Gleason Grading System
The Gleason Grading system is used to help evaluate the prognosis of men with prostate cancer. Together with other parameters, it is incorporated into a strategy of prostate cancer staging which predicts prognosis and helps guide therapy. A Gleason score is given to prostate cancer based upon its microscopic appearance. Cancers with a higher Gleason score are more aggressive and have a worse prognosis.
The scoring system is named after Donald Gleason, a pathologist at the Minneapolis Veterans Affairs Hospital who developed it with colleagues at that facility in the 1960s. In 2005 the Gleason system was altered by the International Society of Urological Pathology. The criteria were refined and the attribution of certain patterns changed. It has been shown that this 'modified Gleason score' has higher performance than the original one, and is currently assumed standard in urological pathology. In this form, it remains an important tool.
Prostate specimens and processing
Most often, a urologist or radiologist will remove a cylindrical sample (biopsy) of prostate tissue through the rectum, using hollow needles, and biomedical scientists in a Histology laboratory prepare microscope slides for H&E staining and Immunohistochemistry for diagnosis by a pathologist. If the prostate is surgically removed, a pathologist will slice the prostate for a final examination.
Histologic patterns 1 through 5
A pathologist microscopically examines the biopsy specimen for certain "Gleason" patterns. These Gleason patterns are associated with the following features:
- Pattern 1 - The cancerous prostate closely resembles normal prostate tissue. The glands are small, well-formed, and closely packed. This corresponds to a well differentiated carcinoma.
- Pattern 2 - The tissue still has well-formed glands, but they are larger and have more tissue between them, implying that the stroma has increased. This also corresponds to a moderately differentiated carcinoma.
- Pattern 3 - The tissue still has recognizable glands, but the cells are darker. At high magnification, some of these cells have left the glands and are beginning to invade the surrounding tissue or having an infiltrative pattern. This corresponds to a moderately differentiated carcinoma.
- Pattern 4 - The tissue has few recognizable glands. Many cells are invading the surrounding tissue in neoplastic clumps. This corresponds to a poorly differentiated carcinoma.
- Pattern 5 - The tissue does not have any or only a few recognizable glands. There are often just sheets of cells throughout the surrounding tissue. This corresponds to an anaplastic carcinoma.
In the present form of the Gleason system, prostate cancer of Gleason patterns 1 and 2 are rarely seen. Gleason pattern 3 is by far the most common.
Primary, secondary and tertiary grades
A pathologist then assigns a grade to the observed patterns of the tumor specimen.
- Primary grade - assigned to the dominant pattern of the tumor (has to be greater than 50% of the total pattern seen).
- Secondary grade - assigned to the next-most frequent pattern (has to be less than 50%, but at least 5%, of the pattern of the total cancer observed).
- Tertiary grade - increasingly, pathologists provide details of the "tertiary" component. This is where there is a small component of a third (generally more aggressive) pattern.
Scores and prognoses
The pathologist then sums the pattern-number of the primary and secondary grades to obtain the final Gleason score. If only two patterns are seen, the first number of the score is that of the tumor's primary grade while the second number is that of the secondary grade, as described in the previous section. If three patterns are seen, the first number of the score would be the primary grade and the second number the pattern with the highest grade. For example, if the primary tumor grade was 2 and the secondary tumor grade was 3 but some cells were found to be grade 4, the Gleason score would be 2+4=6. This is a slight change from the pre-2005 Gleason system where the second number was the secondary grade (i.e., the grade of the second-most common cell line pattern). There could be a Gleason score of 3+4 with a tertiary component of pattern 5 - this would be considered to be more aggressive than a prostate cancer that was Gleason score of 3+4 with no tertiary pattern 5. However, the full significance of the tertiary component on the aggressiveness of a cancer is debatable.
Gleason scores range from 2 to 10, with 2 representing the most well-differentiated tumors and 10 the least-differentiated tumors. Gleason scores have often been categorized into groups that show similar biologic behavior: low-grade (well-differentiated), intermediate-grade, moderate to poorly-differentiated or high-grade. More recently, an investigation of the Johns Hopkins Radical Prostatectomy Database (1982-2011) led to the proposed reporting of Gleason grades and prognostic grade groups as: Gleason score ≤ 6 (prognostic grade group I); Gleason score 3+4=7 (prognostic grade group II); Gleason score 4+3=7 (prognostic grade group III); Gleason score 4+4=8 (prognostic grade group IV); Gleason scores 9-10 (prognostic grade group V). Prostate cancers with a Gleason score ≤ 6 usually have rather good prognoses.
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