Urogenital ImagingAdenocarcinoma, prostatic
(PCa), the most common malignancy in men (excluding skin cancer) and the second leading cause of male cancer deaths (after
lung cancer). The lifetime risk of developing PCa is 18 - 19%. Twenty-five to 33% of men who develop PCa will die of the disease. The latest statistics published by the American Cancer Society indicate that 209,900 new cases of PCa will be diagnosed in 1998, with 41,800 men dying of the disease. Therefore, in 1998, PCa will be responsible for 32% of all new cancer cases and for 14% of estimated cancer deaths in men. The magnitude of the PCa problem continues to grow. Compared to 1985 (incidence 86,000), the incidence of PCa has increased by 144%, and the mortality has increased by 28%. A significant part of the increase in incidence is probably due to an increase in PCa screening using serum prostate-specific antigen (PSA) and transrectal prostate
ultrasound with
biopsy. The American Cancer Society recommends that men over the age of 50 should undergo a yearly digital rectal examination (DRE) and have the PSA assay. If either one is abnormal, transrectal
ultrasound (TRUS) should be performed.
Biopsy is recommended if the
lesion is either palpable or detectable on TRUS, or the PSA density (PSA value divided by the gland volume) is greater than 0.15. Once the diagnosis has been established, the choice of treatment is often empirical, and physician and patient preference are the determining factors.
Staging
The use of imaging in staging evaluation of PCa is a subject of controversy with opinion and practise varying widely. There are evidence-based guidelines on the clinical use of imaging in assessing the risk of distant spread of PCa. Radionuclide bone scans and CT supplement clinical and biochemical evaluation (PSA, prostatic acid phosphates) for suspected metastatic disease to bones and lymph nodes. Guidelines for the use of bone scans (in patients with PSA > 10 ng/ml) and CT (in patients with PSA > 20 ng/ml) have been reported and are in clinical use. No such consensus exists at the current time for the use of imaging in evaluating PCa local tumour extent.
In locally staging prostate cancer either TRUS, CT or MRI have been applied. The reported accuracy of TRUS in the staging of PCa varies. Although some reports have indicated that extension of PCa was detected in as many as 92% of cases, the large multicentre study found an accuracy rate of only 62%. Identification of invasion of the seminal vesicle is even less accurate on TRUS with sensitivity reported to be 22% and specificity 88%. It was hoped that continuing advances in technology would improve TRUS efficacy. However, a 1997 prospective multicentre study showed a continuously poor performance for TRUS, with its ability to detect ECE matching digital rectal examination. In the evaluation of PCa, TRUS remains the most valuable in assisting US-guided biopsy and calculation of PSA density. CT, once considered the mainstay of imaging techniques for PCa, is no longer requested in many institutions. The limitations of CT tumour visualization with respect to extracapsular or seminal vesicle invasion have been recognized. CT today is reserved for advanced stage disease, mostly for detection of lymph node metastasis or planning radiation therapy. In the evaluation of nodal disease, CT (like MRI) relies on tumour size. While 1.0 cm nodal size in the pelvis is often used as a threshold, newer studies advocate that even a nodal size of 0.7 cm should raise the suspicion of a metastasis. Nodal size measurements are reported for the largest transverse diameter of the lymph node. On MR images the signal intensity and the detection of PCa depends on the type of imaging sequence used. On T1-weighted images, the prostate is of homogeneous medium signal intensity. On T2-weighted MR images, prostatic carcinoma is most commonly shown with decreased signal intensity within the high-signal-intensity normal peripheral zo Although controversial, the value of MRI appears to be in the staging of PCa.
Classification
Prostate cancer staging can follow either the TNM or Jewitt classification. The TNM stage 1 or Jewitt stage A are tumours not suitable for MRI detection. Most of those cancers are within the transition zone, the area where MRI can neither depict nor stage the disease. TNM stage 2 disease indicates tumour confined to the prostate gland. The low-signal-intensity tumour is seen within the peripheral zone and while the lateral margin can bulge, the bulge is smooth in contour. With the endorectal coil, direct visualization of the prostate capsule increases the confidence level in the evaluation of TNM stage 2 - Jewitt B disease. In TNM stage 3 and Jewitt C disease, the findings of importance are extracapsular and seminal vesicle invasion. MRI findings of extracapsular extension on endorectal coil MRI (Fig.2) include:
irregular bulge of the prostate margin;
contour deformity with step-off or angulated margin;
breech of the capsule with direct tumour extension;
obliteration of rectoprostatic angle; and
asymmetry of neurovascular bundles.
The seminal vesicle invasion is diagnosed (Fig.3) when there is a) demonstration of contiguous low-signal-intensity tumour extension into and around seminal vesicles; and/or b) tumour extension along the ejaculatory duct resulting in nonvisualization of the ejaculatory duct, decreased signal intensity of seminal vesicles, and loss of seminal vesicle wall on T2-weighted images. While axial planes of section are essential in the evaluation of extracapsular invasion, visualization of the invasion of the seminal vesicles is facilitated by the evaluation of transaxial and coronal plane of section. In a recently reported study by Yu et al., using the Jewitt classification and endorectal coil MRI, the accuracy for extracapsular extension was 82% and for seminal vesicle invasion 97%. These reports are in accordance with previously published results by Schnall et al., although they are higher than that published by Chelsky et al. In the evaluation of lymph node metastases, efficacy data for MRI and CT are similar. The recent developments of MR spectroscopic imaging expand the diagnostic assessment beyond anatomical information. MR spectroscopic imaging provides metabolic information specific to the prostate through the detection of the cellular metabolites citrate, creatine and choline. The information obtained from this new technology may allow an expanded assessment of tumour aggressiveness and the attendant risk of disease progression. In the localization of PCa, positive results from combined MRI and MRS demonstrate 91% specificity, the highest value obtained by a noninvasive method. Furthermore, the combined use of MR imaging and spectroscopy significantly improves evaluation of extracapsular cancer extension and decreases interobserver variability, increasing even further the value of MRI in the evaluation of prostate cancer.
HH