Tumors with a low Gleason score are less likely to show aggressive behavior and therefore are less likely to have spread outside of the gland to lymph nodes (metastases). Gleason's original data showed a progressive increase in death due to the cancer with an increasing Gleason score.
Gleason described five distinct patterns of prostate cancer growth which were based on the pattern that the tumor glands made (as viewed through the microscope on stained tissue sections). These are commonly referred to now as Gleason patterns 1 through 5, with pattern 1 being the most well differentiated and therefore the most favorable. Most, if not all tumors, have a mixture of patterns.
To obtain a Gleason score or grade, the dominant pattern is added to the second most prevalent pattern to obtain a number between 2 and 10. If a tumor has patterns 3 and 2, the score would be 5. If the tumor has only one pattern, or less than 5% of a secondary pattern, then the single pattern is added to itself (e.g. 3+3=6).
The Gleason grading system is not the only grading scheme in existence, but at present it is the most popular one in the United States. The importance of any grading system is that it allows patients with tumors of similar biologic behavior to be compared in studies and provides some useful information for patient treatment options.