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 diffusionweighted imaging (DWI), dynamic contrast-enhanced (DCE) MRI, and sometimes other techniques such as invivo MR spectroscopy. These technologic advances, combined with a growing reader 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 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. The AdMeTech Foundation International Prostate MRI Working Group recommended development of PI-RADS (Prostate Imaging Reporting and Data System) in May 2010.
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, PIRADS 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 widely 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 PIRADS v1. This effort resulted in the development of 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 and/or staging in patients with clinically diagnosed disease. The overall objective is to improve outcomes for patients. The specific aims are to:
• Establish acceptable technical parameters for prostate mpMRI
• Simplify and standardize the terminology and content of radiology reports
• Facilitate the use of MRI data for targeted biopsy
• 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 treatment)
• Enable data collection and outcome monitoring
• Educate radiologists on prostate MRI reporting and reduce variability in imaging interpretations
• 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.