One of the most important and widespread indicators of PC prognosis is the Gleason grading system,
based on examination of prostate biopsy samples by light microscopy (Epstein 2010). Due to the
heterogeneic nature of PC, multiple biopsies of the prostate tumor have to be obtained. The urologist
commonly takes 12 core-biopsies of the prostate (the majority from the peripheral zone), and
prepares hematoxylin & eosin (H&E) stained slides for histological examination (Heidenreich et al.
2011) using a light microscope. The pathologist will examine the architectural pattern and histology
of tumor samples and determine the relative degree of tumor differentiation according to the two
most prevalent tumor patterns within the biopsy (Humphrey 2004). Thus, PC aggressiveness is
determined according to five basic Gleason grade patterns, numbers ranging from 1 to 5: 1 being
very well differentiated (looks most like normal prostate tissue and least aggressive), 5 being poorly
differentiated (most abnormal and most aggressive prostate tissue glandular structure) (Humphrey
2004) (Figure 1).