A Gleason score is given to prostate cancer base upon its microscopic appearance. The Gleason score is important because higher Gleason scores are associated with worse prognosis. This is because higher Gleason scores are given to cancer which is more aggressive. To assign a Gleason score, a piece of prostatic tissue must be obtained (a biopsy). This is done either by removing the entire prostate gland (prostatectomy) or by sampling the gland with a needle introduced through the rectum.