DNA ploidy is basically a test that measures the DNA content within tumor cells. Benign cells and well-behaved tumor cells grow and divide in an orderly fashion. In the resting state, they contain one complete set of chromosomes (this is the diploid condition).
This complete set of chromosomes consists of 23 chromosomes (or N) from Ma and 23 (N again) chromosomes from Pa (equaling a total of 2N). Before a cell can divide it must double the number of its chromosomes, creating two complete sets of chromosomes (this is 4N, or the tetraploid state). After division is completed each new cell gets half the booty of genetic material and so is diploid (2N) once again.
If DNA ploidy analysis were to be performed on a group of these cells, one would see that most of the cells would be diploid and a small fraction of them (those getting ready to divide) would be tetraploid. If we were to measure the amount of genetic material in each cell and create a graph, we would see a dominant diploid peak and a minor tetraploid peak. We can measure the amount of DNA in a cell by staining it with a dye that binds to the genetic material. The concentration and distribution of this dye (Fuelgen stain, if you must know) can be measured by image analysis microscopy.
Now when tumors go from bad to worse they tend to not divide as orderly as they once did. Instead of the resting state having a complete set of chromosomes, they might have a set and a half. Such cells would have a DNA content that was neither diploid or tetraploid but mid-way between. Plotting these cells on our graph above would yield an aneuploid peak midway between the other two.
Some studies have shown that tumors that have a significant aneuploid peak do not behave as well as those that do not. Since there is a strong correlation between ploidy status and nuclear grade, this is not surprising. A nuclear grade can be assessed by any pathologist with enough experience with prostate cancer.
Many but not all doctors consider the value that DNA ploidy analysis adds is that is that it is an objective measurement that can be compared between labs using standardized techniques and that can be used to perform a quick check on the "ball-park" accuracy of Gleason scoring. For instance any Gleason score 2+2=4 or 2+3=5 tumor that has an aneuploid peak, you should consider discussing if a re-evaluation for possible score adjustment is desirable.