Modified clear cell likelihood score and a new CAT score in the assessment of indeterminate small renal masses

In recent decades, small renal masses (SRMs) have become common incidental findings in cross-sectional studies; however, a widely implemented approach for the characterization of SRMs is still lacking. Thus, in this study, we aimed to explore the diagnostic performance of existing algorithms and to propose - as a conceptual framework rather than a clinically verified tool - a new simple radiological scale for estimating the probability of malignancy in SRMs.

Patients with indeterminate solid SRMs (N = 50), discovered using magnetic resonance imaging (MRI) between 2012 and 2023 were included. In 38 cases, the final diagnosis was based on histopathology, while in 12 cases it relied on regression or lack of progression during follow-up. Modified versions of the clear cell likelihood score (ccLS) were calculated, and its diagnostic performance was assessed. Moreover, we analyzed the newly created score, which consisted of selected MRI and clinical features. All of our modified scales used a Likert score for the likelihood of clear cell renal cell carcinoma (ccRCC).

Based on the results of our statistical analyses, we modified the ccLS by adding T1 SI ratio < 0.73, arterial to delayed ratio (ADER) > 0.99, and smoking as independent predictors of ccRCC. We created a new scale, the CAT score, which combined hyperintensity in the Corticomedullary phase, ADER > 0.99, and TI SI ratio < 0.73 (with 1 point being assigned to each of the above-mentioned MRI parameters). In our results, the best diagnostic accuracy was observed for a CAT score ≥ 2, with a sensitivity of 73.9% (51.6-89.8%), a specificity of 77.8% (57.7-91.4%), and an accuracy of 76.0% (61.8-86.9%). Univariate logistic regression analyses demonstrated that all scales created by our group were significant predictors of ccRCC. Importantly, they showed a better predictive ability than the standard ccLS score.

The CAT score appears to improve the prediction of ccRCC compared with both standard and modified versions of the ccLS and may serve as a potential aid in the routine assessment of indeterminate SRMs. Nonetheless, this study should be regarded as a preliminary, proof-of-concept analysis rather than a definitive model-development study. The main limitation of our study is its small, single-center cohort, which limits the statistical power and robustness of model development. Moreover, a substantial proportion of benign diagnoses based only on radiological follow-up rather than histopathology. Therefore, before clinical implementation, the CAT score requires prospective validation in larger, independent, multicenter cohorts.