Prostate: Transrectal Ultrasound
General urological uses of a transrectal ultrasound (TRUS)
- Uses: (5):
- Assess prostate volume
- Locate focal prostate abnormalities
- Assess for obstructive cause of infertility (dilated seminal vesicles secondary to ejaculatory duct obstruction)
- Guide prostate biopsies
- 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
- Click here for image
- 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)
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):
- Granulomatous prostatitis
- Prostatic infarct
- Lymphoma
- BPH nodules
- 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):
- Ipsilateral SV cyst
- Ejaculatory duct obstruction
- Unilateral renal agenesis
- Enlarged prostatic utricle
- Prostatic cysts
- 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
- List uses of a TRUS
- What is the differential diagnosis of a hypoechoic lesion on TRUS?
Answers
- List uses of a TRUS
- Estimate prostate volume
- Locate focal prostate abnormalities
- Assess for obstructive cause of infertility
- Guide prostate biopsies
- Guide placement of brachytherapy seeds
- What is the differential diagnosis of a hypoechoic lesion on TRUS?
- Granulomatous prostatitis
- Prostatic infarct
- Lymphoma
- BPH nodules
- Normal urethra