Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment

Authors: Stuart G. Silverman*, Ivan Pedrosa, James H. Ellis, Nicole M. Hindman, Nicola Schieda, Andrew D. Smith, Erick M. Remer, Atul B. Shinagare, Nicole E. Curci, Steven S. Raman, Shane A. Wells, Samuel D. Kaffenberger, Zhen J. Wang, Hersh Chandarana, Matth

Cystic renal cell carcinoma (RCC) is almost certainly overdiagnosed and overtreated. Efforts to diagnose and treat RCC at a curable stage result in many benign neoplasms and indolent cancers being resected without clear benefit. This is especially true for cystic masses, which compared with solid masses are more likely to be benign and, when malignant, less aggressive. For more than 30 years, the Bosniak classification has been used to stratify the risk of malignancy in cystic renal masses. Although it is widely used and still effective, the classification does not formally incorporate masses identified at MRI or US or masses that are incompletely characterized but are highly likely to be benign, and it is affected by interreader variability and variable reported malignancy rates. The Bosniak classification system cannot fully differentiate aggressive from indolent cancers and results in many benign masses being resected. This proposed update to the Bosniak classification addresses some of these shortcomings. The primary modifications incorporate MRI, establish definitions for previously vague imaging terms, and enable a greater proportion of masses to enter lower-risk classes. Although the update will require validation, it aims to expand the number of cystic masses to which the Bosniak classification can be applied while improving its precision and accuracy for the likelihood of cancer in each class.

© RSNA, 2019

It is hoped that this review of the current Bosniak classification and the proposed update will enhance the classification’s clarity and specificity, be useful in clinical practice, serve as substrate for future study, and lead to future updates that emphasize the clinical significance of the cancers the classification aims to diagnose.

Key Points
■The updated Bosniak classification of cystic renal masses (Bosniak Classification, version 2019) aims to improve the original classification’s ability to predict the likelihood of malignancy in a cystic renal mass.

â– The principal goals of the update are to (a) address data indicating renal cell carcinoma with predominant cystic change is overdiagnosed and overtreated, (b) reduce interreader variability, (c) improve the precision of reported malignancy rates within each Bosniak class, and (d) minimize the number of benign masses undergoing unnecessary treatment by improving specificity (reducing procedural morbidity, loss of renal function, and cost).

â– The Bosniak Classification, version 2019 (a) formally incorporates MRI into the classification, (b) includes specific definitions for individual imaging features and Bosniak classes, (c) incorporates a larger proportion of renal masses encountered in clinical practice (eg, incompletely characterized but highly likely benign cysts), and (d) enables a greater proportion of masses to be placed into lower Bosniak classes.