Platinum Priority – Prostate CancerEditorial by Jelle O. Barentsz, Jeffrey C. Weinreb, Sadhna Verma et al on pp. 41–49 of this issuePI-RADS Prostate Imaging – Reporting and Data System: 2015, Version 2☆
Introduction
Magnetic Resonance Imaging (MRI) has been used for noninvasive assessment of the prostate gland and surrounding structures since the 1980s. Initially, prostate MRI was based solely on morphologic assessment using T1-weighted (T1W) and T2-weighted (T2W) pulse sequences, and its role was primarily for locoregional staging in patients with biopsy proven cancer. However, it provided limited capability to distinguish benign pathological tissue and clinically insignificant prostate cancer from significant cancer.
Advances in technology (both in software and hardware) have led to the development of multiparametric MRI (mpMRI), which combines anatomic T2W with functional and physiologic assessment, including diffusion-weighted imaging (DWI) and its derivative apparent-diffusion coefficient (ADC) maps, dynamic contrast-enhanced (DCE) MRI, and sometimes other techniques such as in-vivo MR proton spectroscopy. These technologic advances, combined with a growing interpreter experience with mpMRI, have substantially improved diagnostic capabilities for addressing the central challenges in prostate cancer care: 1) Improving detection of clinically significant cancer, which is critical for reducing mortality; and 2) Increasing confidence in benign diseases and dormant malignancies, which are not likely to cause problems in a man's lifetime, in order to reduce unnecessary biopsies and treatment.
Consequently, clinical applications of prostate MRI have expanded to include, not only locoregional staging, but also tumor detection, localization (registration against an anatomical reference), characterization, risk stratification, surveillance, assessment of suspected recurrence, and image guidance for biopsy, surgery, focal therapy and radiation therapy.
In 2007, recognizing an important evolving role for MRI in assessment of prostate cancer, the AdMeTech Foundation organized the International Prostate MRI Working Group, which brought together key leaders of academic research and industry. Based on deliberations by this group, a research strategy was developed and a number of critical impediments to the widespread acceptance and use of MRI were identified. Amongst these was excessive variation in the performance, interpretation, and reporting of prostate MRI exams. A greater level of standardization and consistency was recommended in order to facilitate multi-center clinical evaluation and implementation.
In response, the European Society of Urogenital Radiology (ESUR) drafted guidelines, including a scoring system, for prostate MRI known as PI-RADS™ version 1 (PI-RADS™ v1). Since it was published in 2012, PI-RADS™ v1 has been validated in certain clinical and research scenarios.
However, experience has also revealed several limitations, in part due to rapid progress in the field. In an effort to make PI-RADS™ standardization more globally acceptable, the American College of Radiology (ACR), ESUR and the AdMeTech Foundation established a Steering Committee to build upon, update and improve upon the foundation of PI-RADS™ v1. This effort resulted in the development PI-RADS™ v2.
PI-RADS™ v2 was developed by members of the PI-RADS Steering Committee, several working groups with international representation, and administrative support from the ACR using the best available evidence and expert consensus opinion. It is designed to promote global standardization and diminish variation in the acquisition, interpretation, and reporting of prostate mpMRI examinations and it is intended to be a “living” document that will evolve as clinical experience and scientific data accrue. PI-RADS™ v2 needs to be tested and validated for specific research and clinical applications.
PI-RADS™ v2 is designed to improve detection, localization, characterization, and risk stratification in patients with suspected cancer in treatment naïve prostate glands. The overall objective is to improve outcomes for patients. The specific aims are to:
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Establish minimum acceptable technical parameters for prostate mpMRI
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Simplify and standardize the terminology and content of radiology reports
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Facilitate the use of MRI data for targeted biopsy
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Develop assessment categories that summarize levels of suspicion or risk and can be used to select patients for biopsies and management (e.g., observation strategy vs. immediate intervention)
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Enable data collection and outcome monitoring
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Educate radiologists on prostate MRI reporting and reduce variability in imaging interpretations
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Enhance interdisciplinary communications with referring clinicians
PI-RADS™ 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 surveillance, or the use of MRI for evaluation of other parts of the body (e.g. skeletal system) that may be involved with prostate cancer. Furthermore, it does not elucidate or prescribe optimal technical parameters; only those that should result in an acceptable mpMRI examination.
