Dr. Donald Gleason was a pathologist who developed a system for grading prostate cancers according to how aggressive they looked under the microscope. His system went from Grade 1 (barely qualifies as malignant) to Grade 5 (sheets of cancer cells that barely show their prostate origin). He discovered that the higher the grade of tumor, the more likely the man was to die of the cancer. Then he realized that many prostate cancers seem to have two different patterns. He tried giving each tumor a major score and a minor score and adding the two together, so that a tumor could have a score all the way from 1+1=2 to 5+5=10. This combined score was even more effective in predicting whether the cancer would prove to be aggressive.
Dr. Gleason, however, was looking at prostatectomy specimens from the Minneapolis VA Hospital, where he could look at the entire prostate gland. Now, his system is being used to evaluate needle biopsies, where all the tissue available for inspection is a few tiny threads of tissue, perhaps half an inch long, and not even all of that tissue is actually cancerous. Many times a modern pathologist will be faced with twelve of these tiny samples, and find that only a small amount of one sample contains cancer, but he still needs to assign not one but two pattern scores. This is not the situation for which the system was devised, but it is still the best we have.
Not surprisingly, the pathologists find this frustrating. In recent years, they have almost completely stopped recognizing Grade 1 and Grade 2 disease. The lowest score we see now, for practical purposes, is a Gleason 3+3=6, so the Gleason Sum range goes from 6 to 10. Pathologists have also adopted a system of putting the most prevalent score first, so that a 4+3=7 implies a worse looking tumor than a 3+4=7, even though they both get a total of 7. Clinicians today, generally speaking, consider Gleason 6 to be favorable, Gleason 8 to 10 to be unfavorable, and Gleason 7 to be intermediate.