Men with a perceived low risk for prostate cancer may actually not be as low risk as they thought.

Senior author Dr. Leonard Marks, a professor of urology and director of the UCLA Active Surveillance Program, and colleagues discovered that “blind” biopsy which is supposed to identify the extent of prostate cancers failed to reveal the true nature of the disease.

The UCLA researchers involved in a UCLA Active Surveillance program, the largest in Southern California, pioneered active surveillance of prostate cancer using prostate-specific antigen (PSA) tests, ultrasounds and digital rectal exams to determine whether or not tumors are growing.

Almost 400 men are enrolled in the surveillance program, opting for this method of tracking their prostate cancer while forgoing typical treatment for the disease. When targeted biopsy was used on these participants, over one-third of the men had more aggressive cancers than they previously thought.

Other studies conducted by UCLA researchers support these new findings, and have previously demonstrated the value of the new procedure in identifying prostate cancer in men who had negative biopsies.

“These findings are important as active surveillance is a growing trend in this country. It’s an excellent option for many men thought to have slow-growing cancers. But we show here that some men thought to be candidates for active surveillance based on conventional biopsies really are not good candidates,” said Dr. Marks in a statement.

Aside from skin cancer, prostate cancer is the most frequently diagnosed in men in the U.S., with an estimated 233,000 new cases of the disease to be diagnosed in 2014.

The findings of the study is are published in the peer-reviewed Journal of Urology.