I am "Taffy062912" who posted a long post a couple of days ago and would like to add a few things after reading others. First I am 79+. I am in excellent health. I have talked with another doctor friend of mine, not a urologist, who has had patients undergo both types of biopsies and, like those posted, some had excruciating pain from rectal approach and others who have not. It is not clear whether this is do to the skill of the surgeon doing the biopsy or the procedure itself. However, having had the perineal with additional numbing, and my brother who did the rectal, both of us this year, his was a terrible experience. Mine was not but I did insist being numbed both externally and although you have to accept one needle more to numb the prostate, internally. I cannot imagine either being done without numbing. Ask before you are there about which procedure and numbing.
It seems like the side effects and risks are much less with the perineal approach and this is the more modern approach. If the urologist does not offer it, or minimizes numbing need, go somewhere else.
As far as pain threshold, I still suggest you use your dental numbing experience and pinching your bicep as a guide to what you would feel. My dentist says I am in the top 5% getting numb. Also the ability to numb is not the same as a pain threshold. I have a high threshold for pain after a procedure, but not during the procedure. I had a double hernia operation about 15 years ago and was off pain meds and driving 3rd day with only minor pain. However, after they gave a preliminary sedative, they then give the iv for the operation in operating room and ask you to count backwards from 100. I heard the doctors and anesthesiologist check the meds repeatedly and when I was about down to 70, they said, just relax- and I was out. So numbing and sedation are not the same as a pain threshold.
One more thing on the perineal biopsy. They also insert about a Churchill sized instrument up the rectum which sonically guides the surgeon to make sure he is sampling the areas identified as suspicious by the PET scan. This seems like better approach, which I assume is not possible with a rectal approach. The instrument is well lubricated and no more painful than a rectal exam. However, they do "wax" the rectum to the scrotum to remove all hair. I was not expecting that, but at my age, I have so little hair anywhere it was no big deal. The technician did not tell me first and it was quite a surprise.