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, because there is no proof it saves lives.

Prostate cancer treatments are tough, especially on older men. Many men are left with sexual or bladder control problems. Some doctors instead recommend "watchful waiting" to monitor signs of the disease and treat only if they worsen, but smaller studies have given conflicting views of the safety of that approach.

The new study looked at the natural course of the disease in men who chose that option. It is the first involving so many older men — half were over 75 — and so many whose tumors were found through PSA tests.

Using the federal government's cancer database, researchers studied 9,018 men diagnosed from 1992-2002 with early-stage prostate cancer who did not get surgery, radiation or hormone therapy for at least six months. Most never got any treatment at all.

A decade later, 3 percent to 7 percent of those with low- or moderate-grade tumors (rated by how aggressive the cells appear) had died of prostate cancer, versus 23 percent of those with high-grade tumors. Overall, prostate cancer killed 10 percent of them.

"The great majority of patients ... are going to die of something else," so most older men with early-stage tumors could delay treatment, Lu-Yao said.

"If people are younger or have more advanced disease, I wouldn't say this is a safe option," but most cases are diagnosed in men 68 or older, and most are early stage, she noted.

The National Cancer Institute paid for the study. It is not the final word — that usually comes from studies where similar groups of patients are randomly assigned to get one treatment or another, and the results compared. But absent that kind of evidence, this large study "does show that a large number of men do well with no initial treatment and indeed with no treatment long term," Brawley said.

Dr. Howard Sandler, a radiation and prostate specialist at the University of Michigan, agreed, but cautioned, "there are exceptions to every rule," and some very active, healthy older men may do better having treatment right away, along with older men who have higher-grade tumors.

Earlier this month, a scientific review published in the Annals of Internal Medicine concluded that evidence was too thin to recommend treatment over watchful waiting, or one treatment over another. Studies do show that prostate cancer surgery mostly helps men under 65, said Dr. Timothy Wilt of the Minneapolis VA Center for Chronic Disease Outcomes Research, who led the review.

The new study shows that for men older than that, "observation is a very reasonable approach," he said. "Many men do quite well for a long period of time with no treatment."

The cancer conference is sponsored by the American Society of Clinical Oncology and several other groups.

Although routine PSA testing is not recommended for all men, the cancer society does advise giving men information and the option to have it starting at age 50. Screening is recommended starting at age 45 for men with a family history of prostate cancer and for black men, because of their higher risk of the disease.