Gleason score is a number assigned to your prostate cancer tissue based on a microscopic exam of the tissue; the score reflects how aggressive (fast-growing) the cancer is. Doctors use Gleason score in combination with other tests to determine which treatment might be best for the patient, and how soon it should start.

Analyzing prostate tissue samples under a microscope, the pathologist:

• Assesses cell differentiation, that is, how closely the tumor cells resemble normal, healthy prostate cancer cells. Well-differentiated cells tend to be slower-growing. Poorly differentiated cells tend to be more aggressive.
• Grades prostate tissue on the Gleason scale, with 1 being the most differentiated and 5 being the least differentiated.
• Assigns a grade to the two largest areas of the tissue sample. This is done because prostate tumors usually have different levels of differentiation. Grading two areas enables the pathologist to get a more accurate picture of the cancer's aggressiveness.
• Combines the two grades to arrive at the Gleason Score, with 2 being the least aggressive and 10 being the most aggressive.