10 Jul, 2018
Prostate Imaging–Reporting and Data System (PI-RADS) – A Paradigm Shift.
Multiparametric magnetic resonance imaging (mpMRI) is considered the most sensitive and specific imaging technique for localizing clinically significant prostate cancer. However, variation in the performance, interpretation and reporting of prostate mpMRI examinations has been a significant obstacle to its widespread acceptance and use. In order to address this variation, the European Society of Urogenital Radiology (ESUR) published a set of consensus guidelines called Prostate Imaging–Reporting and Data System (PI-RADS).1
In an effort to update and improve upon the original version of PI-RADS, the American College of Radiology (ACR), ESUR and the AdMeTech Foundation collaborated to develop PI-RADS version 2 (PI-RADS v2), which was released in 2015.2 PI-RADS v2 includes information about clinical considerations and technical specifications for mpMRI as well as a lexicon of terminology.
Based on the current uses and capabilities of mpMRI and MRI- targeted procedures, PI-RADS v2 defines clinically significant cancer on pathology as those tumors that have a Gleason score ≥7 (including 3+4 with a prominent, but not predominant, Gleason pattern 4 component) and volume>0.5 cm3 or extraprostatic extension (EPE).
To arrive at one of these 5 PI-RADS v2 assessment categories for each suspicious finding in the prostate, T2WI and DWI are each assessed using a 5-point scale and DCE-MRI is classified as either positive or negative. Then, using the appropriate PI-RADS v2 table for either the peripheral zone (PZ) or the transition zone (TZ), these three parameters (for T2WI, DWI, and ?sometimes DCE-MRI) are integrated and each lesion is assigned a PI-RADS v2 assessment category (PI-RADS 1–5) that indicates the likelihood that it is a clinically significant cancer.
Assignment to a specific PI-RADS v2 assessment category is based solely on mpMRI findings. It does not take into account other factors, such as serum prostate-specific antigen (PSA), digital rectal examination, patient history or choice of treatment. However, these factors, along with local preferences, experience, and clinical history may determine recommendations regarding patient management, including biopsy.
DIFFUSION-WEIGHTED IMAGING SCORING
A score of 1 to 5 is assigned on DWI by comparing the signal intensity in a lesion to the average signal of normal prostate tissue in the histologic zone in which it is located. Nonetheless, the findings on DWI should always be correlated with findings on T2WI, T1WI and DCE-MRI.
Table 1: PI-RADS assessment for peripheral zone on diffusion-weighted imaging (DWI). ADC, apparent diffusion coefficient; PI-RADS, Prostate Imaging–Reporting and Data System v2. (Adapted from the American College of Radiology [ACR] Prostate Imaging–Reporting and Data System, version 2. Available at www.acr.org.)
Table 2 : PI-RADS assessment for transition zone on diffusion-weighted imaging (DWI). ADC, apparent diffusion coefficient; PI-RADS, Prostate Imaging–Reporting and Data System v2. (Adapted from the American College of Radiology [ACR] Prostate Imaging–Reporting and Data System, version 2. Available at www.acr.org.)
T2-WEIGHTED IMAGING SCORING
Scoring of T2WI also utilizes a 5-point scale, although the definitions of each score slightly differ between the PZ and TZ.
On T2WI, clinically significant cancers in the PZ usually appear as round or ill-defined hypointense focal lesions. It is important to note that many benign conditions may mimic this appearance, including prostatitis, hemorrhage, glandular atrophy, benign hyperplasia, biopsy-related scars and post-hormonal therapy or post-ablation changes. Careful inspection of additional sequences may provide clues to the correct diagnosis.
TZ cancers are even more problematic. When benign prostatic hyperplasia (BPH) is present, the TZ is composed of variable amounts of glandular (T2-hyperintense) and stromal (T2-hypo- intense) tissue resulting in heterogeneous signal intensity. Identifying T2 hypointense cancer amongst the regions of benign stromal hyperplasia is challenging. Typical T2W features of TZ tumors that may prove useful include ill-defined moderate hypointensity (“erased charcoal” or “smudgy fingerprint” appearance), spiculated margins, lenticular shape, absence of a complete hypointense capsule and invasion of the urethral sphincter and anterior fibromuscularstroma.
