The survival rate for early-stage prostate cancer is 99 percent after 10 years, regardless of whether men undergo surgery, radiation or are "actively monitored," according to studies published Wednesday. Researchers hailed the results as good news, saying they had been expecting a survival rate of 90 percent.

The two new studies, published in the New England Journal of Medicine, also illustrated the complicated treatment equations facing men with early-stage prostate cancer, and they immediately set off a debate among physicians about how to interpret the results.

Researchers from the universities of Oxford and Bristol found no difference in survival rates among men who were randomly assigned to surgery, radiation or monitoring; it is the largest study of its kind. Those who underwent surgery or radiation cut in half the risk that their disease would spread to bones and lymph nodes, compared with those who were simply monitored. Although the treatment didn't extend life during the first 10 years, a survival benefit might yet emerge in the next five or 10 years, the researchers said. The scientists also found that the surgery and radiation treatments sometimes caused severe side effects, including sexual dysfunction, incontinence and bowel problems, that hurt the patients' quality of life.

"Men with newly diagnosed, localized prostate cancer need to consider the critical trade-off between the short-term and long-term effects of radical treatments on urinary, bowel and sexual function and the higher risks of disease progression" that comes with monitoring, the researchers wrote.

Freddie Hamdy, a professor of surgery and urology at the University of Oxford who was the lead researcher, said his advice to patients is that they should not "rush to receive treatment, but to really digest and really look at the side effects that the treatments produce."

Otis Brawley, chief medical officer for the American Cancer Society, who had no role in the study, agreed. "We have already had an increased number of men in the United States who choose to be observed after diagnosis, and I would hope this would encourage more of that," he said of the study. But he also acknowledged that there would be differences of opinions about the study, adding, "There's a little something for everybody."

Indeed, in an editorial that accompanied the studies, Anthony D'Amico, chief of genitourinary radiation oncology at Brigham and Women's Hospital and Dana-Farber Cancer Institute, emphasized the studies' finding that surgery and radiation reduced the risk of metastasis and disease progression. "If a man wishes to avoid metastatic prostate cancer and the side effects of its treatment, monitoring should be considered only if he has life-shortening coexisting disease" that might prevent him from living for much longer than 10 years.

Jim Hu, a urologic oncologist at Weill Cornell Medicine, said he was struck by the "significant differences in the likelihood of metastatic disease," which he said could eventually translate into a survival benefit for the men who got surgery or radiation.

But other physicians said that might not be the case, and that men who got the treatments might die first from other causes, such as heart attacks.

For the British study, more than 82,000 men between 1999 and 2009, ages 50 to 69 were tested with a blood test called PSA, for prostate-specific antigen. About 1,600 were diagnosed with early-stage cancer and agreed to take part in the study. They were randomly assigned to one of three groups: Those who got surgery to remove the prostate, called a radical prostatectomy; those who got radiation; and those who got active monitoring, which researchers described as "less passive" than the "watchful waiting" stance used by many doctors in the United States. Researchers studied mortality rates at 10 years and whether the cancer progressed and spread; a second study addressed the impact of the treatments reported by the men.

Their conclusion was that the three approaches did not translate into "significant differences" in deaths caused by prostate cancer or other causes. "Thus," they concluded, "longer-term follow-up is necessary."

Part of the conundrum facing researchers and clinicians is that prostate cancer is both overtreated and undertreated. For years, many doctors say, men were overtreated based on the results of a PSA test. The test can result in "false positives," or encourage aggressive treatment of malignancies that are so slow-growing that they aren't a threat. In many cases, men no longer get PSA tests as a routine matter.

However, men still die from prostate cancer — because doctors don't know which cases will turn into a lethal, metastatic form of the disease. More than 180,000 men in the United States will be diagnosed with prostate cancer this year, and more than 26,000 will die from the disease.

"Everybody is searching for the key to identifying cancers that will progress," or spread, said Jenny Donovan, a researcher at the University of Bristol who took part in the studies. "We all want to do that."

Brawley, of the American Cancer Society, noted: "This study has just 10 years of data, and it may take 20 or 25 years to see survival improvements from treatment, so we cannot say yet with certainty whether and how much of an advantage treatment has over no treatment. We can only say that at 10 years, there is no difference."