Abstract

A renal cyst or kidney cyst, is a fluid collection in or on the kidney. There are several types based on the "Bosniak classification". The majority are benign, simple cysts that can be monitored and not intervened upon. However, some are cancerous or are suspicious for cancer and are commonly removed in a surgical procedure called nephrectomy.

Numerous renal cysts are seen in the cystic kidney diseases, which include polycystic kidney disease and medullary sponge kidney.

Epidemiology

Up to 27 percent of individuals greater than 50 years of age may have simple renal cysts that cause no symptoms.

Bosniak classification

Renal cysts are classified by malignant risk using the Bosniak Classification System. The system was created by Dr. Morton Bosniak, a faculty member at the New York University Langone Medical Center in New York City.

The Bosniak classification categorizes renal cysts into five groups.

Category I

Category II

Category IIF

This category includes renal cysts with multiple thin septa, a septum thicker than hairline, slightly thick wall, or with calcification, which may be thick. It also includes intrarenal cysts larger than if there is no contrast enhancement.

Category IIF cysts have a 5-10% risk of being kidney cancer, and therefore follow-up is recommended. However, there is no consensus recommendation on the appropriate interval of follow up.

Category III

Category IV

Ignore-Follow-Excise

This system is more directly focused on the most appropriate management. These alternatives are broadly to ignore the cyst, schedule follow-up or perform a surgical excision of it. When a cyst shows discrepancy in severity across categories, it is the most worrisome feature that is used in deciding about management. There is no established rule regarding the follow-up frequency, but one possibility is after 6 months, which can later be doubled if unchanged.

Workup

The complex cyst can be further evaluated with doppler ultrasonography, and for Bosniak classification and follow-up of complex cysts, either contrast-enhanced ultrasound (CEUS) or contrast CT is used.

Peripelvic versus parapelvic cysts

Parapelvic cysts originate from around the kidney at the adjacent renal parenchyma, and plunge into the renal sinus. Peripelvic cysts are contained entirely within the renal sinus, possibly related to dilated lymphatic channels. When viewed on CT in absence of contrast, they can mimic hydronephrosis. If symptomatic, they can be laparoscopically decorticated - removal of the outer layer or cortex.