<prompt> Carcinoma of the prostate arise fromCentral zonePeripheral zoneTransitional zonePeriurethral zone  <response>: Peripheral zone 1045-CSDT 12th 1354-B L 25thCARCINOMA OF PROSTATE - 90 of tumours are adenocarcinoma the tumour arises primarily in the peripheral zone 70 an area that differs in embryoic derivation from the periurethral transition zone which is the site of formation of benign prostatic hyperplasiaScreening - by transrectal ultrasound TRUSand prostate specific antigen PSA PSA under age 55 years normal 2.5 ngml over age 65 years normal 6.5 ngml. PSA density PSA divided by prostate volume 0.15 suggest cancer Percent free PSA Total PSA minus complexed PSA where when 10 the risk of prostate cancer is 60 only useful with total PSA 2-10 mgml Newer methods for measuring unbound free PSA have substantial promise for differentiating benign from malignant disease 