In a study of older men who had died from causes other than prostate cancer, almost half were found to have prostate tumors.
And up to half of those tumors detected on autopsy would have qualified for treatment had doctors known about them while the men lived, though none had been the cause of death.
That suggests the criteria for treatment "might be worth reexamining," according to the study authors, and adds to a growing body of evidence that a wait-and-see approach might be better than treatment for many prostate cancers.
"The study suggests that the progression of early prostate cancer, including some more aggressive forms of the disease, is far from inevitable within a man's lifetime as many such tumors are found in men who died from other causes when their prostate is analyzed on autopsy," said lead author Dr. Alexandre Zlotta, director of Uro-oncology at Mount Sinai Hospital in New York.
"The study concludes that it is probably worth re-examining our current definitions of clinically unimportant and clinically significant prostate cancer," Zlotta said in an email.
He and his colleagues examined the prostate glands of more than 300 men over 60 who had died from a variety of causes, but not prostate cancer. They autopsied 220 men in Russia and 100 in Japan, both countries where the prostate-specific antigen (PSA) screening test is not as commonly used as in the United States.
Medicare currently covers a yearly PSA test for men over 50 who qualify.
Zlotta and his colleagues selected a Caucasian population of Russian men similar in lifestyle and risk factors for prostate cancer to U.S. men in order to look at how common the cancer is in older men who have not been routinely screened.
In the U.S., the number of tumors they found would have been much lower because many would have been identified by screening and treated or removed before death, Zlotta pointed out.
For comparison, he said, "we chose an Asian population in Japan because clinically detected prostate cancer and mortality are much lower among Asian men compared with Caucasian men and their lifestyles/diet are dramatically different."
Though many fewer Japanese men are diagnosed with or die from prostate cancer than men in the U.S., the new study found that the disease is present in the same proportion of men, and serious tumors are even more common in Japan, based on posthumous prostate examinations.
Almost 40 percent of the Russian men, had tumors of the prostate, compared to 35 percent of the Japanese men, according to the results published in the Journal of the National Cancer Institute.
One in four tumors in the Russian men were deemed serious and would likely have been treated with surgery or radiation in the U.S., compared to one in two tumors in the Japanese men.
But there are still many differences between Russian and American men, so it is difficult to generalize the results, according to Dr. Stacy Loeb, a urologist at NYU Langone Medical Center in New York.
"We should be cautious about trying to apply the results to Americans where we have a very heterogeneous population including very high risk groups such as African Americans," Loeb told Reuters Health.
In the United States, about 239,000 men are expected to be diagnosed with prostate cancer in 2013, but far fewer - less than 30,000 - will die of it, according to the American Cancer Society.
That's still more than will die of the disease in Japan, despite the American men getting more screenings and treatments - a curious inconsistency researchers do not understand and Zlotta called the "million dollar question."
Prostate cancer screening does ultimately reduce the number of painful advanced cancers and deaths from the disease, but it also picks up smaller cancers, and treating those aggressively could lead to unnecessary side effects, Loeb said.
Prostate removal costs about $13,000, and may lead to impotence or incontinence.
The study authors note that the lifetime risk of a man in the U.S. being diagnosed with prostate cancer is 17 percent, but his risk of dying from prostate cancer is 3.4 percent.
That suggests many of those cancers would not advance quickly or at all, and something else would kill the man long before the prostate tumor. The problem is how to know which tumors will be harmless.
Experts said a new screening test may be needed that differentiates between more dangerous cancers and the many tumors that will not eventually kill, thereby avoiding the expense and risks of treatment and removal.
"An example is the Prostate Health Index (phi) which was recently approved by the FDA," Loeb said. "It is a simple blood test that combines PSA with 2 other tests, and was shown to better predict the risk of aggressive cancer."
"The holy grail is to find only those prostate cancers which are life threatening," she said. "The good news is that this is a very active area of research, and the whole process of screening and assessing continues to improve all the time."