically necessary," but it is also reevaluating its policy. A November decision is expected.

Philadelphia-based Cigna Corp. does not cover SBRT for prostate cancer.

The debate pits doctor against doctor and raises tough questions about when insurance should pay for promising new techniques, especially when technology is a key driver of rising costs.

In prostate cancer, new treatments delivered by expensive machines have been embraced without large head-to-head comparisons of effectiveness. To further complicate matters, doctors on both sides say the arguments are clouded by ownership interests in various machines and other financial incentives.

"There's a lot of politics involved in this. There's a lot of self-interest. There's a lot of greed," said Mark Brenner, chief of radiation oncology at Sinai Hospital in Baltimore and a CyberKnife supporter.

Convenience is the big attraction. Men who choose radiation often get about 40 treatments of intensity modulated radiation therapy (IMRT) over eight weeks. SBRT offers five higher doses over a week or two.

Though more than one machine can deliver this kind of therapy, CyberKnife, made by Accuray Inc., is the one most associated with prostate SBRT. Accuray has hired a public relations firm to campaign against the proposed Highmark Medicare rule change, saying that "a change in Highmark's policy would be devastating to local men." It certainly would be hard on owners of CyberKnife machines, which cost about $5 million.

At Philadelphia CyberKnife in Havertown, one of the region's three machines, 19 percent of patients have prostate cancer, administrator Rick Habacivch said. Luther Brady, a well-known radiation oncologist who is the center's medical director, said losing Medicare coverage would be a "dramatic blow."

In South Jersey, about one-tenth of patients treated with Cooper University Hospital's year-old CyberKnife machine have prostate cancer, said Tamara LaCouture, chief of the department of radiation oncology. She said she thought she would have more patients if her center was not competing with a new IMRT facility owned by a large group of urologists, the specialists who typically refer patients for prostate treatment.

Without insurance coverage, LaCouture said, few patients would pick SBRT: "To expect a patient in today's financial climate to choose this over a covered modality, that's just not realistic."

Fox Chase Cancer Center, which has a new CyberKnife, plans to use the machine only for prostate-cancer patients enrolled in clinical trials.

Highmark, which pays for SBRT for about 100 prostate patients a year, says it based its proposal to drop coverage on reports from ASTRO, the American Society for Radiation Oncology. That group's emerging-technology committee released a report last year that called SBRT promising, but not yet well proven.

"What the report said, essentially, is that early data, very immature data, suggest that it may be tolerated and may be effective," said Paul Wallner, a radiation oncologist who co-chaired the panel. "It's interesting. It's clearly seductive. ... We don't think there is yet any mature data."

Prostate-cancer treatment is a big business. Prostate is the second-most-common cancer in men; the American Cancer Society estimates that more than 192,000 will be diagnosed with it this year, and 27,000 will die. Daniella Perlroth, a Stanford University researcher who has studied treatments and their cost, estimates $3 billion a year is spent in the United States.