Illustration of the Gleason grade system. The Gleason system is based on the degree of differentiation of a prostate tumour (graded from 1 to 5). It is the result of a microscopic study of cancer cells, which constitutes a score called the Gleason score. This score is the sum of two grades that are the most frequently represented in the analysed tumour. It varies from 2 to 10. The score 2 corresponds to a tumour that is very similar to benign tissue. The higher the score, the more aggressive the tumour. Grade 1 : Small, well differentiated glands, contained outbreak. Grade 2 : Regular glands in size and shape, ill-defined edges to the outbreak. Grade 3 : Various shaped and sized glands, irregular spacing, extremely ill-defined edges to the outbreak. Grade 4 : Fused glands, massive and with irregular edges. Grade 5 : Gland structure disappears.