LONDON (Reuters) - An extensive study into the merits of screening men between the ages of 50 and 65 for prostate cancer has found it can cut death rates from the disease by as much as half, Swedish scientists said Thursday.

But the findings don’t necessarily mean nationwide prostate screening programs should introduced, experts said, since they run the risk of significant overdiagnosis of tumors in men who would not have suffered any harm from their cancer.

Researchers from the University of Gothenburg conducted a trial involving 20,000 men who were divided equally into a group that was offered prostate screening and a group that was not.

The screening method used was so-called prostate-specific antigen (PSA) testing, which is widely used in the United States and other developed countries to detect early signs of tumors.

Over 14 years of follow-up, prostate cancer death rates were cut almost by half in the screening group compared with the non-screening group, as men were diagnosed and treated in time to stop the cancer from killing them.

Jonas Hugosson, who led the study, said the results showed that PSA screening of all men this age group “can result in a relevant reduction in cancer mortality.”

Screening for cancer -- or for clues such as pre-cancerous cells -- is strongly encouraged in wealthy nations as a way of improving public health. But there are growing doubts about whether the screening’s benefits always outweigh the negatives, with the main concerns centering on the risk of overdiagnosis.

A large U.S. study published last year found that routine prostate screening has resulted in more than 1 million American men being diagnosed with tumors who might otherwise have suffered no ill effects from them. In that study, researchers said that around 20 men had to be diagnosed and treated for every one who benefited.

In the Swedish study, which was published in The Lancet medical journal Thursday, the researchers said the risk of overdiagnosis was less, but still 12 men needed to be diagnosed to save one life.

Prostate cancer is the second most common cancer in men after lung cancer, killing around 254,000 men a year around the world. U.S. doctors have routinely recommended PSA screening in men over 50 based on the assumption that early diagnosis and treatment is better than standing by and doing nothing.

But fears about overdiagnosis, which can lead to treatments such as surgery, radiation or hormone therapy that can cause serious side-effects such as impotence and incontinence, have so far dissuaded many European countries from nationwide screening.

A similar debate over breast cancer screening is also raging among doctors in Europe and United States, with critics of national mammography screening programs saying they needlessly harm thousands of women’s lives by picking up tumors that would otherwise not have caused a problem.

In a commentary on the Swedish study, David Neal from Britain’s Cambridge University, said it showed that “in certain circumstances, PSA testing and early diagnosis reduces death from prostate cancer.”

But he added: “It does not imply that PSA screening programs should now be introduced internationally.”