A Gleason score is the result of a system that rates how closely a prostate tumor resembles normal prostate tissue. A grade is assigned to the two largest areas of cancer within the tissue and then added together to produce the score. A tumor with a low score is likely to be slow-growing; one with a high score is likely to grow more aggressively.
A prostate cancer diagnosis is made by looking at prostate tissue under a microscope. A pathologist (a doctor who identifies diseases by studying tissues under a microscope) will look at the biopsied tissue for prostate cancer cells. Once prostate cancer has been identified, the doctor must determine both the stage and grade of the cancer.
Grade describes how closely the tumor resembles normal prostate tissue. One way of grading prostate cancer is called the Gleason system. This system uses a Gleason score of 2 to 10 to grade prostate cancer. The higher the score, the higher the grade of the tumor. High-grade tumors generally grow more quickly and are more likely to spread than low-grade tumors.
The Gleason score offers a good clue to your tumor's behavior: A tumor with a low Gleason score is likely to be slow-growing, while one with a high score is more likely to grow aggressively or to have already spread outside the prostate (metastasized).
The Gleason grading system assigns a grade to each of the two largest areas of cancer in the tissue samples. Grades range from 1 to 5, with 1 being the least aggressive and 5 the most aggressive. Grade 3 tumors, for example, seldom have metastases, but metastases are common with grade 4 or grade 5.
The two grades are then added together to produce a Gleason score. A score of 2 to 4 is considered low grade; 5 through 7, intermediate grade; and 8 through 10, high grade.
A tumor with a low Gleason score typically grows slowly enough that it may not pose a significant threat to the patient in his lifetime.
(Click Prostate Cancer Diagnosis to read more about how the Gleason score and prostate cancer grade and stage are used when making a prostate cancer diagnosis.)