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

Abstract

<b>BACKGROUND.</b> 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. <b>OBJECTIVE.</b> 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. <b>EVIDENCE ACQUISITION.</b> 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. <b>EVIDENCE SYNTHESIS.</b> 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% (<i>p</i> = .13); III, 74% versus 77% (<i>p</i> = .72); and IV, 79% versus 84% (<i>p</i> = .22). <b>CONCLUSION.</b> 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. <b>CLINICAL IMPACT.</b> The results improve understanding of imaging-based cystic renal-mass classification and may inform development of future renal-mass classification systems. <b>TRIAL REGISTRATION.</b> PROSPERO (International Prospective Register of Systematic Reviews) CRD42023472140.