Prostate: Transrectal Ultrasound

General urological uses of a transrectal ultrasound (TRUS)

• Uses: (5):

1. Assess prostate volume
2. Locate focal prostate abnormalities
3. Assess for obstructive cause of infertility (dilated seminal vesicles secondary to ejaculatory duct obstruction)
4. Guide prostate biopsies
5. Guide placement of brachytherapy seeds

Grayscale TRUS

• Probe frequency
  • Increasing the frequency increases the resolution and decreases the depth of penetration
  • Decreasing the frequency increases the depth of penetration and decreases the resolution
  • The commonly used 7-MHz transducer produces a high-resolution image with a focal range from 1-4 cm from the transducer (best for peripheral zone where most cancers arise)
  • Lower frequency transducers (e.g., older 4-MHz transducers) have a focal range from 2-8 cm but at lower resolution

TRUS Anatomy of the Prostate

• Traditionally described based on a pathologic zonal architecture:
  • Anterior fibromuscular stroma (FS), which is devoid of glandular tissue
  • Transition zone (TZ)
  • Central zone (CZ)
  • Periurethral zone
  • Peripheral zone (PZ)
  • CZ and PZ cannot be distinguished from each other on US, and are often collectively referred to as the PZ on TRUS
  • The TZ is generally hyperechoic compared to the CZ and PZ
  • See Figure 1 and Figure 2
  • See video on GU Sonography of the Urinary Bladder, Scrotum & Prostate
• Calcifications along the surgical capsule, known as the corpora amylacea, highlight the plane between the PZ and TZ (multiple diffuse calcifications are a normal, often incidental finding and represents a result of age rather than a pathologic entity)
• On axial/transverse view, the right side of the prostate is on the left part of the screen, while the left side of the prostate is on the right side of the screen, similar to CT scan[[
• See Figure

Prostate lesions on TRUS

• The peripheral zone is used as the reference for isoechoic when describing lesions
• Most (60%) prostate cancers are hypoechoic; however, can also be iso (39%) or even hyperechoic (1%)§
  • Images of hypoechoic lesions on TRUS
• A hypoechoic lesion contains cancer ≈20% of the time.
  • While there is a need to biopsy hypoechoic lesions, these lesions are not pathognomonic for cancer and do not correlate with the aggressiveness of the disease as measured by Gleason score.
  • Differential diagnosis of hypoechoic lesions on TRUS (5):
    1. Granulomatous prostatitis
    2. Prostatic infarct
    3. Lymphoma
    4. BPH nodules
    5. Normal urethra
• Many cancers, including hematologic malignancies of the prostate, are isoechoic
  • Prostatic cysts
    • May be congenital or acquired but are rarely clinically significant, regardless of cause
    • Congenital prostatic cystic lesions may arise from either müllerian (prostatic utricles and müllerian duct cysts) or wolffian (ejaculatory duct and SV cysts) structures
      • Enlarged prostatic utricle
        • A diverticular projection from the posterior urethra at the level of the verumontanum
        • Appears as a midline anechoic structure
        • Associated with genital anomalies, including hypospadias (most common), ambiguous genitalia, undescended testes, and congenital urethral polyps
      • Müllerian duct cysts
        • Result from failure of the müllerian ducts to fuse with the urethra.
        • Appears as a midline anechoic structure; generally ovoid to pear shaped, with the cyst neck oriented toward the verumontanum.
        • Should prompt renal US to assess for unilateral renal agenesis
      • SV cysts
        • Can be caused by congenital or acquired obstruction of the ejaculatory duct
        • Associated with cystic renal disease
        • Should prompt renal US to assess for unilateral renal agenesis
        • Zinner syndrome (3):
          1. Ipsilateral SV cyst
          2. Ejaculatory duct obstruction
          3. Unilateral renal agenesis
• TRUS appearance after treatment
  • External-beam radiation monotherapy therapy usually results in decreased volume by 6 months after treatment.
  • With brachytherapy, prostate volume declines significantly after treatment, with a 37% size reduction at 1 year after treatment and > 50% reduction 8 years after implantation
  • Irradiated prostates are diffusely hypoechoic
  • Androgen ablation with luteinizing hormone–releasing hormone analogues will cause ≈30% volume decrease with androgen deprivation in prostates with and without cancer
  • Volume decreases by ≈21% at 6 months with 5-ARIs

Estimating Prostate Volume

• Most formulas assume that the gland conforms to an ideal geometric shape: either an ellipse (π/6 × transverse diameter × AP diameter × longitudinal diameter), sphere (π/6 × transverse diameter3), or a prolate (eggshaped) spheroid (π/6 × transverse diameter2 × AP diameter).
  • All formulas reliably estimate gland volume and weight, with correlation coefficients > 0.90 with radical prostatectomy specimen weights, because 1 cm3 = approximately 1 g of prostate tissue.
  • Planimetry is the most accurate means of volume measurement by US
    • Planimetry allows for variation in shape as the area is calculated in consecutive ultrasonographic cross-sections. The area is multiplied with the distance between the cross-sections and the total volume is determined by summation of all contributions.[1]

Questions

1. List uses of a TRUS
2. What is the differential diagnosis of a hypoechoic lesion on TRUS?

Answers

1. List uses of a TRUS
   1. Estimate prostate volume
   2. Locate focal prostate abnormalities
   3. Assess for obstructive cause of infertility
   4. Guide prostate biopsies
   5. Guide placement of brachytherapy seeds
2. What is the differential diagnosis of a hypoechoic lesion on TRUS?
   1. Granulomatous prostatitis
   2. Prostatic infarct
   3. Lymphoma
   4. BPH nodules
   5. Normal urethra

References

• Wein AJ, Kavoussi LR, Partin AW, Peters CA (eds): CAMPBELL-WALSH UROLOGY, ed 11. Philadelphia, Elsevier, 2015, chap 109