In terms of side effects, "In the long term, there's really no difference in outcomes between proton radiation and IMRT for men with prostate cancer," said lead author Dr. James Yu, a radiation oncologist at Yale University School of Medicine in New Haven, Connecticut.

Proton therapy advocates argue that protons blast radiation directly to the tumor and therefore avoid side effects. The more common "intensity-modulated" radiotherapy (IMRT) exposes some healthy tissue to radiation that researchers hypothesized would increase side effects and even additional cancers.

After a year, however, the study found the same number of side effects among men who'd had both treatments.

Prostate cancer, the most common cancer in men, kills about 28,000 Americans each year. However, many men don't die of the disease, because many tumors grow very slowly.

Treatments include chemotherapy, hormone therapy, surgery, and frequent surveillance - aka "watchful waiting."

Although researchers are at odds over which treatment - proton therapy or IMRT - is the better option for men who choose radiation, that hasn't stopped the growth of proton beam centers. There are ten such centers in the U.S., according to the National Association for Proton Therapy, with eight more under development or being built.

Each one can cost more than $125 million, and Medicare pays doctors about twice as much for proton therapy.

For the study in the Journal of the National Cancer Institute, researchers tracked Medicare claims in 2008 and 2009 for treatment-related complications in nearly 28,000 men with prostate cancer for up to a year. Only two percent of the prostate cancer patients underwent proton therapy and the remainder had IMRT.

After six months, nearly 10 percent of IMRT-treated patients, and six percent of proton therapy patients, had side effects including incontinence, a burning sensation while urinating or difficulty getting an erection. However, the difference disappeared a year after treatment, when nearly one in five patients suffered side effects regardless of which radiation treatment they had.

Yu and colleagues found that proton therapy costs nearly twice as much: $32,428 per course of treatment, versus $18,575 for IMRT. That difference was consistent with that found in other studies.

"The ball is in the court of the proton folks in terms of proving a benefit," Yu told Reuters Health.

The study only looked at side effects, and did not compare the effectiveness of the treatments, which proton therapy advocates said was a significant weakness.

If Yu is "willing to make recommendation or clinical judgments based on this sort of data, I think he's at risk to doing a disservice to his patients," said Dr. Andrew Lee, director of the Proton Therapy Center at the University of Texas MD Anderson Cancer Center in Houston. "It's like trying to read a license plate from 30 thousand feet up in the air."

Lee, who was not involved in the new work, said that the study's length - a year - wasn't enough time to look at the full scope of side effects from either treatment. The study also failed to include side effects that didn't require a hospital visit, and couldn't say how long treatments lasted.

Proton therapy isn't for everyone, both noted. Lee said the treatment was best for young healthy patients, while Yu said it is most useful for cancers in children or in sensitive areas where minimizing the radiation is critical.

Yu would not recommend it for prostate cancer.

"The cancer center next door or the radiation oncologist in the community will likely do just as good a job at treating prostate cancer with IMRT as a proton center three times out of the way," Yu said.

SOURCE: bit.ly/V6PkLT Journal of the National Cancer Institute, online December 14, 2012.

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