A Gleason score is a measure of how advanced prostate cancer is, based on microscopic appearance. The Gleason grading system is used in combination with other parameters to stage prostate cancer, which helps to predict patient outcome and guide treatment decisions.

Gleason score is based on the presence of different cellular patterns that are divided into categories numbered 1 to 5. The numbers of the two most common patterns are added together to give the Gleason score which falls somewhere between 1 and 10, and this score is used to assign a low, intermediate or high Gleason grade.

The Gleason patterns are described as follows:

If the sample tissue looks much like normal prostate tissue, it is assigned a number 1 pattern. The glands are tightly packed and small and the carcinoma is classed as well differentiated.

If the tissue shows few or no recognizable glands, with sheets of cells spreading out through surrounding tissue, it is a number 5 pattern.

The patterns in-between (2 and 4) denote increasing levels of cellular spread, with more space accumulating between the glands and the loss of glandular architecture. A number 2 pattern is assigned in cases where the glands are still well-formed but are larger and have more space between them.

However, at this stage the tumor is still classed as moderately differentiated and is not considered cancerous. Gleason pattern 3 is assigned when the cells are darker and have started to invade surrounding tissue. Prostate cancer is most commonly graded 3 and above since grades 1 and 2 are not considered cancerous.

Prostate cancer is a slow growing cancer and a Gleason pattern may be assigned to two different areas of the cancer that may show different rates of progression and growth. The Gleason pattern numbers are added together to obtain the Gleason score and most people with prostate cancer who have a tissue biopsy have a score of at least 6. A Gleason score of 6 or below indicates the cancer is unlikely to spread, while a score of 7 indicates a moderate risk of spread and 8 or more indicates a high risk that the cancer will spread.

If three Gleason patterns are observed, the two numbers that are used to make up the Gleason score include the number of the most common Gleason pattern and the number of whichever of the other two Gleason patterns is higher.