MRI
T2-weighted MRI
Prostate zonal anatomy is best demonstrated with MRI. On T2-weighted sequences, the normal prostate can be divided into the peripheral zone, which shows high signal intensity owing to its high water content, and the central gland (comprising central and transition zones), which has lower signal intensity (Figure 3). Prostate cancer is frequently seen as an area of low signal abnormality in the peripheral zone. Some cancers are isointense and cannot be seen on standard MRI. Other conditions, such as prostatitis, atrophy and calcifications, may also cause low signal intensity and result in a number of false positives. Areas of hemorrhage secondary to biopsy are easily confused with tumor on T2-weighted sequences; for this reason it is important that a T1-weighted sequence is also performed to demonstrate high signal blood products. Ideally, prostate MRI should not be performed for at least 8 weeks following biopsy as hemorrhage may interfere with interpretation of the images.[46]
Figure 3.
Coronal T2-weighted imaging of normal prostate anatomy on MRI. The high signal of the peripheral zone and heterogeneous intermediate signal central gland caused by benign prostatic hyperplasia.
Imaging is usually performed with pelvic phased array coils. The use of endorectal coils improves image resolution on standard 1.5T scanners. Endorectal coils may not confer the same advantages on higher field-strength (3T) systems.[47]
Early studies demonstrated relatively low sensitivity and specificity of MRI for prostate cancer detection and staging;[48] however, recent development of endorectal coils and higher field-strength magnets have led to a significant improvement in image resolution.[47,49]
MRI is useful in the local staging of cancer and treatment planning. Its established role is in patients who are at intermediate or high risk for localized disease progression and who are being considered for radical treatment (surgical prostatectomy or brachytherapy). MRI is used to exclude direct extracapsular tumor extension or invasion into the seminal vesicles; if present, these would usually exclude patients from radical treatment (Figures 4–7). Increasingly, however, surgeons are prepared to operate on patients with early local disease progression (stage T3) and high-dose brachytherapy can also be considered in this patient group. Therefore, accurate documentation of the extent of local disease is essential for treatment planning. Extracapsular tumor extension is usually diagnosed by identification of an abnormal low signal mass extending through the prostate pseudocapsule into the periprostatic fat (T3a) or into the seminal vesicles (T3b). A number of subtle signs of extracapsular spread are also described, including loss of the rectoprostatic angles and retraction or bulge of the prostate capsule.[50]
Figure 4.
Axial T2-weighted imaging of prostate cancer of the right peripheral zone, which appears as a low signal area (arrow).
Figure 5.
Axial T2-weighted imaging of prostate cancer of the left peripheral zone with a bulge of the capsule indicating early extracapsular invasion (arrow).
Figure 6.
Axial T2-weighted imaging of prostate cancer at the right base with extension into the periprostatic fat (arrow).
Figure 7.
Coronal T2-weighted imaging of prostate cancer of the prostate base with invasion of the seminal vesicles (arrow).
The sensitivity of MRI in detecting extra-capsular extension or seminal vesicle invasion has improved to a range of 73 to 80% with a high specificity of 97–100%. Extracapsular extension as small as 0.5 mm at histopathology has been accurately detected.[51]
Magnetic resonance examination is also used to identify enlarged pelvic or abdominal retroperitoneal lymph nodes, which may indicate metastatic disease; it can also identify bone metastases and renal obstruction.
In addition to tumor staging, MRI now has an emerging role in tumor localization within the prostate gland. MRI has a good sensitivity rate for prostate cancer detection, ranging from 60 to 96%, but its poorer specificity has limited its use as a screening tool.[52] However, it has been demonstrated that patients with suspected prostate cancer and negative systematic biopsies will benefit from an MRI examination aimed at locating the tumor. Prostate cancer is detected on MRI in approximately a third of patients with one or more negative systematic biopsies.[53,54] The diagnosis of transitional zone cancer remains challenging owing to the coexistence of nodular BPH, which is an almost invariable finding in patients of this age; fortunately, tumors in this location are uncommon. There are conflicting reports regarding the sensitivity and specificity of MRI in the detection of cancers in the central gland. In one study, MRI diagnosed only one out of 79 cancers,[55] but in another study MRI showed better sensitivity than sextant biopsy throughout the prostate.[56] MRI has also been used to guide targeted prostate biopsy.[57]
MRI has shown excellent results in assisting surgical planning. It is well known that surgery can be complicated by erectile dysfunction following the resection of the neurovascular bundle. The surgical technique will be influenced by the position of the tumor within the gland; a nerve-sparing approach may be selected if the tumor is not in close proximity to the neurovascular bundle. Hricak et al. demonstrated that MRI findings changed the surgical plan in 78% of patients and was accurate in 93% of patients.[58]
The universal use of MRI in the staging of prostate cancer remains controversial. MRI findings have made a significant contribution in the staging of all risk groups but have been greatest in intermediate- and high-risk groups.[59]
A number of newer MRI techniques have recently been developed, which allow the potential for improved prostate tumor detection when added to standard MRI protocols. These include dynamic contrast-enhanced imaging, diffusion-weighted imaging (DWI) and magnetic resonance spectroscopy.