The PI-RADS Steering Committee strongly supports the continued development of promising MRI methodologies for assessment of prostate cancer and local staging (e.g., nodal metastases) utilizing novel and/or advanced research tools not included in PI-RADS™ v2, such as in-vivo MR spectroscopic imaging (MRSI), diffusion tensor imaging (DTI), diffusional kurtosis imaging (DKI), multiple b-value assessment of fractional ADC, intravoxel incoherent motion (IVIM), blood oxygenation level dependent (BOLD) imaging, intravenous ultra-small superparamagnetic iron oxide (USPIO) agents, and MR-PET. Consideration will be given to incorporating them into future versions of PI-RADS™ as relevant data and experience become available.
Section snippets
Timing of MRI Following Prostate Biopsy
Hemorrhage, manifested as hyperintense signal on T1W, may be present in the prostate gland, most commonly the peripheral zone (PZ) and seminal vesicles, following systematic transrectal ultrasound-guided systematic (TRUS) biopsy and may confound mpMRI assessment. When there is evidence of hemorrhage in the PZ on MR images, consideration may be given to postponing the MRI examination until a later date when hemorrhage has resolved. However, this may not always be feasible or necessary, and
Normal Anatomy (Figure 1)
From superior to inferior, the prostate consists of the base (just below the urinary bladder), the midgland, and the apex. It is divided into four histologic zones: (a) the anterior fibromuscular stroma, contains no glandular tissue; (b) the transition zone (TZ), surrounding the urethra proximal to the verumontanum, contains 5% of the glandular tissue; (c) the central zone (CZ), surrounding the ejaculatory ducts, contains about 20% of the glandular tissue; and (d) the outer peripheral zone
Section III: Assessment and Reporting
A major objective of a prostate MRI exam is to identify and localize abnormalities that correspond to clinically significant prostate cancer, and mpMRI is able to detect intermediate to high grade cancers with volumes ≥0.5cc, depending on the location and background tissue within the prostate gland. However, there is no universal agreement of the definition of clinically significant prostate cancer.
In PI-RADS™ v2, the definition of clinically significant cancer is intended to standardize
T1-Weighted (T1W) and T2-Weighted (T2W)
Both T1W and T2W sequences should be obtained for all prostate MR exams. T1W images are used primarily to determine the presence of hemorrhage within the prostate and seminal vesicles and to delineate the outline of the gland. T1W images may also useful for detection of nodal and skeletal metastases, especially following intravenous administration of a gadolinium-based contrast agent (GBCA).
T2W images are used to discern prostatic zonal anatomy, assess abnormalities within the gland, and to
Section V: Staging
MRI is useful for determination of the T stage, either confined to the gland (≤T2 disease) or extending beyond the gland (≥T3 disease).
The apex of the prostate should be carefully inspected. When cancer involves the external urethral sphincter, there is surgical risk of cutting the sphincter, resulting in compromise of urinary competence. Tumor in this region may also have implications for radiation therapy.
High spatial resolution T2W imaging is required for accurate assessment of
Acknowledgements
Administration Mythreyi Chatfield American College of Radiology, Reston Steering Committee Jeffrey C. Weinreb: Co-Chair Yale School of Medicine, New Haven Jelle O. Barentsz: Co-Chair Radboudumc, Nijmegen Peter L. Choyke National Institutes of Health, Bethesda François Cornud René Descartes University, Paris Masoom A. Haider University of Toronto, Sunnybrook Health Sciences Ctr Katarzyna J. Macura Johns Hopkins University, Baltimore Daniel Margolis University of California, Los Angeles Mitchell D. Schnall
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These authors are co-first authors.