Renal cysts

Last updated: October 5, 2022

Summary

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).

Definition

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

Overview

Overview of renal cysts
	Simple cysts	Complex cysts
Description	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)	Fever Pain Hematuria
Diagnostics	Usually a benign, incidental finding during CT or ultrasound examinations Round or oval anechoic cysts (filled with ultrafiltrate) Dorsal sound amplification Minor reduction in sound wave intensity in homogeneous, clear liquids as there is no refraction (i.e., the content of the cyst is anechoic) On the dorsal side of the cysts, the undiminished sound wave results in higher contrast compared to the neighboring tissue (i.e., dorsal sound amplification).	Septated and/or enhanced Internal echoes (indicating solid material inside the cyst) Poorly delimited/thickened cyst walls

Renal cyst Complex renal cyst

Classification

Bosniak classification of renal cysts ^[2]^[3]
Class	Findings on contrast-enhanced CT scan	Likelihood of malignancy	Management
I	Simple cyst: hypodense, thin wall, no septa, no solid components No contrast enhancement	Very low	No follow-up required
II	Mostly thin wall, few septa, fine calcifications in the wall or septa Nonenhancing lesion	Low
II	Small (< 3 cm) homogeneously hyperdense simple cyst (thin wall, no septa, no solid components) Nonenhancing lesion	Low
II-F	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
III	Irregular and/or thick septa and wall segments, coarse calcifications Contrast enhancement	High	Surgery is recommended.
IV	Clearly defined soft lesions in wall or septa Enhancing cyst	Very high	Surgery is recommended.

Bosniak classification of cystic renal masses

Treatment

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).

Complications

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.

References

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
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
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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