Renal cyst

Renal cysts are a common finding in the kidneys. Findings common to all "simple" renal cysts are well-marginated, thin walls with no enhancement of the cyst. They can be diagnosed on ultrasound, CT, or MRI. A cystic lesion in the kidney that deviates from the typical "simple" cyst appearance should be viewed with suspicion and may warrant further imaging to exclude other renal cystic lesions, particularly variants of renal cell carcinoma.

The frequency of renal cysts increases with age and they are present in approximately ~40% of all individuals receiving a CT scan 3. Incidental renal cysts are present in ~0.2% of paediatric patients 4.

  • uncomplicated renal cyst
    • well-marginated anechoic lesion with thin walls
    • a few thin septa may be present (5% of cysts)
    • the back wall should be visible
    • posterior acoustic enhancement may be present, although this finding is non-specific and also may not be seen with smaller cysts
    • a small amount of intracystic haemorrhage/debris may be present, and may require further evaluation (5% of cysts)
  • complicated cyst
    • cystic lesions with thickened or irregular walls or septa are suspicious for renal cell carcinoma and warrant further work up
    • vascularity of the septa on colour or spectral Doppler is suspicious for renal cell carcinoma

Anechoic renal cysts may show some artifactual internal low-level echoes. This may be improved by using harmonic imaging techniques.

Contrast-enhanced ultrasound may be useful to show vascularity of septa or nodular protuberances in a renal cyst, and can help differentiate a benign cyst from an indeterminate cyst or a malignant-appearing cyst 7.

See article: Bosniak classification of renal cysts

  • simple cyst (Bosniak I)
    • well-marginated
    • thin or imperceptible wall
    • water attenuation (<20 HU on noncontrast series)
    • nonenhancing
      • <10 HU increase from non-contrast to postcontrast series is non-enhancing
      • 10-20 HU increase is indeterminate
      • >20 HU increase is enhancement
      • be wary of pseudoenhancement artifact
  • hyperattenuating cyst (Bosniak II)
    • well marginated
    • thin or imperceptible wall
    • hyperattenuating (70-90 HU on noncontrast series)
    • nonenhancing
      • <10 HU increase from non-contrast to postcontrast series is non-enhancing
      • 10-20 HU increase is indeterminate
      • >20 HU increase is enhancement
      • be wary of pseudoenhancement artifact
  • increasing septation of the cyst, thick wall calcification, and wall/septa enhancement are concerning for renal cell carcinoma 

Simple cyst characteristics are similar to ultrasound and CT:

  • T1: hypointense (haemorrhagic debris may mildly increase signal)
  • T1 C+ (Gd): no postcontrast enhancement
  • T2: strongly hyperintense (hemorrhagic debris may mildly decrease signal) and separate from the collecting system
  • DWI: increased signal, but no restricted diffusion

MRI may help clarify possible haemorrhagic cysts on ultrasound and CT. It should show appropriate changes in cyst signal intensity (decreased T2, increased T1) and lack of enhancement.

Renal cystic lesions with postcontrast enhancement and/or restricted diffusion are suggestive of neoplasms and should be viewed with suspicion.

Simple renal cysts are overwhelmingly asymptomatic, although there have been occasional reports of symptomatic giant cysts causing pain from mass effect on adjacent structures. Cysts may occasionally rupture, causing pain.

A symptomatic renal cyst can be aspirated, but cysts have a high rate of recurrence. Percutaneous alcohol ablation has been practiced with some success in selected cases of symptomatic cysts 5.

In a paediatric patient with normal renal function, no follow up is necessary for an incidentally-discovered renal cyst 4.

  • the Bosniak classification should be used only with CT studies as evaluation of calcification is limited on MRI and evaluation of enhancement is limited on ultrasound
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Article information

rID: 32590
System: Urogenital
Synonyms or Alternate Spellings:
  • Renal cyst
  • Simple renal cyst
  • Simple renal cysts
  • Kidney cyst
  • Simple cortical renal cyst
  • Cortical renal cyst

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Cases and figures

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    Figure 1: type 1 Bosniak cyst
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    Case 1: CT
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    Case 2: MRI
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    Case 3: contrast-enhanced CT
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    Hyperdense renal ...
    Case 4: hyperdense renal cyst on CT
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    Hyperdense renal ...
    Case 4: US
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