Our results suggest that the use of intravenous contrast media only improves the diagnostic accuracy of bladder cancer staging, using the VI-RADS system score, when less experienced readers assess small lesions.

In our study, we assumed that a VI-RADS score of only 4 indicates muscle-invasive disease. There were two reasons for this. First, the statistical analysis pointed to this threshold as the optimal point for better accuracy when determining MIBCa. Second, the decision to perform bladder resection should be based on a more specific approach.

Interestingly, despite the low number of cases in each category in our cohort, the false-negative rates were only 4.0 and 8.0% (cases classified as VI-RADS 1, 2 and 3) for the less and more experienced readers, respectively. However, the rates of false-positive cases were relatively high 28.0 and 20.0% for readers 1 and 2, respectively. Our results are aligned with previous studies [15, 17, 22], but slightly better performance was obtained for false-negative cases, although slightly worse results were obtained for false-positive cases [17, 23].

When comparing the accuracy of readers, the results of the less experienced one were consistently inferior to those of the more experienced reader throughout the different sequences and for the final score, but the difference was significant only when assessing small lesions with biparametric approach. One possible explanation for these results is that our study may be underpowered for showing significant difference in other scenarios. Or, biparametric approach may have similar accuracy for experienced readers, which would be not surprising as, in many situations, the diagnostic accuracy of an imaging method is clearly dependent on the experience of the reader [23]. Highly-specialized exams (for instance, prostate MRI) have a known learning curve for interpretation, and some organizations have suggested minimum requirements for independent reporting. However, this is an open issue for bladder cancer imaging [24].

We decided to perform a subgroup analysis assessing the diagnostic accuracy for small bladder lesions (≤ 3.0 cm) because the staging tends to be more complex in these cases. The difference in diagnostic accuracy of this limited sample was not significant, but compared to the whole group, there was a clear trend of lower accuracy for both reader 1 (84.0 vs. 79.2) and reader 2 (86.0 vs. 79.2) for the final mpVI-RADS score and for the bpVI-RADS, 79.6 vs. 75.0 (reader 1) and 85.7 vs. 79.2 (reader 2). This trend was even more conspicuous, although without statistical significance for the less experienced reader, which may indicate, depending on further studies that multiparametric approach should be preserved when radiologists with limited experience were scanning and interpreting MR studies, mainly if small lesions were detected on anatomical sequences. Once the situations where the use of contrast-enhanced sequences are clear, a more rational approach (including these two protocols) for bladder cancer staging would be feasible.

In the pre-VI-RADS years, some relevant studies have placed the contrast-enhanced sequences as essential for the best results on vesical cancer staging [10, 11]. However, recently, the role of contrast sequences has been questioned. Delle Pizzi et al. [19] in a multi-reader, a prospective study found similar accuracies for MRI protocol with and without contrast media, regardless of readers’ experience. Gmeiner et al. [20] pointed out the same direction, although in a retrospective study and using a different protocol than suggested in VI-RADS [12]. Our study also indicates the same direction. Possible explanations for this change in the perception of the value of contrast sequences include the improvement in MRI techniques, including the non-Gradient Echo DWI sequences, the motion-corrected high-resolution T2-weighted images, and the ascending learning curve of all radiologists as Bladder cancer staging is a much more requested exam now than a decade ago.

The VI-RADS scoring system was designed with a focus on the standardization of MR imaging for staging BCa. Based on a multisequence approach, the system also defines an algorithm to conduct cases using findings from different image acquisitions in a hierarchical approach [12]. By using findings from different sequences, one would expect a quite variable interpretation, but the literature has shown excellent reproducibility for the final assessment of the muscle layer by VI-RADS [17, 25, 26] ranging from 0.73 to 0.92. Our data are aligned with the literature in showing substantial agreement for final classification with an ICC of 0.81 (0.60–1.00), p < 0.0001, for all lesions and 0.82 (0.71–0.95), p < 0.0001 for lesions under 3.0 cm. On the other side, in our study, in contrast to others in literature [17, 25, 27], DCE showed almost perfect agreement, slightly higher than those observed for T2, DWI and the final VI-RADS score. This represents an interesting point, as DCE and DWI are the sequences that define the final scores when discrepant findings are present, according to the algorithm proposed by VI-RADS. And, based on it, we could argue that DCE can be preserved in specific situations (small lesions/less experienced readers) as a problem-solving sequence with a high level of reproducibility.

Our study does have some limitations that should be acknowledged. First, it is a retrospective study with all inherent risks of bias, although we carefully applied inclusion and exclusion criteria to minimize them. Second, we had a small cohort, which may limit the analysis of subgroups, such as some of the five categories of VI-RADS. The reason for the small number of patients in our sample was the high number of patients who had cystoscopy/TURB prior to MRI in the first year of our study, which reflects that urologist were not so familiar with VI-RADS at that time. Third, we performed all examinations with a 1.5-T scanner. However, the exclusive use of 3.0-T scanners has not been recommended by VI-RADS, and a recent meta-analysis showed comparable results between studies performed using 1.5 T and 3.0 T [28].

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