Dynamic Contrast-enhanced MRI
During dynamic contrast-enhanced MRI, a bolus of intravenous contrast medium (gadolinium) is injected and serial, rapid sequences are obtained. The objective is to demonstrate the increased enhancement of the prostate cancer compared with normal prostatic tissue, which correlates with tumoral angiogenesis. As with contrast-enhanced ultrasound, tumoral enhancement tends to be most prominent with higher grade, and, therefore more clinically significant tumors.[60] Prostate cancers are characterized by their early enhancement and early washout.[61] Benign prostatic nodules, prostatitis and hemorrhage can all mimic malignancy. Benign prostatic nodules can be particularly challenging and also usually enhance early, but have a slower washout than prostate cancers.
Dynamic contrast-enhanced MRI has demonstrated promising results, improving both specificity and accuracy in prostate cancer localization when added to conventional MRI.[62,63] Dynamic contrast-enhanced sequences, when added to T2-weighted MRI, have also resulted in a significant improvement in the detection of extracapsular extension with reported sensitivity and specificity of 86 and 95%, respectively.[64] Improvements on the accuracy of staging have been more significant for less experienced readers.[65] This technique is also useful in the detection of local tumor recurrence following radiotherapy.[66]
The universal clinical adoption of this imaging modality has been limited by the wide range of sensitivity and specificity reported; this is likely to reflect varying experience of the reporter, protocols, patient demographics and diagnostic criteria. It is accepted that contrast-enhanced MRI is useful for selected cases, particularly when combined with other functional studies.
Diffusion-weighted MRI
Diffusion-weighted imaging assesses the diffusion of water molecules within different tissues. Normal prostate glandular tissue has a higher water diffusion rate than cancer tissue owing to restricted diffusion in tightly packed cancer cells. DWI is an inherently T2-weighted sequence but, unlike conventional T2-weighted imaging, prostate cancer frequently demonstrates increased signal intensity on standard DWI scans, making the tumor difficult to visualize within the normal high-signal peripheral zone. To reduce the effect of this T2 'shine through' effect, the apparent diffusion coefficient (ADC) is calculated, which corresponds to the difference of diffusion. Prostate cancer appears as an area of high signal on DWI images, but as a low-signal region on ADC mapping, owing to restricted diffusion (Figure 8).
Figure 8.
A 59-year-old man with prostate cancer. (A) T2-weighted imaging shows localized, low signal abnormality in the right mid-peripheral zone (arrow). (B–D) Diffusion-weighted images obtained at b values of 0, 250 and 750 s/mm2. The diffusion series show the abnormality (arrows) is difficult to detect on the b = 0 image, but becomes more apparent at the higher b values. (E) Corresponding apparent diffusion coefficient map, showing the tumor as an area of restricted diffusion (arrow) as low signal abnormality.
Images courtesy of Aslam Sohaib, Royal Marsden Hospital, UK.
The advantages of DWI are high-contrast resolution between normal prostate and cancerous tissue, and its short acquisition time. The disadvantages of DWI are its low spatial resolution (due to low signal:noise ratio), increased susceptibility to artifacts and overlap of diffusion values between benign and malignant lesions.[67] There is no ADC threshold value that will reliably differentiate between benign and malignant lesions. In addition, there are age-related changes in diffusion within the prostate and intersubject variability.
Recent studies have demonstrated that DWI, when used as an adjunct to T2-weighted MRI, improved sensitivity and specificity for prostate cancer detection to 84 and 85%, respectively.[68,69] The sensitivity of DWI is better in the peripheral zone than the central gland.[70] DWI has also been shown to be helpful in the identification of prostate cancer in patients with previous negative biopsies and persistently elevated PSA.[71]
Diffusion-weighted imaging is also helpful in differentiating between low-, intermediate- and high-risk patients as there is a significant correlation between the Gleason score of the tumor and the ADC value. There is also an inverse relationship between the ADC value and the percentage of tumor involvement in core biopsies.[72] Pre- and post-radiotherapy ADC values can also be used to assess response to treatment.[73] In patients managed by active surveillance, the reduction of the ADC value by 10% or more indicates disease progression.[74]
A recent study has demonstrated that DWI can be used to accurately differentiate between benign and malignant pelvic nodes in patients with prostate cancer and the ADC value is significantly superior to size criteria.[75]
Future Oncol. 2011;7(5):679-691. © 2011 Future Medicine Ltd.
Cite this: Imaging in Prostate Cancer - Medscape - May 01, 2011.
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