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The prostate gland is the largest accessory gland of the male reproductive system. 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 with a base superiorly and apex inferiorly. It has four surfaces: anterior, posterior and two inferolateral surfaces. The base of the prostate is in continuity with the bladder and the apex ends inferiorly at the urogenital diaphragm 1-3. The prostate gland surrounds the proximal urethra, which traverses the prostate close to its anterior surface at the base and then more centrally at the apex.
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 surfaces rests on the levator ani fascia 3. Its flat triangular 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 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 (of Santorini) lies between and passes through the pseudocapsule 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 a non-glandular anterior fibromuscular stroma and three distinct glandular zones with different embryologic origins:
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 prostate volume 13. It surrounds the proximal and middle portions of the prostatic urethra with the bulk of this zone lying 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.
anteriorly: pubic symphysis, retropubic space of Retzius
posteriorly: rectovesical fascia, rectum
posterosuperior: seminal vesicles
laterally: prostatic venous plexus and levator ani
sometimes supplied by the middle rectal arteries
Venous drainage occurs primarily through the prostatic venous plexus into the inferior vesical vein, which in turn drains into the internal iliac vein.
Venous blood from the prostatic venous plexus also travels via the Batson venous plexus to drain into the internal vertebral venous plexus 2,3,5. The deep dorsal vein of the penis drains into the prostatic venous plexus via its connection with the pudendal venous plexus 3. Therefore, the prostatic venous plexus is a potential route of spreading cancer 13.
drainage mainly to internal iliac, sacral 13 and obturator nodes
some drainage to external iliac, presacral and para-aortic nodes 1-4
The gland is surrounded by the prostatic (nervous) plexus which receives autonomic fibers from the inferior hypogastric plexus:
parasympathetic fibers supplying the inferior hypogastric plexus arise from the pelvic splanchnic nerves (S2-S4) 13
sympathetic fibers supplying the inferior hypogastric plexus primarily arise from the lumbar splanchnic nerves (L1-L2) with some minor contribution from the superior hypogastric plexus via the hypogastric nerves
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
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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