Renal Cyst: Etiology, Classification, Clinical Manifestations, Diagnosis, and Treatment

A renal cyst is a thin-walled, fluid-filled lesion arising from the renal parenchyma and lined by the epithelium. Cysts are considered the most common benign renal lesion and occur in all age groups.

The clinical significance of renal cysts is determined by their impact on kidney function and associated symptoms. Asymptomatic cysts, incidentally detected during imaging studies, account for the majority of cases and require ongoing monitoring.

Symptomatic cysts may present with pain, hypertension, or urinary tract obstruction, necessitating active intervention.

Of particular concern is the potential malignant transformation of cysts, which requires careful monitoring even in asymptomatic patients.

Bosniak Classification of Renal Cysts

The Bosniak classification system, developed in 1986, provides a standardized approach to classifying renal cysts based on contrast-enhanced computed tomography (CT) findings. Key assessment criteria include the presence of septa, calcifications within the cyst walls or septa, and the degree of contrast enhancement.

Category I

Bosniak categories I and II are characterized as benign lesions with a minimal risk of malignancy. Category I includes simple cysts with thin walls and no septa, calcifications, or contrast enhancement.

Category II

Category II includes minimally complex cysts with a few thin septa or small calcifications that do not require active intervention. These lesions require dynamic monitoring with periodic ultrasound examinations.

Category IIF

A Bosniak category IIF cyst is neither definitively benign (as in Bosniak categories I or II) nor clearly suspicious for malignancy (as in Bosniak categories III or IV). Category IIF cysts exhibit certain “suspicious” features that are insufficient to warrant classification into a higher category but are sufficient to recommend active observation. Their defining characteristic is an uncertain malignant potential. The vast majority of these cysts are benign.

Category III

Categories III and IV include cysts with a high risk of malignant transformation. Category III comprises heterogeneous lesions with thickened, irregular walls, multiple septa, or irregular calcifications.

Category IV

Category IV includes cysts with definite solid components demonstrating contrast enhancement, which is pathognomonic of malignancy. These cases require surgical treatment with histopathologic confirmation.

A genetic approach identifies autosomal dominant polycystic kidney disease (ADPKD) as a distinct medical condition.

Epidemiology

Renal cysts are among the most common urologic conditions.

Their prevalence is directly associated with age. Renal cysts are rarely detected in individuals younger than 30 years; however, after age 50, they are found in approximately one in three individuals, and by age 70, in more than half of the population.

Regarding sex-related differences, renal cysts occur twice as frequently in men as in women.

Etiology

Renal cysts are classified as congenital (resulting from developmental abnormalities of the renal tubules) or acquired (developing during person’s lifetime).

The primary pathogenetic mechanism underlying acquired cyst formation is obstruction of the renal tubular lumen due to inflammation, sclerosis, or ischemia. When the secretory activity of the epithelium above the site of obstruction is preserved, fluid accumulates, the tubular walls dilate, and an isolated cavity forms. Congenital forms (e.g., polycystic kidney disease) are associated with genetic defects that lead to uncontrolled formation of multiple cysts arising from nephron tubules.

Clinical Manifestations

Most simple renal cysts are asymptomatic and are detected incidentally during imaging studies performed for other conditions. The absence of clinical manifestations is characteristic of small, solitary lesions that do not compress surrounding tissues. The likelihood of developing symptoms increases with cyst size and proximity to vascular structures or the renal collecting system.

Less commonly, patients may develop hypertension secondary to parenchymal ischemia or impaired urinary drainage due to compression of the ureter or renal pelvis. Differential diagnosis is necessary when malignancy or an association with systemic diseases is suspected.

Diagnosis of Renal Cysts

Ultrasonography is the primary modality for the initial diagnosis of renal cysts because it is noninvasive, widely available, and does not involve exposure to ionizing radiation. It allows detection of cysts and assessment of their location, size, and principal morphological characteristics. Ultrasonography serves as a basic tool for preliminary classification of cysts according to the Bosniak system, thereby guiding subsequent patient management.

Contrast-enhanced CT is used to further characterize complex cystic lesions identified by ultrasonography. This modality provides high spatial resolution and enables detailed assessment of cyst architecture, including the presence of thickened walls, septa, or solid components. CT is the gold standard for definitive Bosniak classification of renal cysts, which determines the need for continued observation or surgical intervention.

Magnetic resonance imaging (MRI) is used when ultrasonography and CT findings are inconclusive, particularly when malignancy is suspected. It provides excellent soft-tissue contrast and allows differentiation of hemorrhagic or proteinaceous cyst contents without exposure to ionizing radiation. MRI is particularly valuable in patients with contraindications to iodine-based contrast agents or when repeated follow-up imaging is required.

