Jeffrey J. Tomaszewski, MD
Robert G. Uzzo, MD
BASICS
DESCRIPTION
• Renal cycts are fluid-filled renal structures not continuous with the nephron or collecting system
• Simple cyst
– Arise from the renal parenchyma
– Size varies, often <2 cm but may be significantly larger
– Typically asymptomatic incidentally detected on CT or US
– Can be single, multiple, and/or bilateral
– If large, may impinge on the renal pelvis causing obstruction
– Diagnostic US findings include a mass that is free of internal echos (anechoic), through transmission with posterior acoustic enhancement
• Complex cyst
– Features not consistent with simple cyst; raise the possibility of malignancy
Increased fluid density, internal thick-walled septations, thickened wall, nodular projections into the lumen, calcifications, and contrast enhancement
• Pyogenic cysts are infected cysts
• Parapelvic cyst (aka peripelvic, parapelvic lymphatic, parapelvic lymphangiectasia, and renal sinus cysts); arise from the renal sinus
ALERT
Parapelvic cysts may be confused with hydronephrosis given their central location.
• Acquired cyst
– Associated with chronic hemodialysis
– Occasionally regress spontaneously
• Bosniak classification used to classify cysts based on CT complexity and likelihood of malignancy
EPIDEMIOLOGY
Incidence
• 0.22% from birth to 18 yr
• 20% by age 40
• 33% by age 60
• In autopsy series, 50% of patients >50 have ≥1 simple renal cysts
• Acquired cystic renal disease is more common among men
• Bilateral simple cysts infrequent <50 yr
Prevalence
N/A
RISK FACTORS
• Age, a known risk factor for simple renal cysts
• Increasing age (7-fold increase from 4th–8th decade or an increased incidence from 5–36%)
• Polycystic kidney disease (autosomal dominant and recessive types)
• Hemodialysis
– In ESRD, cysts in 8–13% prior to hemodialysis (HD)
– 10–20% have acquired cystic renal disease after 3 yr of dialysis, 40–60% after 5 yr, and >90% after 10 yr
Genetics
• ARPKD: PKHD1 gene, chromosome 6, protein product fibrocystin
• ADPKD: PKD1 & PKD2 genes, chromosome 16, protein products polycystin-1,-2
Other genetic cystic diseases: Juvenile nephronoph-thisis, medullary cystic kidney disease, glomerulo-cystic kidney disease, Von Hippel–Lindau syndrome (VHL), tuberous sclerosis, Birt–Hogg–Dubé syndrome
PATHOPHYSIOLOGY
• Simple cysts
– Development of discrete fibrous saccules of clear fluid lined with cuboidal epithelium
– Estimated growth rate: 2.18 mm/yr
– Some will involute and disappear over time although most will not
– It is controversial if renal cysts are causative agents of HTN
• Parapelvic cysts
– Found on <2% of kidneys at autopsy
– Can be confused with hydronephrosis
ASSOCIATED CONDITIONS
• ADPKD (Autosomal dominant polycystic kidney disease)
• ARPKD (Autosomal recessive polycystic kidney disease)
• Birt–Hogg–Dubé syndrome
• ESRD (end stage renal disease)
• Tuberous sclerosis
– 50% have multiple renal angiomyolipomas
– 20–25% of have renal cysts
• VHL disease
– Individuals develop cysts in multiple organs (kidney, pancreas, liver, epididymis)
– Increased risk of clear cell renal cell carcinoma (RCC) in cyst wall
GENERAL PREVENTION
Family members of patients with ADPKD and VHL should be screened
DIAGNOSIS
HISTORY
• Patients may present with an abdominal mass, pain, hematuria, or HTN but most are radiographically incidental
• Family member with polycystic kidney disease or other inherited cystic disease
PHYSICAL EXAM
• Abdominal/flank mass (rare)
• Often a benign exam
DIAGNOSTIC TESTS & INTERPRETATION
Lab
• Urinalysis most often normal unless concurrent medical renal disease present
• Renal function tests—calculate eGFR and stage chronic kidney disease (CKD)
Imaging
• Ultrasound
– Simple cyst
No internal echoes, distinct walls with defined margins, spherical shape with no internal echoes
ALERT
Complex renal cysts including hyperdense cysts may mimic solid renal masses. Doppler US is helpful.
