Pitfalls in Interpreting mp-MRI of the Prostate: A Pictorial Review with Pathologic Correlation
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The purpose of this pictorial review is to present a wide spectrum of prostate multiparametric MRI (mp-MRI) pitfalls that may occur in clinical practice, with radiological and pathological correlation.
All examinations were performed according to ESUR Guidelines protocols.
Results and Conclusion
mp-MRI imaging of the prostate often leads to interpreting doubts and misdiagnosis due to the many interpretative pitfalls that a tissue, whether healthy or treated, may cause. These “false-positive” findings may occur in each stage of the disease history, from the primary diagnosis and staging, to the post-treatment stage, and whether they are caused by the tissue itself or are iatrogenic, their recognition is critical for proper treatment and management. Knowledge of these known pitfalls and their interpretation in the anatomical-radiological context can help radiologists avoid misdiagnosis and consequently mistreatment.
• Some physiological changes in the peripheral and central zone may simulate prostate cancer.
• Technical errors, such as mispositioned endorectal coils, can affect the mp-MRI interpretation.
• Physiological changes post-treatment can simulate recurrence
KeywordsProstate mp-MRI Pitfalls Prostate MRI Differential diagnosis
Pitfalls classification based on categories before treatment
1. Pitfalls in primary diagnosis
1. Bilateral basal hypointense zones (moustache sign)
2. Median posterior hypointense area at the middle third of the gland
3. Transition zone prostate cancer versus BPH foci of stromal hyperplasia
4. Ectopic BPH nodule
5. Granulomatous prostatitis after intravesical instillation of BCG
6. Hypertrophic anterior fibromuscular stroma
7. Periprostatic venous plexus and neurovascular bundle
2. Pitfall in the staging
1. T3 versus T2 (overlapping with pitfalls in primary diagnosis: granulomatous prostatitis, periprostatic venous plexus and neurovascular bundle)
5. Bone findings
6. Controversial on lymph nodes
3. Artifacts/iatrogenic changes
1. Mispositioned endorectal coil
2. Post-biopsy changes
Pitfalls classification based on categories after treatment
4. Pitfalls after surgical treatment
1. Residual glandular tissue
3. Retained seminal vesicles
4. Sealed off veins
5. Prominent periprostatic venous plexus
6. Residual verumontanum
5. Pitfalls after radiation/hormone deprivation therapy and after focal therapies
1. RT-induced capsular irregularity may hinder evaluation of extracapsular extension
2. Focal regions of T2-hypointensity may represent treated tumour and not local recurrence
3. Focal therapies-induced enhancing of reactive prostate tissue may hamper the assessment of persistent/residual disease
Pitfalls in Primary Diagnosis
Bilateral Basal Hypointense Zones (moustache sign)
Median Posterior Hypointense Area at the Middle Third of the Gland
Transition Zone Prostate Cancer Versus BPH Foci of Stromal Hyperplasia
Ectopic BPH Nodule
Granulomatous Prostatitis after Intravesical Instillation of BCG
Hypertrophic Anterior Fibromuscular Stroma
Periprostatic Venous Plexus and Neurovascular Bundle
Pitfalls in the Staging
At the apex: Pseudo T3a stage is caused by an anatomical variation of the shape of the apex, when PZ prostatic tissue is present along the posterior aspect of the distal urethra. When involved by tumour, it can simulate subapical T3 disease.
At the base: bulging BPH nodules along the root of seminal vesicles simulates T3b stage.
Any location: granulomatous prostatitis commonly infiltrates the periprostatic fat and simulates T3a disease.
At the postero-lateral portion of the gland: periprostatic venous plexus and neurovascular bundle.
The pitfalls mentioned above are representative of T3 vs. T2, and overlap with pitfalls in primary diagnosis.
Benign Bone Findings Mimicking Prostate Cancer Metastases
Controversial on Lymph Nodes
Another role of MRI in the evaluation of patients with PCa is the detection of pathologically involved pelvic lymph nodes. According to standard criteria on conventional MRI, lymph nodes are classified as malignant if the short-axis diameter is elongated and exceeds 10 mm, or is rounded and exceeds 8 mm. Unfortunately, it could happen that these kind of lymph nodes are disease free, or that lymph nodes less than 10 mm considered as non metastatic are pathologically involved at histology . The use of lymphotropic, superparamagnetic iron oxide nanoparticle contrast agents can improve the Sensitivity (Se) and Specificity (Spe) of MRI in the detection of lymph node metastases. These nanoparticles have a monocrystalline, inverse spinel, superparamagnetic iron oxide core, contain a dense packing of dextrans to prolong their time in circulation, and are avidly internalized by macrophages within lymph nodes and result in changes in the magnetic properties detectable by MRI  After injection of nanoparticles, nodes could be considered malignant when one of the following three criteria is present: a decrease in signal intensity (SI) of less than 30 % on T2W or gradient-echo sequences; a heterogeneous SI (giving the entire node a mottled appearance), discrete focal defects (isolated islands of high SI), or both; a central area of hyperintensity (excluding a fatty hilum), but a peripheral decrease in SI. In a study of 80 patients with T1-3 PCa who subsequently underwent surgical lymph node resection or targeted lymph node biopsy for a total of 334 lymph nodes evaluated with direct MRI-histological correlations, conventional MRI detected pathologically involved nodes with a Se of 35.4 % and Spe of 90.4 %, compared with 90.5 % Se and 97.8 % Se for MRI with nanoparticle contrast agents .
