Prostatic carcinoma ranks as the commonest malignant tumor in men and the second most common cause of cancer related deaths in men 6. Prostatic adenocarcinoma is by far the most common histological type and is the primary focus of the article.
Prostate gland anatomy
The prostate gland is divided into several anatomic zones. Prostate cancer usually arises within the peripheral zone near the rectum which is why a digital rectal exam (DRE) is a useful screening test.
Epidemiology and clinical presentation
It is primarily a disease of the elderly male. In the United States, approximately 200,000 new cases are diagnosed each year.
Prostate cancer is usually detected by :
- an elevated (greater than 4 ng/dL) prostate-specific antigen (PSA) ; normal is 1-4ng/dL.
- abnormal digital rectal examination
Clinically patients can present with:
- urinary problems
- back pain
Clinical scores that may be useful in assessing symptoms severity include
- IPSS - international prostate symptoms score - 35 point score mainly assessing urinary symptoms
- IIEF - international index of erective function
95% of prostate cancers are adenocarcinomas which develop from the acini of the prostatic ducts 15.
Pathologic specimens are graded using the Gleason score, which is the sum of the most prevalent and second most prevalent types of dysplasia, each on a scale of 1 to 5, with 5 being the most dysplastic.
Transrectal ultrasonography (TRUS) is often intially performed in order to detect abnormalities and to guide biopsy, usually following an abnormal PSA level or DRE.
Ultrasound is used to direct biopsy of suspicious, hypoechoic regions, usually in the peripheral zone. Because of the high incidence of multifocality, systemic sextant biopsies are recommended.
On ultrasound prostate cancer is usually seen as a hypoechoic lesion (60-70%) in the peripheral zone of the gland, but can be hyperechoic or isoechoic (3 -40% of lesions)
Transrectal ultrasound is also the modality of choice for directing brachytherapy seeds into the prostate gland.
The primary indication for MRI of the prostate is in the evaluation of prostate cancer, after a ultrasound guided prostate biopsy has confirmed cancer in order to determine if there is extracapsular extension 1,2,4. Increasingly MRI is also being used to detect and localize cancer when the PSA is persistently elevated but routine TRUS biopsy is negative. Often a PI-RADS score is given to assess the probability of the lesion being malignant.
- using a endorectal coil, on T2-weighted images prostate cancer usually appears as a region of low signal within a normally high signal peripheral zone1,13.
- most significant cancers occur along the posterior portion of the gland abutting the rectum.
- DWI - often shows restricted diffusion
dynamic contrast / DCE -
- shows enhancement - but it can be difficult to distinguish from prostatitis or benign prostatic hyperplasia (especially in the central zone lesions 19).
- more specific than T2 signal 17
Routine use of body 3T magnets now means that endorectal coils have become unnecessary for prostate imaging due to the improved signal to noise and spatial resolution associated with higher field strength.
MRI parameters routinely assessed include the presence of a mass with low T2 signal, restricted diffusion with reduced ADC and increased tissue capillary permeability using dynamic gadolinium contrast enhanced imaging and calculation of the so-called Ktrans (a calculated time constant for permeability). These so called multi-parametric techniques are increasingly being used in assessment of prostate malignancy with MRI 11.
Extra-capsular extension carries a poor prognosis. Assess for :
- asymmetry / extension into the neurovascular bundles
- obliteration of the rectoprostatic angle
- involvement of the urethra
- extension into the seminal vesicles (normal seminal vesicles have high signal on T2)
Lymphadenopathy is best appreciated on T1-weighted images.
Following radical prostatectomy, patients with elevated PSA should also be examined using MRI.
The addition of MR spectroscopy with fast T2-weighted imaging is an area of research that holds promise for the detection of disease. The normal prostate produces a large of amount of citrate from the peripheral zone, which tumors do not 3. In normal prostate tissue citrate and polyamine levels are high and choline levels low. The reverse is the case in a tumor. See MR spectroscopy in prostate cancer.
MRI guided prostate biopsy is also being used, particularly in those cases where TRUS biopsy is negative, but clinical and PSA suspicion remains high 12.
Not accurate at detecting in situ prostate cancer.
Scans of the abdomen and pelvis are normally obtained prior to the onset of radiation therapy to identify bony landmarks for planning
In advanced disease, CT scan is the test of choice to identify enlarged pelvic and retroperitoneal lymph nodes, hydronephrosis and osteoblastic metastases 5.
Tc99 MDP bone scans are usually used to detect metastases.
Generally, patients with a Gleason score of less than 7 and a PSA of less than 10 ng/L are considered to have potentially curable disease. These patients undergo prostatectomy, brachytherapy, or external beam radiation 5.
Patients that do not meet these criteria will usually undergo a combination of hormone therapy and external beam radiation.
General imaging differential considerations include