To date, 10-12 core systematic transrectal ultrasound (TRUS) guided biopsy is the standard to diagnose prostate cancer (PCa). TRUS guided prostate biopsy is performed in men with raised prostate specific antigen (PSA) blood levels or an aberrant digital rectal examination. However, TRUS guided biopsy is not an appropriate tool to select patients suitable for focal therapy. Also, TRUS guided biopsy is associated with the under diagnosis of PCa and it is often incorrectly classifying the aggressiveness of the disease. Recently multiparametric magnetic resonance imaging (mpMRI) has been introduced. In the early years of mpMRI practice for detecting and localizing PCa, a variety of reporting standards were used. Now, reporting is more and more standardized by the use of the Prostate Imaging Reporting And Data System (PI-RADS). Recently, a second version of this classification was introduced. About a decade ago, interest started to rise among radiologists in using MRI for the detection, localization and characterization of PCa. Before that, MRI was primarily used for local-regional staging. The growing interest was motivated by the introduction of the functional MR imaging techniques which allowed for more accurate detection and localization of PCa. Evidently, no agreement or guideline for prostate reporting was available at that time, resulting in a variety of used reporting methods. The then used methods were often based on an ordinal scale. The easiest ordinal scale for oncologic imaging reporting is a binary scale allowing a radiologist to indicate whether a tumor is present or not. However, such a scale does not admit a radiologist to report equivocal findings. To overcome this lacuna, 3-point and 5-point scales were more used for mpMRI. Unfortunately, multiple problems arose with the varying scales used in prostate MR reading. Among scales, even in scales with an equal range, different interpretations were being practiced. For example, a score of 5 (in a 5-point scale) indicated “definitely cancer” in one system whereas it meant “likely malignant” in the other. Another important dilemma was formed by the weighting of the different parameters used in mpMRI. The question arose whether all parameters should contribute in an equal way to a sum score or whether one parameter should be more important than the other. Before PI-RADS, a great amount of uncertainty existed relating to those difficulties. To uniform and standardize prostate MR assessment, following the example of the BI-RADS system in breast cancer, a scoring system was introduced for PCa. PI-RADS version 2 was developed to further improve the accuracy and inter-observer agreement of the first version as that version was limited by variable interpretations. The other main novelties of PI-RADS v2 includes the use of a dominant sequence depending of the location of a lesion (PZ or TZ) and the employment of an overall score estimated from the individual scores of the used sequences. In the appraisal of a lesion, first the location of the lesion has to be selected. This determines the dominant sequence of the overall PI-RADS score. Mainly, the overall PI-RADS score of a lesion located in the PZ follows the score of that lesion as determined with DWI. Only in case of a PI-RADS 3 score, a positive score on DCE can upgrade the score to PI-RADS 4. T2-weighted imaging plays a minor role in lesions located in the PZ. The dominant sequence for lesions located in the TZ is T2-weighted imaging. Now, DWI can upgrade the overall PI-RADS score in case of an equivocal finding (PI-RADS 3) on T2-weighted imaging: a PI-RADS 5 on DWI may upgrade a PI-RADS 3 lesion to PI-RADS 4 . As the functional imaging techniques are susceptible to artifacts, it is not always possible to obtain adequate DWI or DCE data. The other modalities will then play a more important role in the final PI-RADS score. In such a case, T2-weighted imaging is the dominant sequence and depending of which functional imaging technique is missing, a PI-RADS 3 score may be upgraded to PI-RADS 4. The PI-RADS score provides an effective framework for determining the likelihood of prostate cancer on MRI. The DWI PI-RADS score correlates well with the presence of peripheral zone tumor on targeted biopsy, but not with transition zone tumors. By using PIRADS version 2, general body radiologists and prostate specialists can detect high-grade index prostate cancer lesions with high sensitivity and agreement. However, experienced radiologists achieved moderate reproducibility for PI-RADS version 2, and neither required nor benefitted from a training session.

Acknowledgements

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References

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