From Clevelandclinic.org:

• Grade 1: The cancerous cells look a lot like normal cells.
• Grades 2-4: Cancerous cells in the tissue look less like normal cells.
• Grade 5: Cancerous cells look very abnormal.

Each area of prostate cancer may have a different grade, so pathologists pick the two areas that make up most of the cancer. They add the two areas’ grades to come up with a Gleason score.

For example, if the largest area with cancer is Grade 3 and the next largest area is Grade 5, the Gleason score is 8. Any area with a combined Gleason score of 6 or higher is considered cancerous.

What’s a normal Gleason score?

Your Gleason score doesn’t rank potential ranges like ranges set for elevated PSA (prostate-specific antigen) tests. Instead, providers break Gleason scores into three categories:

• Gleason 6: The cells look like healthy cells, which is called well differentiated.
• Gleason 7: The cells look somewhat like healthy cells, which is called moderately differentiated.
• Gleason 8, 9 or 10: The cells look very different from healthy cells, which is called poorly differentiated or undifferentiated.