CHICAGO (Reuters) – Routine screening for prostate cancer has resulted in more than one million US men being diagnosed with tumours who might otherwise have suffered no ill effects from them, US researchers said on Monday.

They said prostate cancer screening is a double-edged sword, catching serious cancers in a few but causing needless worry and expense for the majority of men, who may be getting treatment for tumours growing too slowly to do any harm.

The team looked to see how many additional men have been diagnosed with prostate cancer since the introduction in 1986 of a widely used blood test for prostate cancer that looked for a prostate-cancer specific antigen, or PSA.

"Our estimate is that number is about 1.3 million people in the United States. That is a huge effect," said Dr. H. Gilbert Welch of the VA Outcomes Group in White River Junction, Vermont, whose study appears in the Journal of the National Cancer Institute.

More than one million of those were treated, they found.

"These are men who could not be helped by treatment because their cancer was not destined to cause them symptoms or death," Welch said in a telephone interview.

The increased diagnosis rate more than tripled in men aged 50 to 59 and increased more than a seven-fold in men under age 50.

And while prostate cancer deaths have declined since the introduction of PSA testing, Welch said about 20 men had to be diagnosed and treated for every one who benefited.

Prostate cancer is the second most common cancer in men worldwide after lung cancer, killing 254,000 men a year globally. 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.

All current forms of treatment — surgery, radiation or hormone therapy — can cause harm, resulting in impotence and incontinence in about a third of patients, Welch said.

Just being told you have cancer can do harm, causing anxiety and feelings of vulnerability. And having a cancer diagnosis can mean some people cannot get health insurance, Welch said.

A US expert panel last year urged doctors to stop screening men over 75, but doctors still disagree about the right approach to PSA screening. Two large studies — one in Europe and one in the United States — that aimed to settle the matter produced conflicting results.

Instead of discarding the PSA test, one solution may to simply watch and wait for signs the tumour is growing. A study of more than 51,000 men published on Monday in the Journal of Clinical Oncology found men diagnosed with low-risk tumours who waited were still doing fine an average of eight years after diagnosis — and some as many as 20 years later.