Objective: To assess the diagnostic performance of simplified PI-RADS (S-PI-RADS) with biparametric MRI (bpMRI) vs PI-RADS v2.1 with multiparametric MRI (mpMRI) in identifying lesion index (IL) in patients with multifocal prostate cancer (PCa)., The inter- and intra-observer variability was also determined with two readers. Materials and methods: Retrospective analysis of the prostate MRI of 59 patients with multifocal PCa (two or more tumors ascertained by TRUS/guided MRI targeted prostate biopsy). Two radiologists with experience in prostate MRI identified the IL in each patient according to S-PI-RADS-dedicated bpMRI. According to S-PI-RADS in multifocal intraprostatic lesions the IL was defined as the one with the greatest volume (when lesions with similar signal hypointensity on apparent diffusion coefficient (ADC) maps coexist), or as the lesion with marked hypointensity irrespective of volume (when lesions with moderate and marked signal hypointensity on ADC map coexist). Otherwise, according to PI-RADS v2.1 the IL is the one intraprostatic with the highest score. The agreement between the S-PI-RADS and PI-RADS v2.1 results and the histological data was expressed as a percentage of agreement or sensitivity (percentage of PCa or IL correctly identified on the total of PCas). The intra- and inter-observer agreement was evaluated with Cohen's kappa (k) coefficient. Results: The 133 lesions identified were compared with the biopsy results. Sensitivity in the detection of IL was 79.6–83% and 93.2–94.9% with the PI-RADS v2.1 and S-PI-RADS criteria, respectively, whereas the specificity was 83.8–86.5& and 94.6–96%. With both observers, the overall diagnostic accuracy was significantly higher using S-PI-RADS. Concerning the S-PI-RADS, the concordance between IL and tumor index (PCa with the highest Gleason score) was found in 56/59 patients (93%) both in the peripheral and transition zone. In the detection of the IL the agreement between the two readers was moderate (k = 0.701) for the PI-RADS v2.1 and good (k = 0.844) for the S-PI-RADS. A poor agreement between the two readers 0.38 (95% CI: 0.0653 to 0.693) and 0.41 (95% CI: 0.123 to 0.696) was revealed. Conclusions: In patients with multifocal PCa, S-PI-RADS-based bpMRI showed significantly higher performance in IL detection and good inter-observer agreement.
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