How does the Gleason Score work?

The Gleason grading system is the most widely used system. In this system, because often several different tumor patterns are seen, the most common tumor pattern is assigned a score from 1 to 5 and the second most common pattern is similarly assigned a score, using the same scale. The two scores are added together to give a Gleason sum ranging between 2 and 10. Scores of 2 to 6 designate mildly aggressive prostate cancer, 7 moderately aggressive and scores of 8 to 10 are highly aggressive cancers.