[from the US National Cancer Institute]
Healthy prostate cells are uniform in size and shape, neatly arranged in the patterns of a normal gland. As cancer grows, they lose their healthy look. They change from normal, well-differentiated tissues to more disorganized, poorly differentiated tissue. Eventually, a tumor develops.
If your biopsy shows the presence of prostate cancer, the pathologist assigns each tissue sample a grade, indicating how far the cells have traveled along the path from normal to abnormal. The grade offers a good clue to your tumor's behavior: a tumor with a low grade is likely to be slow-growing, while one with a high grade is more likely to grow aggressively or already to have spread outside the prostate (metastasized). The most widely used grading method for prostate cancer is known as the Gleason grading system (see Gleason Scores, page 7).
Tumor grade is useful as a predictor of outcome. In one analysis, 10 years after prostatectomy for localized cancer, prostate cancer had claimed the lives of 6 percent of the men whose cancers were welldifferentiated compared with 20 percent of those with moderately differentiated cancers and 23 percent of those with poorly differentiated cancers. The
chances of developing metastatic prostate cancer followed a similar pattern. Ten years after surgery, metastasis had been diagnosed in 13 percent of the men with well-differentiated tumors, but in 32 percent of those with cancers that were moderately differentiated and 48 percent of those whose cancers were poorly differentiated.
The Gleason grading system [see diagram] 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.
What is differentiation?
From The Prostate Lab site:
Well, cells that are completely differentiated know exactly what they are doing in life. Perfectly differentiated prostate cells are happy to go about their business in the prostate making PSA and ejaculatory juices. They look and act like the mature prostate cells they are. They interact with other perfectly differentiated prostate cells to create the tiny tubular structures that the prostate requires to deliver its product to the urethra. Once they become malignant, however, the prostate cells become less differentiated and behave less like well-disciplined prostate cells. The tubules they make with other malignant cells are distorted and haphazardly arranged. As they become even less differentiated, they form solid clumps and may become anti-social, preferring to go off on their own ands not even attempt to make tubules with other cells.
What is the Gleason Score?
Again from The Prostate Lab site:
The Gleason score is written as the sum of the two most prominent Gleason patterns. So a Gleason score of 2+3=5 has a dominant well-differentiated pattern (i.e., pattern 2) and a less dominant moderately-differentiated pattern (i.e., pattern 3). A score of 4+3=7 means that a poorly differentiated component (pattern 4) is dominant. If 95% or more of the tumor is composed of one pattern, the corresponding number is counted twice; thus, a wholly moderately-differentiated tumor would be scored 3+3=6.
In other words, not all scores are equal. A score 7 from 3+4 is better than one from 4+3.
Tissues are obtained by biopsy of the prostate but it must be remembered that even with the biopsy, there is no certainty that the sample is fully representative. A tumor may be missed or it may be hit.
It can also be seen that assigning grades is not an exact science. A pathologist who may give a grading that is too high or too low. That is why a second opinion (of the same samples) are highly recommended.
To see sample slides, go to the InfoLink page preserved here at Phoenix5.
National Cancer Institute has a simple essay on tumor grading.