Synopsis

Reporting prostate MRI may be complex and prone to subjective interpretation. In 2012 the Prostate Imaging Reporting and Data System (PI-RADS) was introduced. The aims of PI-RADS were to provide minimal requirements for high-quality image acquisition, to standardize reporting, to simplify interpretation, to reduce inter-observer variability and to improve the accuracy of detecting clinically significant prostate cancer. In this lecture it is demonstrated that PI-RADS does not (yet) achieve these goals successfully and that other prostate MRI scoring systems show similar diagnostic performance. PI-RADS has its merits but requires further refinements.

Introduction: Reporting prostate MRI may be complex and prone to subjective interpretation. In 2012 the ESUR (European Society of Urogenital Radiology) guidelines on multiparametric prostate MRI (mpMRI) were published and the Prostate Imaging Reporting and Data System (PI-RADS) was introduced. In 2016 this system was adopted by the ACR (American College of Radiology) and an updated version of PI-RADS (PI-RADSv2) was developed. The objectives of PI-RADS were to provide minimal requirements for high-quality image acquisition, to standardize reporting, to simplify interpretation, to reduce inter-observer variability and to improve the accuracy of detecting clinically significant prostate cancer. In this lecture it is demonstrated that PI-RADS did not (yet) achieve these goals successfully.

Provide minimal requirements for high-quality image acquisition: In the PI-RADS guidelines Dynamic Contrast Enhanced Imaging (DCE) is recommended in all patients although its added value over T2-Weighted Imaging (T2-WI) and Diffusion Weighted Imaging (DWI) seems currently to be limited.

Standardization of reporting: PI-RADSv2 provides explicit criteria to score each sequence and provides consistent instructions to determine the overall assessment category on a scale from 1 to 5. An overall assessment score of 3 indicates that the findings are indeterminate but clarification of the cutoff value is needed for different clinical scenarios because in the end the clinician needs a binary recommendation for further diagnostic procedures (i.e. to biopsy or not).

Simplify interpretation: Characterization of lesions with mpMRI remains difficult because the different pulse sequences may yield discordant results. The dominant sequence approach which is proposed in PI-RADSv2 may be an oversimplification because all sequences should be taken into account when interrogating a lesion. A possible method of improvement could be to define optimal weighting factors for each sequence.

Reduce inter-observer variability: The inter-observer variability of PI-RADS has been evaluated in several studies but appears to be rather moderate.

Improve the diagnostic accuracy of prostate MRI: PI-RADS aims to detect or exclude clinically significant prostate cancers but prostate MRI has its inherent limitations because small tumors may be missed (even if they are high grade) and benign mimickers may cause false positive results. Moreover the PI-RADS score is purely MRI derived although clinical factors such as age and comorbidity should be taken into account to determine the clinical significance of a prostate cancer.

Alternative scoring systems: many other scoring systems for prostate MRI have been described in the literature, such as the Likert scale (a subjective 5-point scoring system dependent on the radiologist’s overall impression), MLS (a 13-level score based on Morphology, Location and Signal intensity of a lesion), or local institutional scoring systems of e.g. Pinto (in which each parameter is categorized positive or negative in binary fashion), Rastinehad (SQS score) and De Visschere (PI-RADSv2Alt, based on DWI instead of DCE for upgrading T2-WI in the PZ). Reports of direct comparisons of these alternative scoring systems show similar diagnostic performance as PI-RADS.

Conclusion: PI-RADSv2 has its merits but does not (yet) achieve its goals and further refinements are required.

Acknowledgements

References

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