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The most common grading system for prostate cancer is the Gleason classification system. It is used to describe how aggressive a prostate cancer tumour is, and how likely it is to spread. The Gleason classification is used only for adenocarcinoma, the most common type of prostate cancer.
The Gleason classification reflects how different the tumour tissue is from normal prostate tissue. It uses a scale from 1 to 5. The doctor gives the cancer a number based on the patterns and growth of the cancer cells. The lower the number, the more normal the cancer cells look and the lower the grade. The higher the number, the less normal the cancer cells look and the higher the grade. Grades 1 and 2 are not commonly used because the tumour tissue looks and acts like normal tissue. Most prostate tumours are grade 3 or higher.
The cancer cells are well differentiated, which means they still form well-defined glands.
The cancer has grown into, or invaded, the surrounding prostate tissue.
The cancer is less aggressive and has a favourable prognosis.
The cancer cells are between grade 3 and grade 5
The cancer cells are poorly differentiated, which means they are very abnormal and don’t look and act like normal glands.
The cancer is more aggressive and has a less favourable prognosis.
To assign a Gleason score (also called Gleason sum), the pathologist looks at the biopsy sample of the tumour to find the 2 most common types of glandular growth patterns within the tumour. A grade from the scale is given to each of these 2 patterns. The 2 grades are added together to get the total Gleason score. For example, if the grade given to the most common growth pattern is 3 and the grade given to the second most common growth pattern is 4, the total Gleason score is 7.
The Gleason score is always between 6 and 10. Higher Gleason scores indicate more aggressive tumours. Most prostate cancer tumours are low and intermediate grades (Gleason score 6–7). Gleason scores below 6 are not usually given because it is difficult for the pathologist to determine with certainty that the lowest grade tumours are in fact cancer.