Here, look at this study. There is only a 41% chance that a ridge or anything else felt on a DRE will turn out to be positive on a biopsy. So you are better off having a biopsy based on a coin toss than based on a positive DRE - worse than useless. The chance that a negative DRE is correct is only 64% -- it's wrong over a third of the time. Maybe it's time to stop getting the finger.
www.annfammed.org/content/16/2/149