Mr Imaging Of Prostate Cancer Technique

MRI evaluation of the prostate requires optimal technique using phased array pelvic surface coils. Endo-rectal coils (ERCs) used either alone or in combination with pelvic phased array coils have been shown to improve staging accuracy by better depiction of pseudocapsular penetration. ERCs have the drawbacks of causing patient discomfort and therefore possible movement artifact, and producing near field artifact although the latest scanners have software to overcome this problem.

Thin section T2-weighted turbo spin echo sequences are essential for differentiating normal internal zonal prostatic anatomy and pathology. The prostate and seminal vesicles must be covered in their entirety. Off-axis imaging, parallel to the prostate, can be helpful in evaluating extra-prostatic extension.

T1-weighted spin echo images are useful for detecting enlarged pelvic lymph nodes, bone metastases, and in distinguishing tumour from post biopsy haemorrhage. Fat suppressed imaging has no staging benefit over conventional T1 - and T2-weighted spin echo sequences. Dynamic Gd-enhanced imaging can improve tumour recognition and staging accuracy, but is not widely used.

Magnetic resonance spectroscopy is currently under evaluation in prostate cancer. Initial findings indicate that there is increase in the ratio of choline plus creatine to citrate in prostate tumour tissue. This has been used to determine the presence and localisation of tumour within the prostate and to improve the assessment of extra-glandular extension.

Current indications

The indications for MRI are controversial and depend on local surgical and oncological practice, the availability of MRI and local radiological expertise. Many centres do not perform routine MRI, instead relying on clinical assessment (digital rectal examination (DRE)) combined with PSA level and Gleason grade to predict tumour stage.

MRI can provide additional useful information in assessing for extra-capsular tumour and seminal vesicle invasion. Decision analysis studies indicate MRI is most helpful in patients with an intermediate clinical risk of extra-capsular extension (clinically localised at DRE, PSA levels 10-20 ng ml-1, Gleason score of 5-7). In these circumstances, MRI signs of extra-capsular disease can stratify patients into low and high-risk groups for disease progression, based on PSA levels at 3 years post treatment. This information may influence the choice of treatment modality and the decision to offer adjuvant hormonal therapy.

MRI is useful in the selection of patients for brachytherapy. Accurate staging information is essential, as disease must be contained within the prostatic pseudocapsule (T1-2 N0 M0). Despite the low positive predictive value of MRI, features suggestive of extracapsular extension are usually taken as a contraindication to treatment. MRI can also give an accurate assessment of prostatic volume, which should be<50 cm3.

MRI may assist radiotherapy planning for locally advanced disease. The greater soft tissue contrast and multiplanar capabilities of MRI provide a more accurate assessment of disease extent and involvement of adjacent organs. Fusion imaging systems are in development to facilitate radiotherapy planning using MRI data.

MRI may also be useful in evaluating patients with a rising PSA following radical prostatectomy. Locally recurrent tumours or isolated lymph nodes may be suitable for salvage radiotherapy in the absence of more widespread disease.

Imaging features

Prostate cancer occurs in the peripheral zone in 75% of patients and usually has low signal intensity compared to the normal high signal of the peripheral zone on T2-weighted turbo spin echo images. Tumours in the central gland may be indistinguishable from normal tissue or benign prostatic hyperplasia. The prostatic pseudocapsule is seen as a thin band of low signal between the peripheral zone and periprostatic connective tissue. Features suggestive of capsular penetration are extracapsular tumour, capsular retraction, focal capsular bulging, periprostatic stranding, capsular thickening and tumour contiguity with the pseudocapsule>12.0 mm.

Following radiotherapy the prostate gland shrinks and the peripheral zone becomes intermediate signal intensity on T2-weighted imaging. After radical prostatectomy, residual fibrosis in the prostatic bed has low signal intensity on all sequences. This may be differentiated from the intermediate signal intensity of recurrent tumour on T2-weighted images.

Staging accuracy

MRI offers the single most accurate imaging assessment of local disease and regional metastatic spread. The integration of MRI findings with standard clinical diagnostic tests has also been shown to improve the overall accuracy of prostate cancer staging. A metaanalysis of studies evaluating MRI staging accuracy in patients with clinically localised prostate cancer produced a maximum combined sensitivity and specificity on the receiver operating characteristic (ROC) curve of 74%, using pathological stage as the gold standard. At a specificity of 80% on the ROC curve, sensitivity was 69%.

As with any diagnostic test, however, the positive and negative predictive values will vary according to the prevalence in the population studied. Most published results give positive predictive values below 50%, indicating MRI usually overcalls extra-capsular disease extension prior to radical prostatectomy. It is recognised that pathological stage is an imperfect gold standard, however, and selected sections may miss extra-capsular disease extension if the gross specimen is not evaluated throughout.

Pitfalls of MRI

• Signal changes within the prostate should be interpreted with caution as other pathological processes may mimic prostate cancer. Infection, inflammation and haemorrhage will all produce low signal changes within the peripheral zone on T2-weighted imaging. The normal fibromuscular bands of the peripheral zone may also appear thickened as a normal variant, and should not be confused with tumour.

• Staging accuracy is reduced following transrectal ultrasound (TRUS) guided biopsy. Evaluation of extra-capsular extension is particularly difficult if the pseudocapsule or seminal vesicles have been multiply biopsied in an attempt to gain pathological evidence of locally advanced disease. Signal characteristics may be helpful as, unlike tumour, methaemoglobin within haemorrhage is high signal on Tl-weighted imaging. If equivocal findings are present, radical treatment can be deferred and the MRI repeated, as post biopsy changes will resolve with time.

• Similarly, signal changes within and around the seminal vesicles can be misinterpreted on MR imaging. Haemorrhage, inflammatory scarring, stones and amyloid can all produce focal abnormalities and tubular thickening. Normal fibrous tissue around the ejaculatory ducts and infero-medial tips of the seminal vesicles can also be misdiagnosed as tumour. Some published reports have disregarded signal changes in the seminal vesicles unless there is evidence of tumour within the adjacent prostate on TRUS biopsy.

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