The Gleason sum represents the most important indicator of cancer aggressiveness. Definitive treatment decisions (decisions how to treat the newly diagnosed prostate cancer) are predominantly made based on Gleason sum. Gleason sum is obtained from biopsy cores that are examined by the pathologist. The Gleason sum, as its name implies corresponds to the summation of the primary and the secondary Gleason patterns. The primary Gleason pattern represents the dominant (most extensive) pattern that the pathologist can identify in the examined biopsy material. The secondary pattern represents the second most dominant pattern or the high grade component (Gleason pattern 4 or 5), if such pattern can be at all identified in the biopsy material. How well does this sampling reflect the true Gleason sum of the prostate cancer? Only moderately! In as many as 30% of patients, the Gleason sum in the prostate may be higher than that observed on needle biopsy. Why? Biopsy samples some of the areas of the prostate (see figure). The white areas represent benign (non-cancerous prostate). The areas in the red demonstrate highly malignant prostate cancer. Finally, the areas in blue demonstrate favorable prostate cancer. On the right, at biopsy the biopsy needle crosses both the highly aggressive and the favorable prostate cancer. Therefore, the pathologist will be able to report the presence of both grades in the pathology report. On the left, the biopsy needle misses the highly aggressive prostate cancer. The pathology report will only indicate the presence of favorable prostate. This will be incorrect and may lead to inappropriate choice of therapy. For example, interstitial brachytherapy should not be used in presence of aggressive (Gleason sum 7 or higher) prostate cancer. The present (Gleason sum upgrading) nomogram can predict the probability that biopsy underestimates the true aggressiveness of the cancer. This nomogram is of particular interest to individuals with Gleason sum 6 prostate cancers. For example, if your probability of being �upgraded� to Gleason 7 or higher is not negligible, you should consider definitive treatment modalities that provide the highest probability of cure, such as for example the radical prostatectomy or definitive external beam radiotherapy.