MRI enhancement characteristics of septa and walls improve risk stratification of Bosniak III cystic renal masses.

PURPOSE: To evaluate whether MRI characteristics of septa and walls-specifically enhancement patterns during the corticomedullary (CP), nephrographic (NP), and excretory (EP) phases-can improve malignancy risk stratification in Bosniak III cystic renal masses (CRMs).

METHODS AND MATERIALS: This single-center, retrospective study analyzed 120 patients with Bosniak III cystic renal masses who underwent renal MRI between January 2009 and December 2021. The cohort included two subcategories: III-WS (enhancing thick wall/septa ≥ 4 mm; n = 22) and III-OP (enhancing irregular wall/septa or convex protrusion ≤ 3 mm; n = 98). All lesions were confirmed either by histopathology (115 CRMs) or ≥ 5 year stability (5 CRMs). Four radiologists (2 senior, 2 junior), blinded to clinical and pathological data, independently assessed septal and wall enhancement (obvious vs. non-obvious) across CP, NP) and EP, a consensus was reached through discussion. Interobserver agreement was evaluated using Conger's kappa, while diagnostic performance (AUC, sensitivity, specificity, accuracy) of obvious enhancement was assessed via ROC analysis, with AUC comparisons performed using DeLong's test.

RESULTS

The study included 120 patients (mean age: 48 ± 11 years; 94 male), with 95 (79.2%) malignant and 25 (20.8%) benign lesions. Subclassification revealed 22 Bosniak III-WS (10 benign, 12 malignant) and 98 Bosniak III-OP lesions (15 benign, 83 malignant). Obvious enhancement of either septa or walls in corticomedullary phase significantly discriminated benign from malignant lesions across both Bosniak III subcategories and the entire cohort (all P < 0.001). For the entire cohort, this feature demonstrated high predictive power for malignancy, yielding an area under the curve (AUC) of 0.839 (95% CI 0.761-0.900), a sensitivity of 75.8% (95% CI 65.9-84.0%), and a specificity of 92.0% (95% CI 74.0-99.0%). Obvious enhancement of either septa or walls in corticomedullary phase suggested superior diagnostic performance, outperforming isolated septal (AUC = 0.797, 95% CI 0.725-0.869), wall (AUC = 0.717, 95% CI 0.653-0.781), and combined enhancements (AUC = 0.622, 95% CI 0.652-0.682) (all P < 0.05). Notably, all 10 benign III-WS lesions exhibited non-obvious enhancement, whereas obvious enhancement was present in 64 of 83 (77.1%) malignant III-OP lesions. Senior radiologists achieved substantially higher interobserver agreement than junior radiologists in CP assessments (κ = 0.78 vs. 0.54; P < 0.001).

CONCLUSION

Obvious CP enhancement is a significant radiological feature associated with malignancy. Bosniak III-OP lesions with obvious corticomedullary enhancement might warrant management similar to category IV lesions. Conversely, non-enhancing Bosniak III-WS lesions may represent a lower-risk category (such as IIF).