Comprehensive evaluation of contrast-enhanced CT findings and laboratory parameters helps minimize diagnostic errors. Particular attention is given to cysts with thickened walls or heterogeneous contents. Differentiation between simple renal cysts and polycystic kidney disease is based on family history and genetic testing results.

Find more scientifically accurate content on our social media

Subscribe and don’t miss out the latest resources

Treatment

Observation

Conservative management of renal cysts is indicated in patients with small, asymptomatic lesions. The primary criterion is a cyst diameter of less than 4 cm without compression of surrounding tissues or impairment of urinary drainage. An asymptomatic course is confirmed by the absence of pain, dysuria, and abnormalities in laboratory markers of kidney function.

Dynamic observation protocols include regular ultrasound examinations to monitor cyst characteristics. For Bosniak category I–II lesions, ultrasonography is recommended every 6–12 months to assess cyst size and the characteristics of its contents. If these parameters remain stable for two years, the interval between examinations may be extended to 24 months. Changes in the echogenicity of cyst contents or thickening of the cyst walls require reassessment of the Bosniak classification. Contrast-enhanced CT is indicated when malignancy is suspected.

Surgical Approaches to the Treatment of Renal Cysts

Absolute indications for surgical intervention in patients with renal cysts include persistent pain refractory to conservative treatment and impaired urinary drainage resulting in urinary tract obstruction. Additional indications include complications such as cyst infection, intracystic hemorrhage, or suspected malignancy. In these cases, surgical treatment becomes necessary to prevent life-threatening conditions.

Failure to intervene may result in progressive kidney failure and other irreversible changes. Relative indications include situations in which a cyst causes discomfort or functional impairment without posing an immediate threat. These include large cysts (greater than 5 cm) that compress surrounding tissues, as well as lesions demonstrating progressive growth during dynamic observation.

The decision to proceed with surgery is made on an individual basis, considering the patient’s age, comorbidities, and potential intervention risks. The patient’s preference for relief of symptoms affecting quality of life also plays an important role.

Several treatment options are available for renal cysts, each with specific indications. The choice of treatment depends on cyst size, type, location, symptoms, and suspicion of malignancy.

Percutaneous Cyst Aspiration

• Procedure. Ultrasound-guided needle puncture through the skin, followed by aspiration of cyst contents, sometimes with instillation of a sclerosing agent (ethanol).
• This technique is most effective for small cysts located on the posterior surface of the kidney.
• Benefits. Minimally invasive, rapid procedure performed under local anesthesia.
• Disadvantages. High risk of recurrence.

Laparoscopic Cyst Excision (Decortication)

• Procedure. This technique is used to treat Bosniak category I and II cysts. Through 3–4 small abdominal incisions, the cyst is dissected, opened, and its walls are excised and submitted for histopathologic examination. The cyst cavity is left open.
• Benefits. Gold standard for cyst treatment. Low risk of recurrence. The technique is suitable for cysts of any size and location. It provides good visualization and allows biopsy sampling.
• Disadvantages. A surgical procedure requiring general anesthesia. Risks inherent to any surgical procedure include bleeding, infection, and injury to adjacent organs (bowel, blood vessels).

Endoscopic Cyst Fenestration (Marsupialization)

• Procedure. A flexible endoscope is advanced into the kidney through the urethra, bladder, and ureter. The cyst wall protruding into the renal pelvis is incised using a laser, thereby performing cyst fenestration (marsupialization). This allows the cyst contents to drain freely into the renal collecting system. An internal stent is then placed for 7–14 days.
• Benefits. Extremely minimally invasive, with no external incisions. Rapid recovery.
• Disadvantages. Very limited indications: symptomatic parapelvic cysts immediately adjacent to the renal pelvis. The procedure is technically challenging and is not available at all medical centers. Moderate risk of recurrence. Risk of bleeding and infection.

Laparoscopic Resection

• Procedure. This technique is used to treat Bosniak category III and IV cysts. A laparoscope (video camera) and specialized surgical instruments are introduced into the abdominal cavity through 3–4 small incisions (0.5–1 cm). The surgeon separates the kidney from the surrounding adipose tissue, isolates the renal vessels, and visualizes the cyst. A bulldog vascular clamp may be applied to the renal artery for 20–30 minutes to create a bloodless operative field for precise resection. The entire cyst is removed. The resection bed of the excised tumor (cyst) is sutured. The clamp is removed from the artery, and hemostasis is confirmed. The excised cyst is placed in a specialized retrieval bag and extracted through one of the incisions. All instruments are removed, and the incisions are closed with sutures.
• Benefits. Oncologic radicality. Allows complete tumor removal while preserving healthy renal tissue.
• Disadvantages. The most technically complex of the procedures described. High risk of intraoperative bleeding and injury to the renal collecting system and adjacent organs. Requires an exceptionally highly skilled surgeon.