If you have been diagnosed with prostate cancer, the pathologist will have accurately graded your tumor and provided a total number of cores which contain cancer and its percentage. The Gleason score is based upon the degree of loss of the normal glandular tissue architecture (i.e., shape, size and differentiation of the glands) as originally described and developed by Dr. Donald Gleason in 1974.
Unlike colon, breast and lung cancer where a single large mass is noted, prostate cancer is considered a microscopic and multifocal disease - a "salt and pepper" disease. Prostate cancer grading is objectively categorized by the Gleason system, which uses scores of 1 to 5 to describe the cancer. Since 2002, pathologists only use grades 3, 4, or 5 for their scoring system.
Two scores are awarded to each tumor on the biopsy slide: a MAJOR score (the dominant cell type) and a MINOR score. The Major Gleason grade has to be greater than 50 percent of the total pattern seen (i.e., the pattern of the majority of the cancer observed). The Minor Gleason grade has to be less than 50 percent, but at least 5 percent, of the pattern of the total cancer observed. The sum of the primary and secondary Gleason grades is shown as the Gleason sum (i.e., primary grade + secondary grade = GS; i.e., 4+3 or 3+4 = GS 7). The higher the Gleason sum, the higher the grade of the tumor. High-grade tumors (Gleason sum 8-10) can grow more quickly and are more likely to spread than lower-grade tumors.