Bosniak Classification Version 2019: A CT-Based Update for Radiologists

Purpose of Review To review the recent revision of the Bosniak classification: version 2019 (v2019), with emphasis on CT, comparing v2019 to the original Bosniak classification (last updated in 2005) and discuss why this revision was needed and is clinically useful. To present an approach to apply v2019 in practice, reviewing available validation studies evaluating inter- and intra-observer agreement, diagnostic test accuracy, and intermodality comparisons between CT (including dual-energy CT), MRI, and Ultrasound. Recent Findings After the 2019 proposal revising the Bosniak classification of Cystic Renal masses, data have emerged, which support use of the revised system. These include higher prevalence of malignancy in class III and IV cystic masses and slightly improved interobserver agreement with less discrepancy between class assignment comparing CT and MRI. However, studies have shown limitations of v2019 which include high dispersion of agreement regarding wall or septa feature (e.g., irregularity versus nodule), a higher proportion of malignancy in class IIF and persistent upgrading of class comparing MRI to CT. Summary Overall, the recent Bosniak v2019 revision has advanced the field of cystic renal mass imaging by achieving a consensus for terminologies and definitions, slightly improving interobserver agreement (with the opportunity for future iterations to improve upon these preliminary results) and increasing specificity of diagnosis of malignancy in higher Bosniak v2019 classes without impacting sensitivity. Future work is needed to simplify and potentially improve performance of the system and accommodate emerging techniques such as dual-energy CT and artificial intelligence.