A way of describing prostate cancer based on how abnormal the cancer cells in a biopsy sample look under a microscope and how quickly they are likely to grow and spread. It is based on the Gleason score, which is another type of prostate cancer grading system. Grade Group scores range from 1 to 5. The lower the Grade Group score, the more the cancer cells look like normal cells and are likely to grow and spread slowly. The Grade Group system is used to help plan treatment and determine prognosis (outcome).