A grading system devised by Dr. Donald F. Gleason (1920-2008) for prostate cancer. It is based on how the biopsy tissue looks under the microscope. Gleason scores range from 2 to 10 and indicate how likely it is that a tumour will spread. A low Gleason score means the cancer tissue is similar to normal prostate tissue and the tumour is less likely to spread; a high Gleason score means the cancer tissue is very different from normal and the tumour is more likely to spread. Many studies have confirmed that it is a very powerful prognostic factor, both for the prediction of the natural history of prostatic carcinoma and for the assessment of the risk of recurrence after surgical removal of prostate gland (total prostatectomy) or radiotherapy treatment.