Prostate cancer is one of the most prevalent forms of cancer affecting American men and while there are prostate screening tests that attempt to identify the presence of the disease, a new study questions the reliability of these tests in identifying the extent and severity of prostate cancer.

For the study published in the British Journal of Cancer April 10, researchers graded the prostate cancer of 847 men before and after they underwent surgery between July 2007 and October 2011.

They found that more than half of the subjects who were initially told that their prostate cancer was slow-growing and only affected the prostate actually had dangerous tumors. In a number of cases, researchers also found that biopsies which were supposed to determine the severity of the disease were not even able to detect that it has already spread to other areas of the body.

Of the 415 participants who were initially told that their cancer was slow-growing, 209 turned out to actually have the aggressive form of prostate cancer when they were assessed after the surgery. Nearly 131 were also found to have cancer that already spread in areas beyond the prostate.

Study researcher Greg Shaw, from the Cancer Research UK Cambridge Institute, said that the findings of their study suggest that prostate cancer tests could undermine the severity of the disease.

 "In our unscreened population, tools designed to identify insignificant PC are inaccurate. Detection of a wider size range of prostate tumours in the unscreened may contribute to relative inaccuracy," Shaw and his colleagues concluded in their report.

Malcolm Mason from Cancer Research UK said that the study suggests the currently considered best methods for assessing the severity of prostate cancer may not always give the right results.

"Despite the limitations that this study shows, all evidence points to active surveillance being safe provided men are carefully selected" Mason said. "But we need better methods of assigning a grade and stage so that no man has to unnecessarily undergo treatment, while making sure we detect and treat the cancers that really need it."

Data from the U.S. Cancer Statistics Working Group show that as of 2010, more than 196,000 Americans were diagnosed with prostate cancer. Of these, 28,560 died from the disease.