Renal cyst

Last revised by Mohammad Taghi Niknejad on 29 Nov 2022

Renal cyst is a generic term commonly used in description of any predominantly cystic renal lesion. The majority of parenchymal cystic lesions represent benign epithelial cysts; however, malignancy such as renal cell carcinoma may also present as a cystic lesion 8.

Renal cysts are usually evaluated by complexity: a "simple" cyst is confidently diagnosed as a benign finding, while complicated or complex lesions with enhancing components are suspicious for malignancy.

"Renal cyst" is sometimes used as a catch-all term in reference to epithelial cysts, renal sinus (parapelvic/peripelvic) cysts, or calyceal diverticula. This is likely because these have similar appearances on imaging, and distinguishing them is often clinically inconsequential.

The use of the terms complex and complicated to describe renal cysts is now contentious. Originally complex was felt to mean all cysts that were not simple. Complicated was for a cyst that had ruptured, bled or become infected. Therefore both simple and complex cysts could become complicated. However over time the distinction between complex and complicated cysts has become blurred, and experts in the field have suggested that the terms should be avoided 10.

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 pediatric 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 hemorrhage/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 color 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)
    • non-enhancing
      • <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 non-contrast series)
    • non-enhancing
      • <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 (hemorrhagic 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 hemorrhagic 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 practised with some success in selected cases of symptomatic cysts 5.

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

  • 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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Cases and figures

  • 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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  • Case 4: hyperdense renal cyst on CT
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  • Case 4: US
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  • Case 5
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  • Case 6: CEUS
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