Older men with early-stage prostate cancer are not taking a big risk if they keep an eye on the disease instead of treating it right away, suggests the largest study to look at this issue since PSA tests became popular.

Only 10 percent of the 9,000 men in the study who chose to delay or skip treatment had died of prostate cancer a decade later. The vast majority were alive without significantly worsening symptoms or had died of other causes.

Even the 30 percent who eventually sought treatment were able to delay it for an average of 11 years.

"It is important news," said Dr. Otis Brawley, chief medical officer of the American Cancer Society. "It may persuade some middle-of-the-roaders that we are overtreating this disease" and that PSA testing may be amplifying the problem, he said. The PSA blood test to help detect tumors has been widely used since the 1990s.

Grace Lu-Yao of Robert Wood Johnson Medical School in New Jersey led the study and will report results at a cancer conference later this week in San Francisco.

Whether to treat prostate cancer is one of the biggest medical dilemmas today. The disease is the most common cancer in American men — about 220,000 cases will be diagnosed this year — but most tumors grow so slowly they never threaten lives. There is no sure way to tell which tumors will.

PSA tests can help find tumors many years before they cause symptoms, but routine screening of men at average risk of the disease is not recommended.