Grading Prostate Cancer – The Gleason Score
- Cancer staging and grading
- What is the Gleason Score?
- Calculating the Gleason score
- Advantages of the Gleason score
- Limitations of the Gleason score
- Tests used to stage prostate cancer
Cancer staging and grading
The severity and treatment options for a diagnosis of prostate cancer depend on its size and whether it has spread to other parts of the body (metastasised). Staging refers to the range of tests that are used to assess the size and spread of a cancer, which will in turn determine the most effective treatment and the likely outcome for a given individual.
One of the ways to determine the stage of a cancer is called cancer grading. The grade of a cancer is a measurement of how many normal, healthy prostate cells have changed into cancerous cells. Prostate cancers can grow slowly or aggressively, so identifying the grade is an important part of staging a cancer, predicting its response to treatment and the likely prognosis. More aggressive types of cancer have a higher grade and are more likely to spread to other parts of the body. Grading prostate cancer requires taking a sample of the prostate tissue to be examined under a microscope by a pathologist.
Pathologists have developed a system for grading prostate cancer, called the Gleason system or Gleason score. The appearance of prostate cancer cells under a microscope are assigned scores based on different structural features, from which a man’s overall Gleason score is calculated. Higher Gleason scores indicate a higher grade of prostate cancer, indicating a more advanced or more aggressive type of cancer. The Gleason system is the most widely used system of grading prostate cancer.
A Gleason score can be calculated from a sample of prostate tissue, such as in needle biopsies, core biopsies or surgically removed prostate tissue (such as in a radical prostatectomy). Treatments for prostate cancer, such as radiotherapy or hormonal therapy can alter the growth patterns and therefore change the Gleason score. As a result, Gleason scores are generally only calculated before treatment is started. A low Gleason score (≤ 6) indicates a relatively favourable cancer, whereas a high Gleason score (≥ 8) indicates a relatively aggressive cancer.
For more information about prostate cancer types, tests, treatments and lifestyle advice, see Prostate Cancer: Overview.
What is the Gleason Score?
From 1960 to 1975, Dr Donald F. Gleason and the Veterans Administration Cooperative Urological Research Group (VACURG) recruited nearly 5,000 men from across the United States with prostate cancer. By studying these individuals, Dr Gleason and his research group developed the original “Gleason score” – a scoring system based on the appearance of growth patterns in prostate tissue sections under a light microscope. This research group determined that the appearance of these cells was strongly associated with survival rates in the men with prostate cancer. They identified nine patterns of cancer growth, arranged into five grades, with higher grades representing more aggressive cancers.
Calculating the Gleason score
Prostate cancer can have a range of growth patterns. The overall Gleason score is calculated by adding the score of the most common grade (primary grade pattern) and the second most common grade (secondary grade pattern). The primary grade pattern is the predominant pattern by area in the prostate tissue sample.
For example, if the primary grade pattern is 4A and the secondary grade pattern is 3C, the overall Gleason score is 7 (4 + 3). If only one pattern of growth is seen it is multiplied by two for a final score (e.g. score of 4 + 4 = 8). A second grade pattern forming less than 3% of the total tumour is ignored.
The use of two patterns forming the final Gleason score reflects the unique nature of prostate cancer. In Dr Gleason’s original prostate cancer studies, it was shown that prostate cancer behaves in proportion to its average grade, rather than according to the most severe grade.
Advantages of the Gleason score
Using the Gleason score means the outcomes for men with prostate cancer can be more accurately predicted. It can be used in conjunction with other tests (such as PSA blood tests and clinical examinations) to predict how a cancer will respond to types of treatment, such as radiotherapy or surgery. The Gleason score calculated from the prostate tissue after surgery (such as a radical prostatectomy) can also be used to predict the risk of recurrence of prostate cancer.
Limitations of the Gleason score
The current method of taking biopsies of tissue from the prostate is called an “extended” biopsy regimen, where 10 to 12 separate samples are taken from different areas of the prostate. This has improved the accuracy of the Gleason score from previous biopsy approaches that used fewer samples. However, research has shown that the Gleason score calculated from biopsies can potentially under-estimate the level of disease.
Prostate biopsy specimens obtained using the “extended” regimen can also potentially miss small deposits of high-grade disease. There is some controversy regarding the importance of these small deposits of more aggressive cancerous cells that are not factored into the overall Gleason score. Some research has shown that small volumes of high-grade prostate cancer could be associated with poorer outcomes.
As with other grading methods, pathologists do not always agree on the appearance of the prostate cancer tissue and Gleason scores may vary between different pathologists.
Tests used to stage prostate cancer
Men referred for a biopsy of the prostate should have had a digital rectal examination (DRE) and prostate specific antigen (PSA) blood test performed beforehand. Any abnormality on DRE (such as nodularity or asymmetry) and/or elevated or rising PSA scores should prompt an ultrasound-guided prostate biopsy so a Gleason score can be calculated. The presence of any prostate disease can then be staged using a combination of the DRE findings, Gleason score and PSA blood levels.
Guidelines recommend that if a man has a life expectancy greater than 5 years or is experiencing symptoms from prostate cancer, they should have a staging workup using bone scans, pelvic CT and/or magnetic resonance imaging (MRI) depending on the Gleason score. On the basis of these findings, a specialist can calculate the risk of the cancer progressing or recurring after treatment, as well as advise the most appropriate treatment.
For more information about the types tests that can be used to diagnose prostate cancer, see Prostate Cancer Tests.
The overall aim of staging in prostate cancer is to predict as accurately as possible those men that will respond effectively to treatment, and those men at greatest risk of death due to prostate cancer. The Gleason score is combined with clinical findings and a PSA score to determine the most effective treatment strategy for individuals.
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