More U.S. physicians are sparing their low-risk prostate cancer patients from surgery, radiation and hormone therapy in favor of monitoring their patients over time -- a strategy called watchful waiting, a new study shows.

The number of low-risk patients who didn't undergo treatment jumped from as low as 7 percent from 1990-2009 to 40 percent from 2010-2013, the study revealed. These findings indicate that more patients are being monitored to see if their conditions get worse.

This is "excellent news" about the popularity of "active surveillance," said study author Dr. Matthew Cooperberg, the Helen Diller Family Chair in Urology at the University of California, San Francisco.

"We expected to see a rise in surveillance rates, but were surprised by the steepness of the trajectory," he said. "This really does represent a paradigm change, and it's faster than the typical pace of medical evolution."

The reason for the debate over who gets treated is that prostate cancer treatments, such as surgery or radiation, can lead to serious long-term side effects, such as incontinence and impotence.

In addition, some prostate cancers are slow-growing and are less likely to cause problems, especially for older men, the American Cancer Society notes.

Last week, a study in the journal JAMA Internal Medicine suggested that the wide majority of men with low-risk prostate cancer between 2010 and 2011 had treatment.

But, that study defined low-risk in various ways that included between 11 percent and 40 percent of prostate cancer patients.

The new study examines the medical records of more than 10,000 men from 45 urology practices. And, it uses a single definition of low-risk. The current study also looks at information through 2013.

In addition to finding a higher rate of watchful waiting in all men, the study also found that those aged 75 and older were much less likely to get potentially unnecessary treatment.

Among low-risk men aged 75 and older, the rate of watchful waiting shot up from 22 percent in 2000-2004 to 76 percent in 2010-2013, the study found.

As for patients at greater danger, "we're seeing more aggressive management of higher- risk disease with surgery, radiation or both, which is also a trend toward better management," Cooperberg said.

But, the findings aren't necessarily all positive.

"Ultimately, the number of men who will die of prostate cancer because they chose active surveillance cannot be zero by definition," Cooperberg acknowledged. "But it is a very low number, far lower by most estimations than the number of those harmed by avoidable surgery, radiation, etc."

Dr. David Penson, the Hamilton and Howd Chair in Urologic Oncology at Vanderbilt University Medical Center in Nashville, Tenn., agreed with Cooperberg that the statistics are "very good news."

"The net health benefit for men with prostate cancer is likely more positive because we are treating the men who need treatment while we are avoiding the risk of side effects in those who don't," he said.

Also, he said, the findings have implications for the debate about screening men for prostate cancer via "PSA" blood tests.

"One of the arguments against screening is that we are over-detecting and overtreating prostate cancer, and because we are overtreating men who don't need treatment, we are doing more harm than good. By reducing the rates of overtreatment, we are likely increasing the benefit of screening," he noted.

The study appears in the July 7 issue of Journal of the American Medical Association.