Bosniak Classification of Cystic Renal Masses Version 2019: Proportion of Malignancy by Class and Subclass—Systematic Review and Meta-Analysis

BACKGROUND. Bosniak classification version 2019 (v2019) was a major revision to version 2005 (v2005) that defined cystic renal mass subclasses on the basis of wall or septa features. OBJECTIVE. The purpose of the study was to determine the proportion of malignancy within cystic renal masses stratified by Bosniak classification v2019 class and feature-based subclass. EVIDENCE ACQUISITION. MEDLINE and Embase databases were searched on July 24, 2023, for studies published in 2019 or later that reported cystic renal masses that underwent renal-mass CT or MRI, were assessed using Bosniak classification v2019, and had a reference standard (histopathology indicating benignancy or malignancy or ≥ 5 years of imaging follow-up indicating benignancy). Study authors were contacted to provide subclass-stratified data. Pooled proportions of malignancy stratified by v2019 class and subclass were determined using meta-analysis. EVIDENCE SYNTHESIS. The analysis included 12 studies reporting 966 patients with 975 cystic masses. No class I mass was malignant. Pooled proportions of malignancy by class were as follows: II, 9% (95% CI: 5-17%); IIF, 26% (95% CI: 13-46%); III, 80% (95% CI: 71-87%); and IV, 88% (95% CI: 83-91%). Pooled proportions of malignancy by subclass were as follows: IIF with many smooth, thin septa, 10% (95% CI: 2-33%); IIF with minimal wall or septal thickening, 47% (95% CI: 18-77%); IIF with heterogeneous T1 hyperintensity, 26% (95% CI: 8-57%); III with a thick, smooth wall or septa, 78% (95% CI: 60-90%); III with obtuse protrusion(s) 3 mm or less, 84% (95% CI: 77-90%); IV with acute protrusion(s) of any size, 88% (95% CI: 80-93%); and IV with obtuse protrusion(s) 4 mm or greater, 86% (95% CI: 77-91%). The proportion of malignancy was 41% for IIF masses with histopathology reference versus 2% for IIF masses with imaging follow-up reference. In four studies performing intraindividual comparisons of v2005 versus v2019, the proportions of malignancy were as follows: class IIF, 24% versus 42% (p = .13); III, 74% versus 77% (p = .72); and IV, 79% versus 84% (p = .22). CONCLUSION. Bosniak IIF masses had higher malignancy rates when histopathology rather than imaging follow-up was the reference standard, indicating verification bias. All Bosniak III and IV subclasses had high malignancy rates. CLINICAL IMPACT. The results improve understanding of imaging-based cystic renal-mass classification and may inform development of future renal-mass classification systems. TRIAL REGISTRATION. PROSPERO (International Prospective Register of Systematic Reviews) CRD42023472140.