• CT
– Simple cysts
Have sharp walls with smooth margins, spherical shape, homogenous throughout (Hounsfield units (HU) ranging from −10 to +20); no enhancement with IV contrast
CT diagnosis of a simple cyst is almost 100% if study performed properly (bi- or triphasic)
Enhancement is defined as an increase in HU by at least 15–20
If cyst does not meet criteria for being simple, further workup of the lesion is necessary
– Parapelvic cyst
Appears on US as a medially located cystic mass with surrounding echogenic walls (located within the fatty renal sinus)
On US or even CT can be confused with hydronephrosis. An excretory phase CT is most helpful in distinguishing a parapelvic cyst from a simple parcehncymal renal cyst.
• MRI:
– Bosniak criteria can be applied to MRI (exception: Calcifications may not be well seen)
– Low signal T1, high signal T2 is consistent with benign simple cyst
– Hyperdense cysts can be high signal on T1 and low on T2 but appearance depends on hemoglobin breakdown
– MRI may have a role in a subset of patients (VHL, multiple renal masses) if concern exists regarding excessive radiation due to multiple long-term imaging studies
– May be superior in characterization of internal cyst contents (blood, mucin)
• Bi- or triphasic CT represents the gold standard for distinguishing renal cysts:
– Discriminate between cysts and collecting system on excretory phase
– Particularly important in assessing hydronephrotic systems
US may be misleading/difficult to interpret
• Bosniak classification system of cystic renal masses originally based on CT (image) (1):
– Category I: Benign simple cysts; thin wall without septa, calcifications, or solid components, water density, and no contrast enhancement; No further imaging needed
Nearly all are benign
– Category II: Benign cysts with a few thin septae; the wall or septa may contain fine calcification, sharp margins, nonenhancing
– Category IIF: Well marginated and may have thin septae or minimal smooth thickening of the septa or wall, which may contain calcification that may also be thick and nodular; no contrast enhancement; includes totally intrarenal nonenhancing complex lesions >3 cm
These require follow-up (designated by the F designation)
5–20% of Bosniak II/IIF cysts contain malignancy in wall
– Category III: Indeterminate cysts with thickened irregular or smooth walls or septae; enhancement present
40–60% of these are malignant (cystic RCC and multiloculated cystic RCC)
Other class III lesions are benign (infected cysts and multiloculated cystic nephroma)
– Category IV: Characteristics of category III cysts plus they contain contrast-enhancing soft tissue components that are adjacent to and independent of the wall or septum
Risk of malignancy is 85–100%
Diagnostic Procedures/Surgery
• Cyst aspiration is rarely curative as fluid reaccumulates. Infected cysts may require aspiration and catheter placement.
– Infected cysts often represent calyceal diverticula
• Cyst biopsy is difficult and frequently results in indeterminate pathology
• Cytologic evaluation of fluid for malignancy or culture based on the indication and characteristics of the cyst
Pathologic Findings
• Simple renal cyst
– Single layer of cuboidal epithelium
• Not continuous with the collecting system
DIFFERENTIAL DIAGNOSIS (2)
• ADPKD (Autosomal dominant polycystic kidney disease)
• ARPKD (Autosomal recessive polycystic kidney disease)
• Calyceal diverticulum (evaluate for connection to the collecting system)
• Cystic degeneration (necrosis) of RCC
• Cystic malignancy (cystic RCC; sometimes called papillary cystadenocarcinoma)
• Hydronephrosis (parapelvic cysts)
• Juvenile nephronophthisis
• Medullary sponge kidney
• Multicystic dysplastic kidney
• Renal abscess
• Urinoma
• Pararenal (retroperitoneal or adjacent mesenteric/liver/splenic/adrenal cyst)
• Xanthogranulomatous pyelonephritis
TREATMENT
GENERAL MEASURES
• The major issue with renal cysts is differentiating a simple cyst from more serious diseases: Malignancy (RCC), polycystic kidney disease, complex cysts, and solid masses (such as a renal carcinoma or abscess)
• Risk of RCC with Bosniak III and IV lesions
• Bosniak I (benign cyst)
– No action necessary
• Bosniak II (septae and/or wall calcifications)
– Some clinicians consider 1 follow-up study (US) to confirm stability or if unable to differentiate from IIF cyst
• Bosniak IIF (increased, thicker walls and calcifications compared to type I)
– Require follow-up studies for 2–3 yr
• Bosniak III (irregular thick walls with calcification)
– Excision vs. alternative diagnostic evaluation (needle biopsy, MRI). May be observed in elderly and infirmed.
