# Bosniak classification of cystic kidney masses

Source: https://onco.cc/terms/bosniak-classification/  
OnCo record `bosniak-classification` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

The Bosniak class on a CT or MRI report says how likely a fluid-filled lump in the kidney is to be cancer, which decides between leaving it, watching it and removing it.

## Summary

For more than 30 years the Bosniak classification has graded the risk of cancer in cystic kidney masses. Cystic masses are more often benign than solid ones and, when malignant, less aggressive, and the 2019 update proposal (Silverman and colleagues, Radiology 2019) starts from the observation that cystic kidney cancer is overdiagnosed and overtreated, with many benign masses removed without clear benefit.

Version 2019 formally brings in MRI, defines imaging terms that had been vague, and lets more masses enter the lower-risk classes. The authors state it still needs validation and cannot fully separate aggressive from indolent cancers.

## Fields

- Kind: Term
- Last checked: 2026-11-04
- Also known as: Bosniak classification; Bosniak category; Bosniak IIF; Bosniak III; Bosniak IV; Bosniak 2019

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Bosniak_classification
- Silverman et al., Bosniak classification of cystic renal masses, version 2019: an update proposal and needs assessment (Radiology 2019): https://doi.org/10.1148/radiol.2019182646

## Connected records

- technologies: [CT (computed tomography)](https://onco.cc/technologies/ct/), [MRI](https://onco.cc/technologies/mri/), [Partial nephrectomy, ablation & active surveillance of small renal masses](https://onco.cc/technologies/partial-nephrectomy-active-surveillance/)
- cancers: [Renal cell carcinoma](https://onco.cc/cancers/rcc/)
- terms: [PI-RADS (Prostate Imaging Reporting and Data System)](https://onco.cc/terms/pi-rads/)

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