MpMRI:
MpMRI is the method of choice to evaluate the prostate in clinically significant adenocarcinoma.
Images are obtained using a multiparametric technique including High-resolution T2 weighted images,
a dynamic contrast study (DCE),
and DWI. Secuences T1W-images determine the presence of post-biopsy hemorrhage.
Indications MpMRI in prostate gland (fig.2):
The principal indication for MRI of the prostate is in the evaluation of prostate cancer after an ultrasound guided prostate biopsy has confirmed cancer in order to determine if there is extrcapsular extension.
MRI is also being used when the TRUS biopsy is negative but the PSA is persistently elevated.
MRI-guided prostate biopsy is also being used,
particularly in those cases where TRUS biopsy is negative but clinical and PSA suspicion remains high.
Following radical prostatectomy,
patients with elevated PSA should also be examined using MRI.
PI-RADS (Table 1)
A major objective of a prostate MRI exam is to identify and localize abnormalities that correspond to clinically significant prostate cancer.
It is designed to be used in a pre-therapy patient.
PI-RADS score is given to assess the probability of the lesion being malignant.
For PI-RADS™ v2 clinically significant cancer is defined on pathology/histology as Gleason score ≥7,
and/or volume ≥0.5cc,
and/or extra prostatic extension.
The PI-RADS assessment categories are based on the findings of multiparametric MRI,
which is a combination of T2-weighted (T2W),
diffusion weighted imaging (DWI) and dynamic contrast-enhanced (DCE) imaging.
Although there is debate about the value of using DCE imaging,
it is still included in the PIRADS version 2.0.
Post-biopsy changes,
(i.e.
hemorrhage and inflammation),
may adversely affect the interpretation of multiparametric MRI whereas signal intensities might be altered.
PROSTATE CANCER (staging)
Correct staging of prostate carcinoma is important for therapeutic management.
TNM-staging is based on clinical and pathological findings,
and if indicated on additional imaging findings.
(fig.3)
N-STAGE
Regional lymph nodes are below the level of the common iliac junction and are staged N1: pelvic,
hypogastric,
sacral and iliac (internal,
external)
In MRI the DWI is the best sequence for detection of lymph nodes.
T1W series are useful for interpretation of the border contour and signal characteristics of lymph nodes.
The characteristics are considered suspicious:
- Round shape and short axis of ≥8mm.
- Oval shape and short axis of ≥10mm.
- Heterogeneous appearance.
- Irregular margins
Distant lymph nodes are outside these regions and are staged as metastatic disease M1a.
Detection of extracapsular extension is an essential part of tumor staging.
PI-RADS ASSESSMENT CATEGORIES
Assignment of a PI-RADS assessment category for each lesion is based on the scoring of T2w,
DWI/ADC,
and DCE sequences,
according to zonal anatomy:
Peripheral zone (PZ): (fig.4)
For the peripheral zone the DWI whith corresponding ADC map is the dominant sequence that will determine the overall suspicious score.
A lesion with DCE positive can increase the category PIRADS from 3 to 4.
Examples: (fig 5,
6,
7,
8,
9,
10,
11,
12,
13).
Transition zone (TZ): (fig.14)
T2w imaging is the dominant sequence in the transition-central zone that will determine the overall suspicion score.
In the transitional zone an equivocal lesion (PI-RADS category 3) is assigned to PI-RADS category 4 if the DWI corresponds with category 5 (markedly intense > 1.5 cm).
The lesion remains assigned to PI-RADS category 3 if the DWI corresponds to DWI category 4 (markedly intense but < 1.5cm) or a lower category.
Examples in our hospital: (fig 15,
16,
17,
18,
19,
20,
21,
22).
EXTRACAPSULAR EXTENSION:
MRI is superior to transrectal ultrasound,
CT,
and digital rectal examination in detecting extracapsular extension as well as seminal vesicular invasion in prostate cancer.
The prostate does not have a true capsule.
However on MRI the outer border of the prostate does have a thin,
hypointense line,
which is histopathologically composed of a fibromuscular band.
This hypointense line can be used to assess extraprostatic tumor growth.
The extracapsular extension (ECE) is found in a significant number of patients with clinically localized prostate cancer who undergo radical prostatectomy.
ECE,
if present,
poses a risk for an increased likelihood of postoperative biochemical recurrence. Furthermore,
there are data endorsing the use of radiation therapy in patients with T3 disease.
Therefore,
it would be valuable to assess the risk of ECE before surgery for therapeutic planning.
The European Society of Urogenital Radiology (ESUR) has suggested a scoring system for predition of ECE.
Boesen et al have evaluated this model and found that a cutoff level of ≥ 4 has the best relation of sensitivity and specificity (0,81 and 0,78,
respectively) but that a cutoff level of ≥3 has a higher predictive negative value (0,95).
The radiologist must accordingly be familiar with the MRI signs associated with ECE.
(Table 2).
Examples: (fig.
9,
10,
11,
12,
19, 20,
21).