Transrectal Prostate Ultrasonography
General procedure
In transrectal ultrasonography (TRUS) of the prostate, scanning begins in the axial plane, and the base of the prostate and seminal vesicles are visualized first. A small amount of urine in the bladder facilitates the examination.
Seminal vesicles are identified bilaterally, with the ampullae of the vas deferens on either side of the midline (see the image below). The seminal vesicles are convoluted cystic structures that are darkly anechoic. Men who have abstained from ejaculation for a long period may have dilated seminal vesicles.
Axial images of the seminal vesicles. White arrows indicate the ampulla of the vas deferens.
Next, the base of the prostate is visualized. The central zone comprises the posterior part of the gland and is often hyperechoic. The midgland is the widest portion of the gland. The peripheral zone forms most of the gland volume. Echoes are described as isoechoic and closely packed.
The transition zone is the central part of the gland and is hypoechoic. The junction of the peripheral zone and the transition zone is distinct posteriorly and is characterized by a hyperechoic region, which results from prostatic calculi or corpora amylacea. The transition zone is often filled with cystic spaces in patients with benign prostatic hyperplasia (BPH).
Scanning at the level of the verumontanum and observing the Eiffel tower sign (anterior shadowing) help identify the urethra and the verumontanum. The prostate distal to the verumontanum is composed mainly of the peripheral zone. The capsule is a hyperechoic structure that can be identified all around the prostate gland.
Several hypoechoic rounded structures can be identified around the prostate gland. These are the prostatic venous plexi.
The position of the neurovascular bundles can often be identified by the vascular structures. Imaging in the sagittal plane allows visualization of the urethra. The median lobes of the prostate are often visualized.
Volume measurement
Volume assessment of the prostate is an important and integral part of TRUS. Of the several formulas that have been developed for this purpose, the most commonly used is the ellipsoid formula, which requires measurement of 3 different prostate dimensions.
First, the transverse dimension and the anteroposterior dimension at the estimated point of the widest transverse dimension are measured in the axial plane. Next, the longitudinal dimension is measured in the sagittal plane just off the midline (because the bladder neck often obscures the cephalad extent of the gland). The ellipsoid volume formula is then applied, as follows:
Volume = height × width × length × 0.52
Biopsy
Directed biopsies are obtained from any area that is considered suggestive on the basis of ultrasonographic findings or palpable abnormalities found on DRE. Because the incidence of nonpalpable isoechoic prostate tumors is high, limiting biopsy sites to either ultrasonographically hypoechoic lesions or to areas of palpable abnormality tends to miss many malignancies.
Obtaining separate biopsy samples from each sextant of the prostate improves the odds of sampling clinically unapparent tumors. Originally, these biopsy sites included the midlobe parasagittal plane at the apex, the midgland, and the base bilaterally. Subsequently, however, changes to this protocol were recommended.
Various authors suggested that the 6 biopsy samples should be obtained from the lateral third of each lobe rather than from the mid lobe or that 2 lateral biopsy samples should be obtained from each lobe in addition to the original sextant samples. Some authors recommend obtaining even larger numbers of biopsy cores to increase the diagnostic sensitivity. (See Prostate Biopsy.)
Complications of prostate biopsy include hematuria, rectal bleeding, hematospermia, urosepsis, and perineal pain. [38] Although most of these complications subside within 48-72 hours, patients should be warned that hematospermia can last for 3-4 weeks. In rare cases (< 1%), bacteremia develops that necessitates hospitalization and administration of intravenous antibiotics.
Early diagnosis of prostate cancer
Advances in TRUS coincided with the development of prostate-specific antigen (PSA) testing. The PSA has proved the most valuable tumor marker test for early diagnosis of carcinoma of the prostate (CAP).
TRUS was also evaluated to determine whether it could be used for CAP screening. However, it was not found to be highly effective for this purpose, because of its lack of specificity. CAP lesions may appear hypoechoic, hyperechoic, or isoechoic on TRUS. Therefore, TRUS is used primarily to direct the physician to suggestive areas in the prostate (see the images below) or to guide the performance of prostate biopsies.
Sagittal image of the prostate showing a hypoechoic area (white arrow). This area was a focus of cancer on biopsy findings.
A large hypoechoic area in the left peripheral zone suggestive of prostate cancer.
Axial and sagittal images of the prostate showing extensive hypoechoic areas. This patient had a prostate-specific antigen level of 17 ng/mL and digital rectal examination findings highly suggestive of cancer. Biopsy revealed granulomatous prostatitis.
Prostate volume is assessed during the TRUS examination. The decision to perform biopsy in patients with abnormal PSA levels can be bolstered by PSA density (PSAD), which is defined as the PSA level divided by the prostate volume. The sensitivity of PSAD is enhanced by a cutoff value of 1.5.
BPH tissue produces one tenth as much PSA per gram as cancer tissue does; accordingly, a gland with a large amount of BPH tissue indicates an elevated PSA level. CAP is diagnosed in 30% patients with a PSA value of 4-10 ng/mL and in 60% of patients with a PSA value of 10-20 ng/mL. PSAD has been used to decrease the number of prostate biopsies performed.
