The Bosniak classification of renal cysts

The Bosniak renal cyst classification system was initially reported in 1986, using CT scan findings. Although other imaging modalities are frequently used in the evaluation of renal masses, such as ultrasound and magnetic resonance imaging (MRI), CT scan (with and without contrast enhancement), remains the primary diagnostic technique. Ultrasonography is helpful for simple cyst identification,
but provides limited information with increasingly complex renal cysts and solid masses.

However, MRI canbe helpful with increasingly complex cyst identification.
Recent developments with MRI scanning allow shorter breath holds and increased contrast resolution with gadoliniumenhanced images. As such, the cysts may be characterized in greater detail, compared with CT scan. The MRI may demonstrate poorly identified septa on CT scan and show enhancements that are not otherwise clearly perceived. Also, MRI may differentiate between hemorrhagic cysts and solid enhancing masses.

renal cyst ct scan

renal cyst ct scan

The Bosniak system consists of four categories based on triphasic CT findings, ranging from simple to complex cysts (Table 1). Category I cysts have no malignant potential and, as such, no follow-up is required. However, there is a large difference in potential malignant risk, between category II and category III. These are 0% to 5% and about 50%, respectively.
To clarify this further, a subcategory of II was developed, IIF (for “follow-up”). Category IIF identifies the category II cyst which was slightly more complicated, but not necessarily suspicious enough to warrant surgical exploration.
Category IIF includes cysts which have multiple thin septa, slight wall-thickening without measurable contrast enhancement. They may have calcification, including thick, nodular or irregular calcification. Ultimately, 95% of category IIF cysts are proven to be nonmalignant. While the importance of calcification has diminished over the years since the original classification, enhancement with CT contrast has not. Any mass studied with CT thin slice scanning, that increases between 10 to 20 Hounsfield units (i.e., Category III and IV), is a renal cell carcinoma until proven otherwise.

Category I: Malignant risk less than 1%; no follow-up required
– uncomplicated, simple benign cyst
– anechoic, posterior enhancement ( through transmission), round or oval shape, thin, smooth wall
– homogeneous water content, sharp delineation with the renal parenchyma, no calcification, enhancement or wall-thickening

Category II: Malignant risk less than 3%; no follow-up required
Cystic lesion with some abnormal radiological features
– <1 mm septations (hairline thin)
– fine calcifications within the septum or wall
– <3 cm in diameter
– hyperdense cysts (>20 Hounsfield units)

Category IIF: Malignant risk 5-10%; follow-up recommended
Cystic lesion with increased abnormal findings
– multiple thin septum
– septa thicker than hairline or slightly thick wall
– calcification, which may be thick
– intrarenal, >3 cm
– no contrast enhancement

Category III: Malignant risk 40-60%; surgical excision recommended
More complicated
– uniform wall thickening/nodularity
– thick/irregular calcification
– thick septa
– enhances with contrast

Category IV: Malignant risk greater than 80%; surgical excision recommended
– large cystic components
– irregular margins/prominent nodules
– solid enhancing elements, independent of septa


Category I: Malignant risk less than 1%; no follow-up requiredCategory II: Malignant risk less than 3%; no follow-up required
Cystic lesion with some abnormal radiological features
Category IIF: Malignant risk 5-10%; follow-up recommended
Cystic lesion with increased abnormal findings
Category III: Malignant risk 40-60%; surgical excision recommended
More complicated
Category IV: Malignant risk greater than 80%; surgical excision recommended
– uncomplicated, simple benign cyst
– anechoic, posterior enhancement ( through transmission), round or oval shape, thin, smooth wall
– homogeneous water content, sharp delineation with the renal parenchyma, no calcification, enhancement or wall-thickening
– <1 mm septations (hairline thin)
– fine calcifications within the septum or wall
– <3 cm in diameter
– hyperdense cysts (>20 Hounsfield units)
– multiple thin septum
– septa thicker than hairline or slightly thick wall
– calcification, which may be thick
– intrarenal, >3 cm
– no contrast enhancement
– uniform wall thickening/nodularity
– thick/irregular calcification
– thick septa
– enhances with contrast
– large cystic components
– irregular margins/prominent nodules
– solid enhancing elements, independent of septa

Thomas F. Whelan, MD, FRCSC Can Urol Assoc J 2010;4(2):98-9

Source.

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