What can multiparametric MRI reveal about the local stage of prostate cancer, including tumor extent, seminal vesicle involvement, neurovascular bundle involvement, and nodal disease?

Multiparametric MRI for Prostate Cancer Staging

Multiparametric MRI is highly specific but poorly sensitive for local prostate cancer staging, with excellent ability to rule in extracapsular extension and seminal vesicle invasion when positive, but limited ability to exclude these findings when negative. 1

Local Tumor Extent Assessment

Extracapsular Extension (ECE)

• MRI demonstrates 91% specificity but only 57% sensitivity for detecting ECE, meaning a positive finding is highly reliable, but a negative MRI cannot exclude microscopic capsular penetration 1
• T2-weighted imaging is the critical sequence for assessing capsular integrity, showing cancer as low signal intensity areas that may breach the prostatic capsule 2, 3
• Look specifically for: irregular bulging of the capsular contour, obliteration of the rectoprostatic angle, direct tumor extension into periprostatic fat, and asymmetry of the neurovascular bundles 3, 4

Seminal Vesicle Invasion (SVI)

• MRI achieves 96% specificity but only 58% sensitivity for SVI, making it excellent for confirming invasion when present but unreliable for excluding it 1
• Positive findings include: low T2 signal within normally hyperintense seminal vesicles, focal or diffuse enlargement, disruption of the seminal vesicle wall, and restricted diffusion on DWI 3, 4
• Direct tumor extension along the ejaculatory ducts is the most common pathway for SVI 4

Neurovascular Bundle Involvement

• Neurovascular bundle asymmetry, thickening, or direct tumor contact on T2-weighted imaging suggests involvement, though this finding has lower accuracy than ECE or SVI assessment 3
• Bilateral neurovascular bundle involvement significantly impacts surgical planning and nerve-sparing approaches 1
• MRI can help localize dominant disease for focal therapy planning in high-risk patients 1

Nodal Disease Evaluation

Limitations of MRI for Lymph Nodes

• Both MRI and CT rely on size criteria to detect nodal metastases, conferring poor sensitivity because most metastatic nodes are normal-sized 1
• Standard size threshold is typically >8-10mm in short axis, but this misses micrometastatic disease 1
• MRI or CT for nodal staging is generally appropriate when the a priori risk exceeds 10%, which occurs in all high-risk patients 1

Diffusion-Weighted Imaging for Nodes

• DWI can improve nodal detection beyond size criteria alone, with reported sensitivity of 61-94% and specificity of 90-99% 5
• Whole-body DWI can serve as a screening examination for both nodal and bone metastases 1

Optimal MRI Protocol for Staging

Essential Sequences

• T2-weighted imaging (axial, sagittal, coronal) is mandatory for anatomic detail and local staging assessment 2, 5
• Diffusion-weighted imaging with ADC maps is critical, with high b-value images (b=1400-2000) most reliable for cancer detection 2
• Dynamic contrast-enhanced (DCE) imaging adds modest incremental value for initial staging but becomes critically important in specific scenarios like post-treatment surveillance 2

Technical Specifications

• 3T MRI is preferred over 1.5T for improved signal-to-noise ratio and spatial resolution 2, 5
• Pelvic phased-array surface coil should be used; endorectal coil is not routinely required 2
• Follow PI-RADS version 2.1 standards for all sequence parameters 2

Clinical Application Algorithm

For Intermediate-Risk Disease:

1. Obtain multiparametric MRI (with and without IV contrast) for local staging 1
2. Use T2-weighted findings to assess ECE and SVI risk
3. If MRI shows definite ECE or SVI (high specificity), adjust treatment planning accordingly
4. If MRI is negative, recognize that microscopic disease may still be present (low sensitivity)

For High-Risk Disease:

1. Obtain bone scan AND either pelvic mpMRI or CT at diagnosis 6
2. mpMRI is preferred over CT for local tumor staging 6
3. CT of abdomen and pelvis should be performed to evaluate nodal metastases 6
4. Consider molecular imaging (PSMA PET) if conventional imaging is negative but treatment decisions would be altered by detecting occult metastases 6

Critical Pitfalls to Avoid

• Do not rely on negative MRI to exclude ECE or SVI - the 57-58% sensitivity means nearly half of cases with pathologic extension will appear organ-confined on imaging 1
• Perform MRI at least 6 weeks after prostate biopsy to avoid hemorrhage artifact that degrades image quality 2
• Do not use MRI without contrast for staging - DCE sequences significantly improve accuracy 5
• Recognize that post-biopsy changes can mimic extracapsular extension on T2-weighted imaging, leading to overstaging 3
• The interpreting radiologist should read at least 20 prostate MRI examinations annually for adequate experience 2

Comparison with Other Modalities

• MRI is superior to CT for detecting detrusor muscle invasion and periprostatic tissue involvement 5
• Bone scintigraphy remains the standard for bone metastasis detection, though MRI and PSMA PET are superior for both diagnosis and follow-up 1
• CT is useful for following response of known enlarged lymphadenopathy but has the same size-based limitations as MRI for initial nodal detection 1