Renal cysts

Last updated: October 5, 2022

Summarytoggle arrow icon

Renal cysts are sacs of fluid that form in the kidneys, either as a single cyst or multiple cysts. They are categorized as simple or complex. Simple cysts are the most common renal masses, consisting of a thin wall surrounding water-like fluid, and are usually asymptomatic. Complex cysts are septated and/or enhanced on imaging, with solid material inside. They may manifest with pain, fever, and hematuria, and should raise suspicion of malignancy. Cysts can be diagnosed via imaging such as ultrasound, CT scan, or MRI, and are usually incidental findings. In most cases, treatment is unnecessary. Indications for surgery include symptomatic cysts or cysts that carry a risk of complications. Complex cysts require follow-up and must be removed if renal cell carcinoma is suspected. Possible complications include cyst rupture, infection, and compression of an adjacent structure (e.g., the ureter).

Definitiontoggle arrow icon

  • Sacs of fluid that form in the kidneys
  • Solitary or multiple renal cysts

Overviewtoggle arrow icon

Overview of renal cysts
Simple cysts Complex cysts
  • Very common in the general population
  • Most common renal masses [1]
  • Thin wall and only water-like fluid inside
  • Should raise suspicion of kidney cancer until proven otherwise
  • Thicker wall and solid material inside
Clinical features
  • Usually asymptomatic
  • In the case of enlargement:
    • Dull pain (back, side, or upper abdomen)
    • Changes in urinary habits (e.g., impaired urinary flow)
  • Septated and/or enhanced
  • Internal echoes (indicating solid material inside the cyst)
  • Poorly delimited/thickened cyst walls

Classificationtoggle arrow icon

Bosniak classification of renal cysts [2][3]
Class Findings on contrast-enhanced CT scan Likelihood of malignancy Management
  • Very low
  • No follow-up required
  • Mostly thin wall, few septa, fine calcifications in the wall or septa
  • Nonenhancing lesion
  • Low
  • Small (< 3 cm) homogeneously hyperdense simple cyst (thin wall, no septa, no solid components)
  • Nonenhancing lesion


  • Not clearly categorized as class II or III
    • Many thin septa (enhancing or nonenhancing)
    • Coarse calcifications in wall or septa without clear enhancement
    • Large (> 3 cm) homogeneously hyperdense nonenhancing cyst
  • Unclear significance
  • Follow-up to assess for changes


  • High


  • Clearly defined soft lesions in wall or septa
  • Enhancing cyst
  • Very high

Treatmenttoggle arrow icon

  • Not indicated in most cases
  • Indication for surgery: symptomatic cysts (e.g., impaired urinary flow, pain, bleeding) or risk of further complications (e.g., infection, malignancy)
  • Complex cysts require follow-up and must be removed if renal cell carcinoma is suspected (e.g., due to changes in echogenicity or shape).

Complicationstoggle arrow icon

Complications are rare.

  • Cyst rupture
  • Cyst infection
  • Compression of adjacent tissue (e.g., of the ureter causing impaired urinary flow)

We list the most important complications. The selection is not exhaustive.

Referencestoggle arrow icon

  1. Yu Y, Ma L, Wang Z, Zhang Z. Renal cell carcinoma presenting as a simple renal cyst: A case report. Molecular and Clinical Oncology. 2017; 6 (4): p.550-552.doi: 10.3892/mco.2017.1173 . | Open in Read by QxMD
  2. Muglia VF, Westphalen AC. Bosniak classification for complex renal cysts: history and critical analysis.. Radiologia brasileira. 2014; 47 (6): p.368-73.doi: 10.1590/0100-3984.2013.1797 . | Open in Read by QxMD
  3. Silverman SG, Pedrosa I, Ellis JH, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. Radiology. 2019; 292 (2): p.475-488.doi: 10.1148/radiol.2019182646 . | Open in Read by QxMD

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