Renal cysts Q61.3

Hippocrates first described what was probably a polycystic kidney disease as early as 460 BCE (Ganten 2013). Konert (2004) names Felix Platter (1536 - 1614) as the first describer. Thomas Willis (1621 - 1675) was the first to describe the polycyst (Konert 2004). The first comprehensive description of a cyst was made in 1829 - 1835 by Cruveillhier (Ganten 2013).

A renal cyst is a unilateral or bilateral, solitary or multiple round cyst filled with amber-colored (Risler 2008), clear contents (Hegele 2015). It is usually located in the cortical region of the kidney (Keller 2010) and has no connection to the renal pelvicocaliceal system (Manski 2019).

Renal cysts are divided into

• -simple benign renal cysts
• complex potentially malignant renal cysts (Hegele 2015).

For morphological assessment and subsequent treatment planning (Lerchbaumer 2018), cystic space lesions are classified according to Bosniak into:

• Bosniak I: The cyst is clearly benign. There is no risk of malignancy.
• Bosniak II: The cyst is < 3 cm in size. There is mild septation, hyperdense lesions without contrast uptake, and thin calcifications. There is no risk of malignancy, and controls are not required.
• Bosniak IIF: These cysts are not clearly classified as group II or III. There is a 5% risk of malignancy. The F stands for "follow up": regular controls are required.
• Bosniak III: There are septa > 1 mm thick. The cyst wall shows irregular thickening with contrast enhancement. Primary indistinct densities are seen on CT. The cyst is not clearly benign, the risk of malignancy is 50%. Further diagnostic workup is absolutely necessary.
• Bosniak IV: The cyst appears suspicious for malignancy: It is sonographically not primarily recognizable as a cyst, shows a protruding solid process and / or a centrally disintegrating tumor. The risk of malignancy is 75 % - 90 %. Further diagnosis or surgical measures are urgently indicated (Manski 2019 / Kuhlmann 2015 / Bartels 2013).

The historical classification of cystic kidney disease according to Potter (Potter I - IV) is no longer in use (not to be confused with the Potter sequence, which describes characteristic phenotypic changes (Deeg 2014; Manski 2019).

The simple renal cyst is one of the most common space-occupying lesions of the kidney. Men are affected twice as often as women. The incidence increases with age (Hegele 2015). In 60-year-olds, for example, about 33% have one or more simple renal cysts (Manski 2019).

The etiology of the simple renal cyst has not yet been clarified. It is suspected that there is a drainage disorder in the area of the nephrons, which leads to a bulging and ultimately to the formation of a cyst (Geiger 2003).

A renal cyst results from a cystic dilatation originating from a nephron segment (Manski 2019).

A simple renal cyst usually does not cause any symptoms. With very large cysts, patients sometimes complain of flank pain or abdominal displacement symptoms (Manski 2019).

The simple renal cyst is largely discovered as an incidental finding during sonography (Herold 2021). In the diagnosis, it is important to assign the cyst to a simple benign or a complex - potentially malignant - cyst (Manski 2019).

Sonography

Sonographic criteria for a simple renal cyst are according to Hegele 2015:

• no thickening of the round to oval cyst wall
• anechoic content of the cyst
• dorsal sonographic enhancement
• no septa detectable
• no calcifications

The kidney itself appears normal in size, and cysts may encompass up to 10 cm in diameter (Kuhlmann 2015). If internal echoes, calcifications or cyst conglomerates are presentable, a CT or MRI is recommended for further clarification (Keller 2010).

Computed tomography: A CT is indicated in case of an unclear sonographic finding, the simple renal cyst is not an indication (Keller 2010). Lack of contrast of the cyst wall and, after contrast administration, of the cyst contents are suspicious. The density of the simple renal cyst is -10 to 20 HU or between 20 - 90 HU for hyperdense simple cysts (Manski 2019). From Bosniak III onwards, primary indeterminate densities can be seen (Bartels 2013).

MRI: Since ionizing radiation can be avoided with MRI, it is more suitable for follow-up. There are no diagnostic advantages over CT (Manski 2019).

Haematuria: If the cyst ruptures into the hollow system, haematuria may occur (Manski 2019). There is usually no restriction of renal function (Herold 2021).

Histologically, there is a fibrous cyst wall lined with a simple cuboidal to flat epithelium (Manski 2019).

Multicystic dysplasia (MCDK) (Manski 2019).

• Renal abscess
• renalcell carcinoma (Risler 2008)
• cystic malignancy
• secondary renal cysts in chronic renal insufficiency especially under renal replacement therapy
• hematoma
• cystic dilatation of the calyces
• tuberculous cavern
• abscess
• echinococcus cyst
• dermoid cyst
• genetic cystic kidney disease (Herold 2021)

arterial hypertension in compression of the renal artery or one of its branches

haematuria in case of rupture of the cyst

hydronephrosis in case of compression of the hollow system or the ureter (Hegele 2015)

Infection of the cyst (although this is a rarity - Risler 2008).

The simple renal cyst usually does not require any therapy (Manski 2019).

However, if the renal cyst becomes symptomatic, puncture with aspiration of the contents and subsequent sclerotherapy can be performed to reduce the recurrence rate (Herold 2021). In the punctate, in addition to cytology, determination of creatinine is recommended (Manski 2019). The creatinine value is serum-identical in the simple cyst (Hegele 2015). Laparoscopic cyst ablation is also possible, but corresponds to the 2nd choice, as percutaneous sclerotherapy of the cyst shows good results (Manski 2019).

Surgical treatment in the form of tumor nephrectomy or partial kidney resection is required for cystic masses from Bosniak III onwards (Manski 2019).

The prognosis of the simple renal cyst is good. There is usually no loss of renal function (Herold 2021).

Approximately 25 % of cysts show an increase in size within 3 years.

After puncture with subsequent sclerotherapy, recurrences occur in 10 % - 30 % (Hegele 2015).

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