The Gleason system assigns a grade based on how much the cancer looks like normal prostate tissue:
Grade 1. The cancer looks a lot like normal prostate tissue.
Grades 2 through 4. The cancer has features between normal and very abnormal.
Grade 5. Cancer looks very abnormal.
Almost all cancers are grade 3 or higher. Because prostate cancers often have areas with different grades, a grade is assigned to the two areas that make up most of the cancer. These two grades are added to yield the Gleason score or Gleason sum. The first number assigned is the grade that is most common in the tumor.
Cancers with a Gleason score of 6 or less are well differentiated or low-grade. Cancers with a Gleason score of 7 are moderately differentiated or intermediate-grade. Cancers with Gleason scores of 8 to 10 are poorly differentiated or high-grade.