Last revised by Dr Calum Worsley on 22 May 2022

The prostate gland is part of the male reproductive system and is the largest male accessory gland. It typically weighs between 20-40 grams with an average size of 3 x 4 x 2 cm. The prostate is comprised of 70% glandular tissue and 30% fibromuscular or stromal tissue 1-3 and provides ~30% of the volume of seminal fluid.

The prostate gland is an inverted pyramid that surrounds the proximal urethra, which traverses the prostate close to its anterior surface at base and then more centrally. It has a superior base, inferior apex and three surfaces: anterior, inferolateral and posterior. The base of the prostate is in continuity with the bladder. It ends inferiorly at the apex at the urogenital diaphragm 1-3.

The anterior surface forms the posterior limit of the retropubic space. The prostate is connected to the pubic bone by the puboprostatic ligaments 1. Its inferolateral surface rests on the levator ani fascia 3. Its triangular and flat posterior surface is anterior to the rectum and has a vertical median groove, which is palpable via a digital rectal exam. It is separated from the rectum by the rectovesical fascia (Denonvilliers fascia) 13.

The seminal vesicles are superior and posterior to the prostate gland. Their ejaculatory ducts pierce the posterior surface of the prostate below the bladder 1-3 and drain into the prostatic urethra

The prostate gland lacks a true capsule and the so-called prostate capsule is a pseudocapsule formed from fibromuscular tissue surrounding 3 distinct layers of fascia; the anterior, lateral, and posterior fasciae. Anteriorly and apically this pseudocapsule is deficient. Laterally the fascia fuses with the levator fascia. The prostatic venous plexus (Santorini plexus) lies between and passes through the pseudocapsule and fascia 1-3,9-11.

Neurovascular bundles travel posterolaterally at 5 and 7 o'clock and give off branches into the prostate at the apex and base 5.

The prostate is comprised of three distinct zones with different embryologic origins:

  1. peripheral zone
  2. central zone
  3. transition zone

The large cup-shaped peripheral zone (PZ) encompasses the central and transition zones and accounts for approximately 70% of the total prostate volume in a young adult 13. The peripheral zone is deficient anteriorly where it is replaced by the anterior fibromuscular stroma (AFMS) 13. The peripheral zone surrounds the distal prostatic urethra at the apex of the prostate and extends posterolaterally to the base 13. The peripheral zone is separated from central and transitional zones by a fibrous layer13. The majority (70%) of prostatic tumors occur in the peripheral zone 13.

The small wedge-shaped central zone (CZ) constitutes up 25% of the prostate volume and contains the ejaculatory ducts 13. It is posterior to the prostatic urethra and forms the base of the prostate.

The smaller transition zone (TZ) makes up the remaining 5% of the prostatic volume 13. It is predominantly anterolateral to the prostatic urethra 2-4.  Benign prostatic hypertrophy occurs in the transition zone 13. About 20% of prostatic cancers come from transition zone 13. The transition zone is occasionally written incorrectly as the transitional zone.

Superiorly, the anterior fibromuscular stroma is continuous with detrusor muscle. Inferiorly, it blends with levator muscles and puboprostatic ligaments 13.

With aging, the central zone atrophies and the transition zone becomes hypertrophic 13

Historically, the prostate was described as having five lobes, the anterior, posterior, median and two lateral lobes. The anterior lobe which joined the two lateral lobes to each other was also known as the isthmus 12.

Some radiologists and urologists refer to the central gland (CG) which consists of both the central and transition zones. These zones are discernable on MRI.

  • prostatic venous plexus in communication with the pudendal plexus to the deep dorsal vein (to the internal iliac vein) with some communication to the Batson vertebral venous plexus 2,3,5 Prostatic venous plexus is a potential route of spreading cancers13.
  • drainage mainly to internal iliac, sacral 13 and obturator nodes
  • some drainage to external iliac, presacral and para-aortic nodes 1-4
  • absence of the middle lobe
  • presence of a 4th lobe 
  • best assessed with transrectal ultrasound
  • some zonal anatomy distinguishable
  • outer gland (central and peripheral zones) - uniform low echogenicity but usually more echogenic than the inner gland 6,7
  • 30 mL is a commonly used upper limit for normal volume
  •  poor for assessment of prostate zonal anatomy and pathology
  •  with adjusted window settings
    • the central zone appears hyperdense between 40-60 HU
    • the peripheral zone appears hypodense between 10-25 HU
    • useful for nodal and metastatic staging 4,6
  • preferred imaging modality
  • T1: homogeneous intermediate signal intensity
  • T2
    • anterior fibromuscular stroma is low T1W and T2W signal
    • peripheral zone is high T2W signal 13, similar to or greater than adjacent fat
      • there are age-related decreases in T2W signal
    • central and transition zones are lower T2W signal than peripheral zone 13
    • "capsule" is a thin rim of low signal intensity
    • the distal urethra is a small ring of low signal intensity 4,5

The central zone forms from the Wolffian duct whereas both the transition and peripheral zones arise from the urogenital sinus 3.

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Cases and figures

  • Case 1 : normal prostate gland - 3 T MR
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  • Figure 1: normal prostate MRI
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  • Figure 2: prostate gland and seminal vesicles (Gray's illustration)
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  • Figure 3: anatomy of the penis (Gray's illustration)
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  • Figure 4: lymphatics of the prostate (Gray's illustration)
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  • Figure 5: prostate anatomy
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