Table 3: PI-RADS assessment for peripheral zone on T2-weighted imaging (T2WI). PI-RADS, Prostate Imaging–Reporting and Data System v2. (Adapted from American College of Radiology [ACR] Prostate Imaging–Reporting and Data System, version 2, 2015. Available at www.acr.org.)
Table 4: PI-RADS assessment for transition zone on T2-weighted imaging (T2WI). BPH, benign prostatic hyperplasia; PI-RADS, Prostate Imaging–Reporting and Data System v2. (Adapted from American College of Radiology [ACR] Prostate Imaging–Reporting and Data System, version 2, 2015. Available at www.acr.org.)
DYNAMIC CONTRAST-ENHANCED MRI SCORING
Dynamic contrast-enhanced MRI is considered “positive” when there is focal enhancement that occurs before or contemporaneous with enhancement of adjacent normal prostatic tissue and that corresponds to a signal abnormality on DWI and/or T2WI.
Some benign processes are DCE-MRI positive. BPH nodules serve as the most common example of a benign process exhibiting occasional early enhancement, although their benign morphology on T2WI (round shape, circumscribed and encapsulated margins) often suggests the correct diagnosis. Diffuse non-focal early enhancement not localized to a specific T2WI or DWI abnormality is often seen in the setting of prostatitis and also considered a benign finding on DCE-MRI.
When T2WI and DWI are of diagnostic quality, DCE-MRI plays a minor role in determining the PI-RADS v2 assessment category. Thus, DCE-MRI does not contribute to determination of the assessment category when the finding in the PZ has a low (PI-RADS 1 or 2) or high (PI-RADS 4 or 5) likelihood of clinically significant cancer. However, in the PZ, a positive DCE-MRI score upgrades a DWI score of 3 to the final PI-RADS v2 assessment category of 4. The DCE score does not influence the final PI-RADS v2 assessment category for lesions in the TZ.
Table 5: PI-RADS assessment for dynamic contrast-enhanced (DCE) MRI. ADC, apparent diffusion coefficient; DWI, diffusion-weighted imaging; PI-RADS, Prostate Imaging–Reporting and Data System; T2WI, T2-weighted imaging. (Adapted from American College of Radiology [ACR] Prostate Imaging–Reporting and Data System, version 2, 2015. Available at www.acr.org.)
PERIPHERAL ZONE LESION
In the peripheral zone (PZ), assignment of a PI-RADS v2 assessment category for a lesion is based predominantly on the DWI score.
For example, if the DWI score is 4 and the T2WI score is 2, then the PI-RADS assessment category should be 4. The only exception to this direct relationship of the DWI score and PI- RADS assessment category in the PZ is a positive (+) DCE score, which upgrades a DWI score of 3 to the final PI-RADS assessment category of 4.
Fig. 1 Flowchart for deriving final PI-RADS assessment category for peripheral zone lesion. ADC, apparent diffusion coefficient; DWI, diffusion- weighted imaging; PI-RADS, Prostate Imaging–Reporting and Data System v2. (Adapted from American College of Radiology [ACR] Prostate Imaging– Reporting and Data System, version 2, 2015. Available at www.acr.org.)
TRANSITION ZONE LESION
In the transition zone (TZ), assignment of a PI-RADS v2 assessment category for a lesion is based predominantly on the T2W.
For example, if the T2WI score for a TZ lesion is 4 and the DWI score is 2, then the PI-RADS assessment category should be 4. The only exception to this direct relationship between the T2WI score and the final PI-RADS assessment category occurs with the T2WI score of 3, in which case the DWI score serves as a tiebreaker (namely, a DWI score of 5 leads to an upgrade of a TZ lesion with a T2WI score of 3 to a PI-RADS assessment category of 4).
Fig. 2 Flowchart for deriving final PI-RADS assessment category for transition zone lesion. BPH, benign prostatic hyperplasia; DWI, diffusion- weighted imaging; PI-RADS, Prostate Imaging–Reporting and Data System; T2WI, T2-weighted imaging. (Adapted from American College of Radiology [ACR] Prostate Imaging–Reporting and Data System, version 2, 2015. Available at www.acr.org.)
