Diagnostic Performance and Interreader Agreement of the MRI Clear Cell Likelihood Score for Characterization of cT1a and cT1b Solid Renal Masses: An External Validation Study.

Background: The clear-cell likelihood score (ccLS) has been proposed for the noninvasive differentiation of clear-cell RCC (ccRCC) from other renal neoplasms on multiparametric MRI, though further external validation remains needed. Objective: To evaluate the diagnostic performance and interreader agreement of the ccLS version 2.0 (v2.0) for characterizing solid renal masses as ccRCC. Methods: This retrospective study included 102 patients (67 men, 35 women; mean age, 56.9 ± 12.8 years) who underwent multiparametric MRI between January 2013 and February 2018, demonstrating a total of 108 (≥25% enhancing tissue) solid renal masses measuring ≤7 cm [83 cT1a (≤4 cm), 25 cT1b (>4 cm and ≤7 cm)], all with a histologic diagnosis. Three abdominal radiologists independently reviewed the MRI examinations using ccLS v2.0. Median reader sensitivity, specificity, and accuracy were computed for predicting ccRCC by ccLS ≥4, and individual reader AUCs were derived. The percentage of masses that were ccRCC was calculated, stratified by ccLS. Interobserver agreement was assessed by the Fleiss' kappa statistic. Results: The sample included 45 ccRCCs (34 cT1a, 11 cT1b), 30 papillary renal cell carcinomas (RCCs), 13 chromophobe RCCs, 14 oncocytomas and 6 fat-poor angiomyolipomas. Median reader sensitivity, specificity and accuracy for predicting ccRCC by ccLS ≥4 were 85%, 82%, and 83% among cT1a masses, and 82%, 100%, and 92% among cT1b masses. The three readers' AUCs for predicting ccRCC by ccLS for cT1a masses were 0.90, 0.84, and 0.89, and for cT1b masses were 0.99, 0.97, and 0.92. Across readers, the percentage of masses that were ccRCC among cT1a masses was 0%, 0%, 20%, 68%, and 93% for ccLS of 1, 2, 3, 4, and 5, respectively; and among cT1b masses was 0%, 0%, 32%, 90%, and 100% for ccLS of 1, 2, 3, 4, and 5, respectively. Interobserver agreement among cT1a and cT1b masses for ccLS ≥4 was 0.82 and 0.83, and for ccLS 1-5 overall was 0.65 and 0.61, respectively. Conclusion: This study provides external validation of the ccLS, finding overall high measures of diagnostic performance and interreader agreement. Clinical Impact: The ccLS provides a standardized approach to the noninvasive diagnosis of ccRCC by MRI.