Pitfalls after Treatment
Radical prostatectomy has been performed for more than a century , and remains the most common treatment choice in patients with organ-confined prostate cancer . It involves the removal of the entire prostate, the seminal vesicles, and the ampullary portions of the vasa deferentia, with the formation of a vesicourethral anastomosis. Whenever possible, the surgical procedure is tailored to preserve the neurovascular bundles responsible for erectile function as well as the external sphincter responsible for urinary continence. The likelihood of simultaneous achievement of all three desired outcomes (cancer-free status, continence, and potency, sometimes called the “trifecta”) after radical prostatectomy has been reported to be 60 %–91 % after 18–24 months [39, 40, 41].
Residual Glandular Tissue, Fibrosis and Granulation Tissue
Retained Seminal Vesicles
Sealed Off Veins
Prominent Periprostatic Venous Plexus
Pitfalls after Radiation/Hormone Deprivation Therapy
The first important aspect to consider when evaluating possible recurrence after Radiation Therapy (RT) is that recurrence tends to occur at the site of the primary (pre-RT) tumour [49, 50]. The relative signal intensity difference between the treated gland and a cancerous lesion is typically less than in the untreated gland, however, often making the lesions less conspicuous .
Pitfalls after Focal Therapies
After cryotherapy, heterogeneous enhancement intermixed with areas of necrosis and thickening of the prostatic capsule, urethra and rectal wall are seen on T1-weighed (T1w) images .
After HIFU, ablation-induced changes in the region of the lesions appear on contrast-enhanced T1w images as non-enhancing hypointense regions with 3–8 mm thick peripheral rims of enhancement that resolve within 3–5 months showed that at 6 months, the prostate is of predominantly low signal intensity on T2w images and that there is a median volume reduction of 61 % . They also concluded that the volume of enhancing prostate tissue on the initial image after treatment correlated well with serum PSA level nadir (Spearman’s r = 0.90, P < 0.001) and with volume at 6 months (Pearson’s r = 0.80, P = 0.001). After photodynamic therapy, MRI may be used to assess the extent and distribution of the expected necrosis in the target region. In one study , most patients showed marked irregularity at the treatment boundary, that was best appreciated on T1w images after i.v. administration of contrast material, with areas of enhancement (viable tissue) interposed between non-enhancing low-signal-intensity regions (necrosis) .
Enhancing soft tissue lesions after focal treatments should be considered suggestive of residual/recurrence, just as they are after other forms of treatment. It is important to be aware that a recurrent lesion may present in conjunction with normal post-treatment appearances. Furthermore, the characteristics typically associated with recurrence on T2w images may not represent recurrence in some cases. In some cases of recurrence, these features simply fail to appear . Some authors suggest that MRSI is superior to MRI for the differentiation of cancer voxels from necrosis voxels , but at present, MRSI is not widely used to aid clinical decision making, and is therefore insufficient to give a conclusive statement. After HIFU, the detection of recurrent or residual disease could be hindered by diffuse or multifocal areas of low signal intensity on T2w MRI . A short time to peak enhancement, early washout, and other pharmacokinetic parameters seen on DCE-MRI in patients with untreated prostate cancer can also be present in cases of recurrence after HIFU . A study showed that for prediction of local tumour progression of prostate cancer after high intensity focused ultrasound, dynamic contrast-enhanced MR imaging was more sensitive but less specific than the combination of T2-weighted and DW MR imaging . Despite the sensitivity of dynamic contrast-enhanced imaging for the detection of progressive tumours after ablative therapy, areas of residual benign prostatic hypertrophy may show hypervascularity, thereby yielding false-positive findings and limiting the specificity of dynamic contrast-enhanced imaging . Diffusion-weighted imaging, on the other hand, may provide greater specificity, albeit lower sensitivity for detection of viable tumours after ablative therapy . Therefore, suspicious lesions should always be confirmed by (targeted) biopsy.
Artefacts and Iatrogenic Changes
Mispositioned Endorectal Coil
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