The Gleason Grade/Score has two main points: 1: Based on architectural patterns, rather than cytological ones. The Gleason System separates architectural features into 1 of 5 histological patterns. These are in decreasing differentiation order but increasing in number: Pattern 1 is the most differentiated and pattern 5 is the least differentiated. (with less differentiation in "cancer" typically being a worse prognosis. Differentiation is the degree in which the tissue, usually referring to the neoplasm, resembles the native tissue. So less resemblance is typically a worse prognosis.) 2: The second feature of Gleason grading is that the grade is not based on the highest (least differentiated) pattern within the tumor , instead it is a combination of the most often and second most often patterns seen. Gleason realized that prostatic carcinomas have multiple patterns and that the prognosis of prostatic carcinoma was split between the most prevalent and the second most prevalent neoplasm pattern. So using the Gleason system, the  most prevalent and second most prevalent pattern, if at least 5\% of the neoplasm , are added together equaling the overall Gleason score. If for example the most prevalent pattern/grade is 2, and the second most prevalent is a 1, then the Gleason score is 2+1=3. If the neoplasm has only one pattern, the number of the pattern is doubled to obtain the score. For example a tumor is all pattern 1, the Gleason score would be 1+1=2. The most differentiated tumor will have , at highest, a Gleason score of 2 (1+1 being all of type 1 architecture it just gets doubled ), while the worst differentiated neoplasm (not resembling the native prostate tissue at all) will have a score of 10 (5+5 also being all type 5 tissue). So a Gleason Score/Grade can only be between 2 and 10, there is no 0 or 1 by definition. The main way to tell the difference between a normal prostate gland and neoplastic glands is that prostatic glands normally have two cell layers in the glands, in adenocarcinoma(adeno- being that it is glandular cancer, carcinoma- being that its ectoderm derived because only ectodermal tissue makes glands) the basal ( bottom-usually cuboidal ) cell layer is lost so that you only have one top layer of cellular tissue usually ranging from columnar- pseudostratifiedin appearance . Score descriptions Using this system the most well-differentiated tumors have a Gleason score/grade of 2, and the least-differentiated tumors a score of 10. Range by definition is from 2-10, with architectural type being from 1-5 and always doubled or added together accordingly as described above. Gleason scores are often grouped  based on similar behaviour: Grade 2-4 being well-differentiated neoplasm, Grade 5-6 intermediate-grade neoplasm, Grade 7 moderately - poorly differentiated grade neoplasm, Grade 8-10 high-grade neoplasm. The majority of treatable/treated cancers are of Gleason scores 5 - 7 and are detected due to biopsy after abnormal digital rectal exam or prostate specific antigen evaluation. A Gleason score of 7 can be 3+4= 7, where the majority is pattern 3, or it can be 4+3=7  which pattern 4 dominates  resulting in a more advanced cancer . The cancer is typically located in the peripheral zone usually the posterior portion, explaining the rationale of performing the digital rectal exam. Tumors with Gleason scores 8-10 tend to be advanced neoplasms that are unlikely to be cured. Although some evidence suggests that prostate cancers will become more aggressive over time, Gleason scores usually remain stable for a few years.
The Gleason grade is based on tissue architectural patterns rather than purely cytological changes. These tissue patterns are classified into 5 grades, numbered 1 though 5. Lower numbers indicate more differentiation, with pattern 5 being the least differentiated. Differentiation is the degree to which the tissue, in this case the tumor, resembles native tissue. Greater resemblance (lower grade) is typically associated with a better prognosis.) However, the Gleason score is not simply the highest grade (least differentiated) pattern within the tumor . Rather, it is a combination of the most two most frequent patterns seen. This recognizes that prostatic carcinomas have multiple patterns and that prognosis is more accurately determined by adding the scores of the two most prevalent patterns. Using this system, the grades of the most prevalent and second most prevalent patterns ( if at least 5\% of the total) , are added together to yield the overall Gleason score. For example, if the most prevalent pattern/grade is 2, and the second most prevalent grade is 1, then the Gleason score is 2+1=3. If the neoplasm has only one pattern, the grade of that pattern is doubled to obtain the score. For example a tumor is all pattern 1, the Gleason score would be 1+1=2. The most differentiated tumor would have the lowest score, Gleason 2 (1+1 ), while the most differentiated neoplasm (not resembling native prostate tissue ) would have a the higest score, Gleason 10 (5+5 ). Gleason scores range from 2 to 10, there is no 0 or 1 by definition. Cytological differences between normal prostate to neoplastic glands are evident in changes to the typical two cell layers of the gland. In prostatic adenocarcinoma, the basal ( bottom, usually cuboidal type ) cell layer is lost , with only the top layer (usally columnar to pseudostratified) remaining . Score descriptions Using this system the most well-differentiated tumors have a Gleason score/grade of 2, and the least-differentiated tumors a score of 10. Range by definition is from 2-10, with architectural type being from 1-5 and always doubled or added together accordingly as described above. Gleason scores are often grouped together, based on similar behaviour: Grade 2-4 being well-differentiated neoplasm, Grade 5-6 intermediate-grade neoplasm, Grade 7 moderately - poorly differentiated grade neoplasm, Grade 8-10 high-grade neoplasm. The majority of treatable/treated cancers are of Gleason scores 5 - 7 and are detected due to biopsy after abnormal digital rectal exam or prostate specific antigen evaluation. A Gleason score of 7 can be 3+4= 7, where the majority is pattern 3, or it can be 4+3=7 in which pattern 4 dominates , resulting in a more advanced stage . The cancer is typically located in the peripheral zone usually the posterior portion, explaining the rationale of performing the digital rectal exam. Tumors with Gleason scores 8-10 tend to be advanced neoplasms that are unlikely to be cured. Although some evidence suggests that prostate cancers will become more aggressive over time, Gleason scores typically remain stable for a several years.
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