After a prostate cancer biopsy, a pathologist compares tissue cells to cancer cells under a laboratory microscope, grading the first and second most common tumor patterns from 1 to 5, explains WebMD. The numbers are then added together, giving a Gleason score indicating the likelihood of the cancer spreading.
Determining the two most common tumor patterns requires grading the abnormal appearance of the tissue cells, according to WebMD. The first most common tumor pattern carries more weight than the second most common when calculating the Gleason score. A score of 7 derived from a grade 3 for the first most common tumor pattern, and 4 for the second, indicates a less aggressive form of cancer than the same score derived from a grade 4 for the most common, and 3 for the second.
A Gleason score ranging from 2 to 4 indicates the cancer cells are not likely to spread and have a normal appearance, says WebMD. A range of 5 to 7 indicates an intermediate risk of the cancer cells spreading in the body. With a Gleason score of 8 to 10, the tissue cells have very little resemblance to normal cells and are in an aggressive stage. The lower the Gleason score, the better the prognosis for the patient.