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Diagnosis and Management of Complex Renal Cysts

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Renal cysts are a common finding in routine radiological studies. This article gives a detailed description of diagnosis and management.

Published At February 2, 2023
Reviewed AtFebruary 2, 2023


Earlier, renal cysts were discovered in 33% of patients of the same age with the help of computed tomography scan technology. Today, ultrasound and cross-sectional imaging studies are used to obtain the diagnosis of abdominal complaints. With improved technology and newer-generation diagnostic equipment, renal masses are more frequently identified.

The identification of incidental findings with the help of imaging techniques have become common. Sometimes, managing the incidental findings becomes tricky. Complex renal cysts are representative of such a situation.

In 1986, Morton Bosniak published a review article in which he suggested a classification for the management of cystic lesions of the kidneys based on the findings of contrast-enhanced computed tomography (CT). Urologists and imaging specialists gradually adopt this classification, which is currently used as a reference in medicine.

What Are Renal Cysts?

A renal cyst is a well-defined round or oval fluid-filled sac; some cysts grow on the surface, while others grow inside the kidney. They may be associated with disorders that impair kidney function. Renal cysts are very common, and complex mass detection has increased dramatically over the past few decades with the help of cross-sectional imaging. Cystic renal cell carcinoma represents five to seven percent of all renal tumors. The ability to differentiate between benign and malignant cysts remains a major challenge.

What Is the Classification of Renal Cysts?

  • Simple Cyst: They are thin-walled and have fluid-filled sacs.

  • Complex Cyst: Some renal cysts are complex and have a thicker wall that contains solid material instead of fluid.

What Are the Symptoms of Complex Cysts?

  • Fever.

  • Changes in urinary habits.

  • Pain between the ribs and pelvis.

  • Upper abdominal pain.

  • Hematuria- Blood in the urine.

What Is Meant by the Bosniak Classification?

The Bosniak classification for renal cysts was developed in the late 1980s to standardize the management of complex cystic renal lesions. Alterations were made during 1990, and the last one in 2005. Despite the standardized description suggested by Bosniak, there remained a subjective component for distinguishing between minimally complex and benign lesions that come under category two (comprises homogeneous hyperdense cysts lesser than or equal to 3.0 cm), which does not require surgical approach and malignant cysts that come under category three (comprises lesions with thick septa and irregular wall with coarse calcifications), for which surgical approach is recommended. Bosniak and his collaborators suggested introducing a fifth category, called 2-F ("F" as follow-up), in the classification.

How Complex Cysts Are Classified Based on Their Management?

According to the cystic degree of complexity and likelihood of malignancy, the five categories of cystic renal lesions include 1, 2, 2-F, 3, and 4.

  • Category 1: Corresponds to simple cysts with thin and smooth walls, without septa or vegetations, and no contrast enhancement after the administration of intravenous contrast agents.

  • Category 2: Comprises homogeneous hyperdense cysts lesser than or equal to 3.0 cm. This includes minimally thick wall cysts with thin septations and fine parietal calcifications with no contrast enhancement after intravenous contrast agent injection.

  • Category 3: Comprises lesions with thick septa and irregular walls with coarse calcifications with clear enhancement after intravenous contrast injection.

  • Category 4: Comprises cysts with well-defined solid components that demonstrate contrast enhancement after intravenous contrast injection.

  • Category 2- F: Corresponds to indeterminate lesions. Although the finding is not sufficient to indicate surgical exploration, it may suggest a slight risk of malignancy.

What Are the Ways to Diagnose Complex Cysts?

  • Abdominal Ultrasound and Pelvic Ultrasound: Helps to confirm the presence of fluid inside the renal cysts and monitor simple renal cysts for any changes over time.

  • Abdominal and Pelvic Computed Tomography: Helps to distinguish between benign cysts and tumors in the kidneys by injecting contrast material.

  • Body Magnetic Resonance Imaging (MRI): This test uses a magnetic field and radio frequency pulses to produce detailed pictures of the kidneys.

  • Biopsy: Percutaneous biopsy is useful in diagnosing complex cystic lesions. Image-guided biopsy has increasingly been used to evaluate intermediate complex cysts.

What Is the New Prospectus for Diagnosing Complex Cysts?

The use of intravenous sonographic contrast agents allows for detecting the enhancement in complex cystic lesions, which possess very thin septa (hair-like enhancement), with an accuracy superior to computed tomography. Limitations of such a technique include the following:

  • Low reproducibility of the method.

  • Ultrasound operator dependence.

  • The cost of the contrast agent is four times higher than the iodinated contrast agent.

  • Differences might occur in multiple cysts requiring repeated contrast injections.

  • Other techniques have also been emphasized to improve the characterization of complex renal cystic lesions.

  • Due to the indirect evaluation of the cellularity of neoplasms, diffusion-weighted magnetic resonance imaging has gained more attention.

How Are Renal Cysts Treated?

The management of renal cysts is done with the help of an interprofessional team that includes a urologist, geneticist, nephrologist, and a primary clinician with a physician assistant.


  • A long needle is inserted through the skin and then enters the cyst under ultrasound guidance.

  • The physician drains the cyst filled with an alcohol-based solution.

  • This makes the tissue harden and shrink, reducing the recurrence chance.

  • Usually performed as an outpatient procedure under local anesthesia.


  • In the case of larger cysts, access is made with a small incision under the guidance of laparoscopy.

  • Then the cyst is drained and burned out.

  • This procedure is usually done under general anesthesia.


The Bosniak classification has allowed for the standardization of the management of renal cystic lesions. Initially, the classification was described for computed tomography, which is now used with magnetic resonance imaging too. Introducing the intermediate category 2-F has created conditions to reduce the number of benign lesions treated with surgery. Although Bosniak classification is not utilized to determine the lesion, ultrasound remains an excellent method for detecting and defining the complexity of cystic lesions.

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Dr. Samer Sameer Juma Ali Altawil
Dr. Samer Sameer Juma Ali Altawil



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