• Bosniak IV (enhancement with contrast, malignancy likely); surgical management
MEDICATION
First Line
Specific to those cystic diseases noted that cause HTN or renal insufficiency
Second Line
N/A
SURGERY/OTHER PROCEDURES
• Radical or partial nephrectomy for complex or suspicious cysts
• Cyst decortication with marsupialization should be reserved for very select cases
– Laparoscopic, open, and percutaneous approaches
ADDITIONAL TREATMENT
Additional Therapies
• Cyst aspiration and sclerotherapy (3)
– Rarely employed for large simple symptomatic renal cysts
– Pain often does not resolve with surgical management of renal cysts
– Not for parapelvic/peripelvic cysts
– Many consider this approach as primary therapy for large symptomatic cysts before surgical management; others support laparoscopic management initially
– Simple aspiration rarely leads to resolution; reaccumulation common
– Negative cyst fluid cytology required
– Sclerosing agents increase success but associated with pain and infection
– Multiple sessions are usually required, and use of indwelling percutaneous catheter may increase success rates
– Sclerosing agents: Ethanol, bismuth phosphate, n-butyl cyanoacrylate, povidone-iodine, and tetracycline; No agent superior
ONGOING CARE
PROGNOSIS
• Up to 15% of ADPKD will require hemodialysis
• Benign simple cysts demonstrate little risk of progressing to malignancy
• Bosniak classification and risk of malignancy
– Bosniak I: No risk
– Bosniak II/IIF: 5–20% risk depending on imaging characteristics
– Bosniak III: 50% risk
– Bosniak IV: 75–90% risk
COMPLICATIONS
• Rupture and hemorrhage with simple renal cyst; usually associated with flank pain and hematuria
• Infected renal cyst
• ADPKD
ALERT
• Associated with cerebral berry aneurysms.
– In up to 40% of patients.
– 9% mortality (subarachnoid hemorrhage).
FOLLOW-UP
Patient Monitoring
• Follow-up imaging of Bosniak type IIF cysts
• Multicystic dysplastic kidney/VHL
– Periodic sonography to monitor for neoplastic changes
• Acquired renal cystic disease
– Periodic imaging (US) on dialysis
Patient Resources
http://kidney.niddk.nih.gov/kudiseases/pubs/cysts/
REFERENCES
1. Bosniak MA. The Bosniak renal cyst classification: 25 years later. Radiology. 2012;262(3):781–785.
2. Minor LD, Picken MM, Campbell SC, et al. Benign renal tumors. AUA Update Series. 2003;22:170–175. Lesson: 22.
3. Skolarikos A, Laguna MP, de la Rosette JJ. Conservative and radiological management of simple renal cysts: A comprehensive review. BJU Int. 2012;110(2):170–178.
ADDITIONAL READING
• Feiner HD, Katz LA, Gallo GR. Acquired renal cystic disease of kidney in chronic hemodialysis patients. Urology. 1981;17:260–264.
• Terada N, Ichioka K, Matsuta Y, et al. The natural history of simple renal cysts. J Urol. 2002;167:21–23.
See Also (Topic, Algorithm, Media)
• Acquired Renal Cystic Disease
• Birt–Hogg–Dubé Syndrome
• Cystadenocarcinoma, Genitourinary
• Medullary Sponge Kidney
• Multicystic Dysplastic Kidney
• Polycystic Kidney Disease, Autosomal Dominant
• Polycystic Kidney Disease, Autosomal Recessive
• Renal Cell Carcinoma, General
• Renal Mass
• Tuberous Sclerosis
• VHL Disease
• Renal Cysts (Intrarenal, Peripelvic, and Parapelvic) Images 
CODES
ICD9
• 593.2 Cyst of kidney, acquired
• 753.12 Polycystic kidney, unspecified type
• 753.16 Medullary cystic kidney
ICD10
• N28.1 Cyst of kidney, acquired
• Q61.3 Polycystic kidney, unspecified
• Q61.5 Medullary cystic kidney
CLINICAL/SURGICAL PEARLS
• Bosniak cyst type malignancy risk:
– II/IIF (5–20%), III (50%), IV (up to 90%).