# Cystic Renal Masses: Use Bosniak Features in Context

First establish the correct imaging category; then choose surveillance or intervention using tumor features and the person’s competing risks.

![Infographic: Cystic Renal Masses: Use Bosniak Features in Context](https://urinenephrology.org/visual-reference/images/bosniak.png?v=20261003c)

## Characterize the enhancing tissue

Use a renal-mass CT or MRI protocol and ask the radiologist to specify Bosniak version 2019. The category depends on enhancing walls, septa, or nodules—not size alone. Ultrasound showing a complex cyst usually requires further characterization.

## I and II: usually no surveillance

Class I is a simple cyst; II includes specified minimally complex benign patterns. When confidently classified, neither generally needs follow-up. Treat symptoms if attributable to the cyst. Thick calcification can obscure enhancement and may require MRI before declaring a lesion benign.

## IIF: define the follow-up clock

IIF includes specified enhancing septal or mild wall complexity without class III/IV features. CUA guidance suggests imaging at 6–12 months in the first year, then annually if stable, generally for 5 years. New enhancing complexity matters more than a small isolated size change.

## III versus IV

Class III has thick or irregular enhancing walls/septa without an enhancing nodule; IV includes enhancing nodules. Neither category alone dictates urgent surgery in every patient. Confirm the imaging interpretation, lesion size, comorbidity, renal reserve, and preference with urology.

## Surveillance can be an active choice

For predominantly cystic small lesions, especially ≤2 cm, surveillance may be appropriate. The 2023 CUA guideline allows surveillance or surgery for III/IV lesions 2–4 cm. These are conditional recommendations; record imaging intervals, clinician ownership, and triggers to reconsider treatment.

## Avoid false percentages and shortcuts

Surgical series overrepresent concerning lesions, so their malignancy rates are not an individual risk calculator. Do not use a generic “less than 30 HU” rule without acquisition phase and homogeneity. If intervention is chosen, consider nephron preservation when feasible.

## Supporting evidence

- [Clinical evidence and guidance](https://pmc.ncbi.nlm.nih.gov/articles/PMC10263289/)
- [PubMed 34115531](https://pubmed.ncbi.nlm.nih.gov/34115531/)

## Source lessons

- [bosniak classification](https://urinenephrology.org/2025_UDPA_Lectures_Live/kidney-cysts-masses/cystic-diseases/bosniak-classification.html)
- [index](https://urinenephrology.org/2025_UDPA_Lectures_Live/kidney-cysts-masses/index.html)

Read alongside the full lessons; the findings and decisions shown here require the stated clinical context.