Although it is not the final word, a large new study suggests that older men live longer if their prostate cancer is treated rather than just observed to see if it spreads.

Median survival was more than 13 years for men who had surgery or radiation, but only 10 years for those who chose "watchful waiting," according to an analysis of Medicare and federal cancer registry records on nearly 50,000 men.

Looked at another way, men who were treated were half as likely to die — 59 percent of them were alive at the end of the study compared with only 27 percent of the others.

"Even when you adjust for all the differences between the groups, there still is a survival advantage to being treated," said Dr. Yu-Ning Wong of Fox Chase Cancer Center in Philadelphia. She reported the results Saturday at a conference in San Francisco.

Cancer of the prostate, a gland at the base of the penis that makes seminal fluid, is the most common major malignancy in American men. Each year in the United States, there are more than 232,000 new cases and 30,000 deaths from it. Worldwide, there are 680,000 cases and 221,000 deaths.

In most men, the disease grows so slowly that it doesn't threaten their lives, but in others it is fatal. This makes deciding whether to undergo treatment a dilemma: Doctors know they're treating many men who don't need it, but they can't accurately predict who does.

Treatment also can cause problems in having sex or controlling the bladder, and there's little evidence that it saves the lives of older men with small tumors that don't seem to grow rapidly. For these reasons, many men choose "watchful waiting" with regular monitoring and treatment only if it gets worse.

Studies conflict on whether waiting is safe. A study last year of 767 Connecticut men found little harm in waiting or using hormone treatment instead of surgery. But a more rigorous study in Scandinavia found clear benefit from surgery, at least for those 65 and younger.

Wong's new study looks at men older than 65 diagnosed in the 1990s with small or not very aggressive tumors confined to the prostate. Within six months of diagnosis, 14,560 chose watchful waiting and 34,046 had surgery or radiation.

After an average of 13 years of follow-up, she compared their survival, taking into account differences in age, tumor size, general health and other factors.

"Even among the men aged 75 to 80, there still was a benefit to treatment," she said.

Cancer specialists urged caution in interpreting the findings because they come from just observing a large group of men, not scientifically assigning some to treatment and others to observation.

Even though Wong tried to adjust for health factors, men who choose watchful waiting often do so because they're not well enough to withstand treatment, said Dr. Peter Albertsen, chief of urology at the University of Connecticut, who led the earlier study on Connecticut men.

"You'd expect these guys to die sooner," he said of watchful waiters.

Still, the new study adds important information, said Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society.

"If you're a healthy 77-year-old or 80-year-old man, this study might tip the balance" toward treatment, though it's not strong enough to dictate treatment decisions, he said.

A more definitive answer should come from two studies now under way — one by the Veterans Administration in the United States and another in England — that are randomly assigning men to watchful waiting or treatment.

Also, a slightly different approach — "active waiting" — is increasingly being advised for men over 70, said Dr. Howard Parnes, director of prostate cancer prevention at the National Cancer Institute.

"Active surveillance is replacing watchful waiting for men healthy enough to undergo treatment who wish to defer therapy. This means close follow-up, including annual prostate biopsies," he said.