Any history of PCa in your family? Have there been any other abnormal findings on previous exams? Has your doctor put you on an antibiotic regimen to rule out a chronic prostate infection?
Depending on other indicators in your case, maybe it's time for a "saturation biopsy." A prostate saturation biopsy typically entails 40-80 core samples taken from the prostate under general anesthesia. This technique is used on certain high risk patients, typically with elevated prostate specific antigen levels, abnormal findings on previous biopsies, or abnormal rectal exams.
Just a thought. I am by no means a doctor.