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Transitional zone of prostate

The transitional zone of the prostate is one of the four histological zones of the gland, located centrally around the proximal prostatic urethra. It is the smallest zone at birth, but undergoes significant enlargement with age due to benign prostatic hyperplasia (BPH), making it clinically and radiologically important. It is also a site where prostate cancer can occur, although cancers more commonly arise in the peripheral zone.

This zone is of special significance in imaging because it is the main site of nodular enlargement leading to urinary obstruction in older men and is directly evaluated in PI-RADS (Prostate Imaging Reporting and Data System) MRI protocols.

Synonyms

  • TZ of prostate

  • Periurethral zone (older term)

  • Central periurethral gland zone

Location and Boundaries

  • Anteriorly: Anterior fibromuscular stroma of the prostate

  • Posteriorly: Central zone of the prostate

  • Medially: Proximal prostatic urethra (encircles it)

  • Laterally: Peripheral zone of the prostate

  • Superiorly: Bladder neck

  • Inferiorly: Apex of the prostate, where it blends with peripheral zone

Relations

  • Closely surrounds the proximal prostatic urethra

  • Lies medial to the peripheral zone (major site of carcinoma)

  • Related anteriorly to the fibromuscular stroma

  • Posteriorly adjacent to the ejaculatory ducts and central zone

Function

  • Contributes glandular secretions to semen, helping to nourish and transport sperm

  • Provides structural support around the prostatic urethra

  • Undergoes nodular hyperplasia with age (leading to urinary outflow obstruction)

  • Site of origin for most benign prostatic hyperplasia (BPH) nodules

Clinical Significance

  • Benign Prostatic Hyperplasia (BPH): Transitional zone is the primary site of enlargement, compressing the urethra and causing urinary obstruction

  • Surgery: Important landmark in transurethral resection of prostate (TURP)

MRI Appearance

T1-weighted images:

  • Normal transitional zone: homogeneous low-to-intermediate signal intensity

  • Fat within the glandular tissue: bright signal outlining nodules or septa

  • Hemorrhage (post-biopsy or TURP): bright hyperintensity

T2-weighted images:

  • Normal: heterogeneous low-to-intermediate signal with indistinct capsule

  • BPH nodules: appear as circumscribed round or oval lesions, some with high T2 signal (glandular), some with low signal (fibromuscular)

  • Carcinoma: ill-defined low T2 signal, often lacking a capsule

STIR (Short Tau Inversion Recovery):

  • Normal glandular tissue: low signal

  • BPH nodules with edema/inflammation: bright signal

  • Tumors may also show bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal TZ: mild diffuse enhancement

  • BPH nodules: show heterogeneous but usually benign enhancement

  • Malignant lesions: focal, asymmetric, irregular enhancement

CT Appearance

Non-Contrast CT:

  • Transitional zone is difficult to delineate separately from the prostate gland

  • Appears as part of soft tissue density of the prostate

  • Enlarged TZ in BPH causes central gland expansion, compressing urethra and indenting bladder base

Post-Contrast CT:

  • Normal gland enhances homogeneously

  • BPH nodules appear as focal enlargements with variable enhancement

  • Malignant lesions may enhance irregularly but CT is less sensitive than MRI for TZ pathology

MRI images

CT image

Transitional zone of prostate CT sagittal anatomy image-img-00000-00000