How likely is it that my prostate biopsy results were a false positive?

Very rare. Prostate biopsies are the only way to initially diagnose prostate cancer. Although a false positive result may occur, it is very unlikely and unlikely to occur. The grading of prostate cancer is fairly uniform known as the gleason grade and can be readily identified by a qualified pathologist.
Prostate biopsy. While a negative biopsy does not guarantee that prostate cancer is not present in some unsampled area, pathologists are generally very cautious about "over calling" suspicious tissue. If only a low % of a single core biopsy (of 10-14 samples) is positive for low grade (gleason 3) cancer you may want to request a tissue review by an outside pathologist.