If we believe in the auto-immune reaction hypothesis, it's clear that the anal region is just beside the prostate region. So, it there are tons of pro-inflammatory immune messengers released in the prostate region, it would not be a surprise to find some inflammation in the anal region near by - in fact, the prostate is so close that it is leaning against the rectum wall, and that is why a prostate exam is made by introducing a finger through the anus in order to sense the prostate shape. Actually, some members have reported anal itching as a POIS symptom.