Other methods that have been used to aid in the decision whether to perform biopsy include the following:
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Expected PSA value for a given prostate volume
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Volume and PSAD of the transition zone
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A PSA value that increases at a velocity greater than 0.75 ng/mL per year
The volume of the prostate gland can also be used to determine treatment options. Both perineal prostatectomy and brachytherapy are easier to perform when the gland is smaller than 50 g. In large glands, the anterolateral portion of the gland is behind the pubic arch, and these areas cannot be reached with the perineal brachytherapy needles. Hormonal downsizing is useful in such cases, and TRUS is used to monitor gland size.
Measuring prostate volume is also useful in large BPH glands to help determine whether transurethral resection or an open procedure is appropriate for prostatectomy.
Whether TRUS has a role in staging prostate cancer is debatable. Most early cancers are confined to the organ. Lee et al popularized staging biopsies of the neurovascular bundles and the seminal vesicles. Positive results from biopsy of the neurovascular bundles and seminal vesicles signified extracapsular disease and a poor outcome.
Currently, however, the greatest amount of information is available on staging approaches using the PSA value and the Gleason score of prostate cancer based on several published nomograms (eg, Partin nomograms). Moreover, biopsy of the periprostatic venous plexus may result in pelvic hematoma. Perineural invasion found on prostate biopsy samples should not be considered an indicator of extraprostatic spread. TRUS can help identify extraprostatic CAP in advanced-stage T3 cases.
Brachytherapy
Localized prostate cancer can be treated by means of brachytherapy using permanent radioactive iodine seeds, with or without preimplant external beam irradiation (depending on the tumor grade). After initial volume assessment, the seeds are placed according to a computer-generated grid under ultrasonographic guidance. [39]
To exclude violation of the urethra or bladder, cystoscopic evaluation is necessary at the end of the procedure. Although iodine seeds are most commonly used, palladium seeds are often employed to treat more aggressive cancers (usually defined as those with a Gleason score higher than 7 and a PSA value higher than 10 ng/mL).
Alternatively, patients with more aggressive tumors may receive high-dose radiation therapy consisting of external beam irradiation during the second and fourth weeks along with a brachytherapy boost with temporary implants. With temporary seeds, trocars are placed under ultrasonographic guidance according to a computer-generated grid, and the radioactive source is threaded in and out of each of the trocars.
Cryotherapy
Gonder and associates were the first to use cryoablation in urologic disorders in the 1960s. In 1988, Onik et al used real-time ultrasonography to monitor the freezing process during radical cryoablation of the prostate. Currently, cryotherapy is acceptable as salvage therapy for radiation failures.
Radical cryoablation is defined as the freezing of the entire prostate, the periprostatic tissue, the neurovascular pedicles, and the proximal seminal vesicles. Probes are placed in the prostate gland via the perineum under ultrasonographic guidance, and cryotherapy is begun. The ice ball, which is an anechoic lesion with a hyperechoic edge that can be seen advancing or receding, is directly monitored as it occupies the entire prostate gland.
Most centers use a urethral warming device to prevent urethral necrosis. The following 3 techniques have been used:
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Single freeze-thaw cycle
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Double freeze-thaw cycles
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Pullback freeze technique
A suprapubic catheter is kept in place until the patient is able to void satisfactorily with minimal residual urine. Follow-up biopsies are performed at 6 months, 1 year, and 2 years.
Using the current double freeze-thaw technique, Cohen et al reported an 11% positive biopsy rate after 4 years of follow-up. All positive results occurred in patients with a PSA value higher than 10 ng/mL or tumors at stage T3. [40]
Ghafar et al reported the results of salvage cryotherapy for recurrence after external beam radiation therapy. [41] In this study, 38 men were treated with the double freeze-thaw technique on an argon-based system. The biochemical recurrence-free survival rate was 86% at 1 year and 74% at 2 years. Complications included rectal pain (39.5%), urinary tract infection (2.6%), incontinence (7.9%), hematuria (7.9%), and scrotal edema (10%). None of the patients developed rectourethral fistula, urethral sloughing, or retention.
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Transverse image of the prostate showing a hypertrophied transition zone (yellow arrows) and a compressed peripheral zone (blue arrows).
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Large hypoechoic area along the left peripheral zone, suggestive of carcinoma.
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Sagittal image of the prostate showing a hypoechoic area (white arrow). This area was a focus of cancer on biopsy findings.
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A large hypoechoic area in the left peripheral zone suggestive of prostate cancer.
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Axial and sagittal images of the prostate showing extensive hypoechoic areas. This patient had a prostate-specific antigen level of 17 ng/mL and digital rectal examination findings highly suggestive of cancer. Biopsy revealed granulomatous prostatitis.
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Axial image of a prostate. White arrows show the asymmetrical anterior prostate. This could only be appreciated on TRUS images.
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Sagittal image of a prostate. White arrows show darkly hypoechoic areas suggestive of periprostatic veins.
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Axial images of the seminal vesicles. White arrows indicate the ampulla of the vas deferens.
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Sagittal image of the prostate. White arrows indicate calcification in the prostatic urethra.
Tables
Table 1. PI-RADS Assessment Categories |
|
Assessment categories |
Likelihood for cancer |
PI-RADS1 |
Very low |
PI-RADS2 |
Low |
PI-RADS3 |
Intermediate |
PI-RADS4 |
High |
P-IRADS5 |
Very high |