The major objectives of PI-RADS v2 are to improve prostate cancer detection, localization, characterization and risk stratification in patients with suspected tumor. In order to meet these goals, it is imperative to communicate results of an mpMRI examination in a clear, concise and structured fashion. Prostate Imaging–Reporting and Data System v2 contains a number of recommendations to help reduce variability in image interpretation, simplify terminology and standardize content.
Prostate volume should always be reported, as it may be useful to calculate PSA density (PSA/prostate volume) and influence various management decisions.
For PI-RADS v2, a lesion in the PZ should be measured on DWI (the “dominant” sequence in the PZ) and a lesion in the TZ should be measured on T2WI (the “dominant” sequence in the TZ). If lesion measurement is difficult or compromised on DWI (for PZ) or T2WI (for TZ), then measurement should be made on the sequence that shows the suspicious finding best.
As prostate cancer is usually multifocal, up to four lesions with a PI-RADS assessment category of 3, 4 or 5 may be assigned on the sector map. If there are more than four suspicious lesions, then only the four with the highest likelihood of clinically significant cancer (i.e. highest PI-RADS assessment category) should be reported.
From a clinical perspective, in a patient with multifocal tumors, the index lesion is the tumor focus that will drive any adverse oncologic outcome in the patient.3,4,5On MRI, the lesion designated as the index lesion is anticipated to yield the highest Gleason score, contribute to extraprostatic extension or produce positive margins at surgery. The lesion with the highest PI-RADS v2 assessment category should be designated as the index lesion. If the highest PI-RADS v2 assessment category is assigned to two or more lesions, then the index lesion should be the one that shows extraprostatic extension.
Each reported lesion should be assigned to a prostate sector(s) on a sector map consisting of 39 sectors: 36 for the prostate, 2 for the seminal vesicles and 1 for the external urethral sphincter.
Fig . 3 Prostate sector map for indication of location of lesions. AFS, anterior fibromuscularstroma; CZ, central zone; PZ, peripheral zone; TZ, transtion zone; US, urethral sphincter. (The prostate sector map, modified courtesy of David A. Rini, Department of Art as Applied to Medicine, Johns Hopkins University, is based on previously published figures by Villers et al [CurrOpin Urol. 2009;19:274–282] and Dickinson et al [Eur Urol. 2011;59:477–494] with anatomical correlation to the normal histology of the prostate by McNeal [Am J SurgPathol. 1988;12:619–633].)
?The Prostate Imaging–Reporting and Data System v2 is not a comprehensive prostate cancer diagnosis document and should be used in conjunction with other current resources. For example, it does not address the use of MRI for detection of suspected recurrent prostate cancer following therapy, progression during active surveillance or the use of MRI for evaluation of other parts of the body (e.g., the skeletal system) that may be involved with prostate cancer. ?In addition, there are multiple new and emerging imaging techniques for prostate cancer assessment and local staging that ?will undoubtedly affect the proposed interpretation scheme in the future. As relevant data and experience become available, these additional techniques may be incorporated into future versions of PI-RADS. However, PI-RADS has fuelled a paradigm shift towards global standardization in the acquisition, interpretation and reporting of prostate mpMRI examinations.
 Barentsz JO, Richenberg J, Clements R et al. European Society of Urogenital Radiology. ESUR prostate MR guidelines 2012.EurRadiol 2012; 22(4):746– 757
 American College of Radiology. MR Prostate Imaging Reporting and Data System version 2.0. Accessed June 2015, from http://www.acr.org/Quality- Safety/Resources/PI-RADS.
 Ahmed HU. The index lesion and the origin of prostate cancer. N Engl J Med 2009; 361(17):1704–1706
 Karavitakis M, Winkler M, Abel P, Livni N, Beckley I, Ahmed HU. Histological characteristics of the index lesion in whole-mount radical prostatectomy specimens: implications for focal therapy. Prostate Cancer Prostatic Dis 2011; 14(1):46–52
 Weinreb JC, Barentsz JO, Choyke PL et al. PI-RADS Prostate Imaging - Report- ing and Data System: 2015, Version 2. EurUrol 2016; 69(1):16–40
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