Many men with low-risk prostate cancer get aggressive treatment, increasing the risk of serious side effects, U.S. researchers said on Monday.

They said more than 40 percent of men who fell below the current standard for getting a biopsy had their prostates removed surgically, and a third had radiation therapy.

They argue that current efforts to lower the threshold of what is considered an abnormal prostate cancer screening test would add significantly to the number of men who are overtreated for cancers that might never harm them.

"The tremendous improvement in survival has been attributed to early detection and treatment," Yu-Hsuan Shao of the Cancer Institute of New Jersey in New Brunswick and colleagues wrote in the Archives of Internal Medicine.

"However, there have been concerns about the potential overdiagnosis and overtreatment of localized prostate cancer."

A 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.

Prostate cancer screening is most often done with a blood test that measures blood concentrations of prostate-specific antigen or PSA, a protein made in the prostate that becomes elevated men with prostate cancer.

Generally, a PSA reading of 4 nanograms of PSA per milliter of blood is considered normal. But in one large study, prostate cancer was diagnosed in 15.2 percent of men with a PSA level at or below 4 nanograms per milliter, or about 2.4 percent overall.

Shao and colleagues studied treatment practices among 124,000 men in this low-risk group who were newly diagnosed with prostate cancer between 2004 and 2006.

In men with PSA values of 4 nanograms per milliliter of blood or lower — the current threshold for getting a biopsy — 44 percent had their prostates removed, and 33 percent of men had radiation treatment.

"Despite their lower risk of having clinically significant disease, treatment rates for men with PSA values of 4.0 nanograms per milliliter or lower were comparable to those of men presenting with PSA values between 4.0 and 20.0 nanograms per milliliter," Shao and colleagues wrote.

They estimate that lowering the treatment threshold from 4.0 to 2.5 nanograms per milliliter would double the number of men considered to have abnormal PSA levels to 6 million.

And they estimate that 82.5 percent, or 1.9 million of these men would get an aggressive treatment, even though only 2.4 percent would have high-grade cancer.

"These results underscore the fact that PSA level, the current biomarker, is not a sufficient basis for treatment decisions," Shao and colleagues wrote.

They said without technology that allows doctors to distinguish aggressive from mild cancers, lowering the biopsy threshold "might increase the risk of overdiagnosis and overtreatment."

Prostate cancer is the second most common cancer in men after lung cancer, killing around 254,000 men a year around the world. Overdiagnosis can lead to treatments such as surgery, radiation or hormone therapy that can cause serious side effects such as impotence and incontinence.