Simple Renal Cysts: Benign Lesions of the Kidney
Definition of a Simple Renal Cyst
Simple renal cysts occur unilateral or bilateral, single or multiple. Renal cysts are usually circular, filled with clear fluid and have no connection to the pyelocaliceal system.
The prevalence rate of kidney cysts increases with age: 20% with 40 years, 33% with 60 years of age.
Etiology and Pathology of Kidney Cysts
Causes of simple renal cyst:
Simple renal cysts are based from a segment of the nephron
Pathology of a simple renal cyst:
Simple renal cysts have a fibrous cyst wall lined with a simple flat to cubical epithelium.
Signs and Symptoms
Kidney cysts usually do not cause any symptoms. Simple kidney cysts may cause complications like:
- Abdominal or lumbar pain: due to the displacement of abdominal organs, rupture or bleeding of the renal cyst
- Hematuria: due to rupture of a renal cyst into the pyelocaliceal system.
- Arterial hypertension: by compression of the renal artery.
- Hydronephrosis: due to compression of the ureter or renal pelvis.
Simple or Complex Kidney Cyst: Diagnosis
Most important with imaging of renal cysts is the classification into simple (benign) or complex (potential malign) renal cysts.
Sonographic criteria for a simple renal cyst are:
- Round lesion of the kidney with echo-free content
- No septa
- No thickening of the round to oval cyst wall, no calcifications
- Posterior acoustic enhancement
Computed Tomography (CT)
CT imaging is indicated for suspicious ultrasound findings such as septation, clustering of cysts, wall thickening or calcification or echogenic contents of the cyst.
The criteria for a simple renal cyst in CT are similar to the ultrasound (see above), the density is −10 to 20 HU. A simple hyperdense cyst with 20–90 HU is also possible. The most important criteria is no contrast media enhancement of the cyst or the cyst wall. Calcifications are not obligate suspect [fig simple renal cysts].
- Bosniak I: clearly benign cyst. No malignancy.
- Bosniak II: high density, harmless septation, little calcification. Frequent checks are necessary. The chance for malignancy is 0–5%.
- Bosniak IIF: this group subsumes cysts, which cannot be clearly assigned to Bosniak II or III. Close follow-up is necessary.
- Bosniak III: not clearly benign cyst with wall thickening, increased calcification or complex septation. Surgical excision is necessary, the chance for malignancy is 50%.
- Bosniak IV: cyst with clear malign criteria, partial nephrectomy or radical nephrectomy are necessary. The chance for malignancy is 75–90%.
CT of renal cysts: Photo left: bilateral simple renal cysts. Right: renal cyst with small calcifications in the cyst wall (Bosniak II). Used with permission, Dr. G. Antes, Kempten.
Magnetic Resonance Imaging (MRI)
MRI imaging has no diagnostic benefits compared to the combination of renal ultrasound imaging and CT. MRI has no routine role in the evaluation of renal cysts.
Treatment of Simple Renal Cysts
Percutaneous Puncture and Aspiration of Renal Cysts
Pain due to a kidney cyst should disappear after diagnostic puncture and aspiration, at least for a while. A small proportion of patients are cured by this unique puncture and aspiration. Simultaneously, the cyst fluid is examined (cytology and creatinine concentration). A simple renal cyst should not reveal malignant cells; the creatinine concentration is near the serum concentration. With the recurrence of the cyst (and symptoms), definitive treatment is done by a percutaneous or laparoscopic approach (see below).
Percutaneous Sclerotherapy of Renal Cysts
A thin nephrostomy tube is placed into the renal cyst with ultrasound control. As described above, the cyst volume and creatinine concentration is measured. The creatinine concentration should be near the serum concentration. A cytological investigation is necessary, if not done with the first puncture.
Before sclerotherapy, contrast media is injected via the nephrostomy tube into the cyst. The contrast media should not enter the pyelocaliceal system. The most commonly used sclerosant is ethanol (99%), the dosage is 20% of the cyst volume to a maximum of 100 ml. The alcohol is left 90–120 min in the cyst: the longer the alcohol contact (several fillings, contact time), the lower the recurrence rate. Alternatively, polidocanol can be used as a sclerosant. A relapse of the cyst occurs in approximately 10–30%. Percutaneous sclerotherapy can be repeated.
Laparoscopic Unroofing of Kidney Cysts
The kidney is approached with the laparoscopic or retroperitoneoscopic technique. If the cyst is in anatomic proximity to the ureter, the ureter has to be identified and separated from the cyst wall. The cyst wall is circularly incised near the renal parenchyma (unroofing), the base of the cyst remains in situ. The cyst roof is sent for pathology. Hemostasis is achieved with bipolar cauterization of the remaining cyst wall. Due to the good results of percutaneous sclerotherapy, laparoscopic cyst removal is the treatment of second choice.
Prognosis of Simple Kidney Cysts
25% of kidney cysts show a trend to enlargement within three years.
Index: 1–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Israel, G. M. & Bosniak, M. A.
An update of the Bosniak renal cyst classification system.
Urology, 2005, 66, 484-488