A Gleason score—on a scale of 2 to 10—is a way to describe how prostate cancer cells look under a microscope. It helps predict how likely the cancer cells are to grow and spread. It's called a score because it adds together the two grades of cancer seen in the largest areas of the biopsy sample.
The higher the Gleason score, the more likely it is that the prostate cancer will grow rapidly and spread (metastasize) to other parts of the body.
Prostate biopsy results include both a Gleason score and a Grade Group number. The Grade Group numbers are based on the Gleason scores and range from 1 to 5. For example, Gleason scores 2 through 6 are equal to Grade Group 1, where the cancer cells look a lot like normal prostate cells and usually grow slowly. Gleason scores 7 through 10 (Grade Groups 2 through 5) describe cancer that is more likely to grow and spread.