Prostate cancers when looked at under the microscope can look like normal prostate tissue or they can look wild and disorganized. When these disorganized-looking cells (cancer cells) are found, they are graded on a scale called the Gleason grade of between 2 and 10. Two is the very best – a low grade, slow growing prostate cancer. Ten is the worst – a rapidly progressing, very aggressive prostate cancer. It turns out that only about 10 percent of tumors are grade 2, 3, and 4; 10 percent are 8, 9 and 10; and the vast majority are Gleason grades 5, 6 and 7.