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Renal Cysts: Definitions, Diagnosis and Treatment

Definitions of Renal Cysts

Simple Renal Cyst

A simple renal cyst is a solitary or multiple, unilateral or bilateral, round lesion of the kidney filled with clear fluid, without communication with the pyelocalyceal system and without suspicion of malignancy.

Parapelvic Renal Cyst

A parapelvic renal cyst is a renal cyst located adjacent to the renal pelvis.

Peripelvic Sinus Cysts

Peripelvic sinus cysts are usually multiple and often bilateral dilatations of lymphatic vessels in the renal sinus. Because they lack an epithelial lining, they are not true renal cysts.

Complicated Renal Cyst

A complicated renal cyst is a cystic renal lesion with at least one atypical feature that does not permit secure classification as benign. Examples include septa, wall thickening, calcifications, hemorrhagic or proteinaceous contents, contrast enhancement, or nodular components.

Bilateral parapelvic renal cysts: Abdominal CT images on the left and center and ultrasound of the same patient on the right. Multiple cysts in the renal sinus cause elongation of the calyceal system without hydronephrosis. On ultrasound, parapelvic renal cysts can easily be misinterpreted as hydronephrosis. CT images with kind permission, Prof. Dr. K. Bohndorf, Augsburg.

Epidemiology

The prevalence rate of kidney cysts increases with age: 20% at 40 years and 33% at 60 years of age.

Etiology:

Renal cysts are cystic dilatations arising from a segment of the nephron. Typical multiple peripelvic sinus cysts arise from lymphatic vessels, often after nephrolithiasis or urinary obstruction.

Pathology:

Simple renal cysts have a thin fibrous wall lined with simple flat to cuboidal epithelium.

Signs and Symptoms

Renal cysts are usually asymptomatic; otherwise, the following symptoms are rare:

• Flank pain or abdominal symptoms caused by displacement
• Hematuria due to rupture into the collecting system
• Arterial hypertension due to compression of the renal artery or one of its branches
• Hydronephrosis due to compression of the collecting system
• Infection with fever and flank pain

Diagnosis

The aim of imaging is to reliably distinguish a simple benign renal cyst from a complex cystic renal mass with a potential risk of malignancy.

Ultrasound Imaging

Criteria for a simple renal cyst are:

• Anechoic contents without septa
• No thickening of the round to oval cyst wall and no calcifications
• Posterior acoustic enhancement and lateral edge shadowing

If ultrasound findings are atypical, contrast-enhanced CT or MRI is indicated.

Computed Tomography

Abdominal CT is indicated for suspicious ultrasound findings such as septa, clusters of cysts, wall thickening or calcifications, or echogenic cyst contents. The morphologic criteria for a simple renal cyst on CT are similar to those on ultrasound. Density is usually −10 to 20 HU; however, hyperdense simple cysts with 20–90 HU also occur. Absence of enhancement of the cyst wall and cyst contents after contrast administration is important. Calcifications are not necessarily suspicious.

Bosniak Classification of Cystic Renal Masses (Israel and Bosniak, 2005):

• Bosniak I: Clearly benign cyst. No risk of malignancy
• Bosniak II: Few thin septa (<1 mm), thin calcifications, or well-circumscribed hyperdense lesions without contrast enhancement and smaller than 3 cm. Risk of malignancy 0 %; follow-up is unnecessary
• Bosniak IIF: Cysts that cannot be clearly assigned to Bosniak II or III and therefore require follow-up; the risk of malignancy is 5 %
• Bosniak III: Cyst wall or septa more than 1 mm thick with contrast enhancement. The risk of malignancy is approximately 50 %
• Bosniak IV: Cystic renal mass with enhancing solid components. The risk of malignancy is 75–90 %

CT of renal cysts: Bilateral uncomplicated renal cysts on the left. On the right, a renal cyst with borderline wall thickening and small calcifications (Bosniak IIF). With kind permission, Dr. G. Antes, Kempten.

Contrast-Enhanced Ultrasound:

Contrast-enhanced ultrasound can improve the diagnostic value of ultrasound examination, although it is operator-dependent, and may reduce the need for CT in patients with suspicious ultrasound findings.

Abdominal MRI:

MRI is a good alternative to CT, especially for repeated follow-up of suspicious lesions and in patients with contraindications to iodinated contrast medium. MRI avoids the high radiation exposure of multiphase CT and provides higher soft-tissue contrast with better differentiation of hemorrhagic or lipomatous components. Patient cooperation is important to avoid artifacts caused by respiratory motion of the kidneys.

Treatment of Simple Renal Cysts

Asymptomatic simple renal cysts do not require treatment or routine follow-up. Indications for treatment are symptoms, urinary obstruction, recurrent hemorrhage, infection, or, rarely, treatment-refractory hypertension with a plausible causal relationship.

Percutaneous Renal Cyst Puncture

To establish the diagnosis of a symptomatic renal cyst, symptoms should disappear for at least some time after diagnostic puncture and aspiration. A small proportion of patients are cured by a single puncture and aspiration. Cytology and creatinine measurement in the aspirate are not mandatory for an unequivocally simple cyst. If the cyst and symptoms recur, definitive treatment with percutaneous cyst sclerotherapy or laparoscopic cyst deroofing is required.

Percutaneous Renal Cyst Sclerotherapy

Percutaneous aspiration followed by sclerotherapy is an established minimally invasive treatment for symptomatic simple renal cysts. Under ultrasound guidance, a thin nephrostomy tube is placed in the renal cyst. The cyst contents are drained and the volume is measured. Instillation of contrast medium confirms the diagnosis; contrast medium must not opacify the collecting system. Creatinine concentration may optionally be determined and cytologic examination performed.

The most commonly used sclerosant is ethanol (99 %). The dose is 20 % of the cyst volume, to a maximum of 100 ml, and is left in place for 90–120 min. The longer the alcohol remains in contact with the cyst, the lower the recurrence rate. Polidocanol is an alternative. Recurrence occurs in approximately 10–30 % of cases. Treatment can be repeated.

Laparoscopic Renal Cyst Marsupialization

Laparoscopic or retroperitoneoscopic marsupialization is particularly useful for large, recurrent, or unfavorably located symptomatic simple renal cysts. Compared with percutaneous aspiration and sclerotherapy, it has a lower recurrence rate but is more invasive. Laparoscopic or retroperitoneoscopic exposure of the kidney is selected according to cyst location. Identify the ureter when it is anatomically close to the cyst. The cyst roof is removed close to the renal parenchyma, while the cyst base remains in situ. The cyst wall is sutured for hemostasis and/or treated with electrocautery.

Marsupialization (deroofing) of a renal cyst: schematic drawing.

Prognosis of Simple Renal Cysts

Simple renal cysts often show slow growth over years. An increase in size alone does not prove malignancy; morphologic criteria, and particularly newly developing or increasing complexity according to Bosniak classification, are decisive for risk assessment.

References

Israel, G. M. & Bosniak, M. A.
An update of the Bosniak renal cyst classification system.
Urology, 2005, 66, 484-488

  Deutsche Version: Nierenzyste

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