Bosniak classification system differentiates benign renal cysts from cystic carcinoma [Classics Series]

This study summary is an excerpt from the book 2 Minute Medicine’s The Classics in Medicine: Summaries of the Landmark Trials

1. The presence of calcification, septa, irregularities or thickness in cyst walls on ultrasound (US) or computed tomography (CT) can be used to identify potentially malignant lesions requiring surgery.

2. Hyperdense cysts can be considered benign without further imaging if they demonstrate a specific appearance (smooth, round, sharply marginated, and homogeneous), lack of enhancement, and size < 3 cm.

Original Date of Publication: January 1986

Study Rundown: Renal cysts are a common incidental finding on imaging studies done to evaluate potential abdominal or pelvic pathologies. Most often, renal cysts are discovered through US or CT. While many simple or uncomplicated cysts do not pose any difficulty in diagnosis, the management of complicated cystic lesions has often been met with a difference of clinical opinion. Bosniak proposed an approach towards the diagnosis of renal cysts using US and CT imaging. This includes a classification system for renal cysts and cystic lesions. Category I lesions include simple benign cysts of the kidney diagnosed definitively by sonography and/or CT. Category II lesions are minimally-complicated cysts that are benign but have some concerning radiologic findings (i.e. septa, minimal calcification, infection or high density). Category III lesions are complicated cystic lesions exhibiting some but not all radiologic features of malignancy, and therefore require surgical exploration. Category IV lesions are malignant cystic carcinomas.

Click to read the study in Radiology

In-Depth [review]: Bosniak provides a detailed diagnostic algorithm towards the radiological imaging of renal cysts. For masses first identified through urography without clear characteristics of malignancy (i.e. fat or calcification within the mass, increased tissue density, irregularity of margins, or invasion of collection system), US should be performed with results of previous imaging available for review. If the mass does not meet all criteria for a simple cyst on US, CT examination including contrast-enhanced and non-contrast-enhanced scans should be performed. Non-contrast-enhanced scans allow for evaluation of a lesion’s contrast enhancement, which is of particular value in identifying vascular renal masses. The Bosniak classification system (Table I) was made on the basis of the most common and pertinent radiological findings made through US and/or CT with respect to renal cysts. This includes calcification seen within a renal lesion, which should always be interpreted as a sign of possible malignancy. However, if all other US and CT criteria for a cyst are present, a small amount of calcium or lining in the wall or septa of a lesion may indicate a complicated cyst without malignancy (category II lesion). However, extensive calcification in the wall of the lesion warrants category III classification requiring surgical exploration to rule out malignancy. Calcification associated with an enhancing soft-tissue mass indicates malignancy (category IV) and necessitates radical nephrectomy. In addition, while many benign cysts have fine septa, lesions with numerous septa, irregular septal walls, septa over 1 mm in thickness, or with associated solid elements at attachment sites should be explored surgically (category III). If very irregular and numerous septa are present and associated with solid areas, a cystic carcinoma (category IV) can be diagnosed. Thickening or irregularity of the wall of a lesion also excludes benignity, placing such lesions in categories III or IV, depending on the severity of findings.

Bosniak also states the importance of assessing fluid density in cysts. Hyperdense renal cysts, usually containing old blood and often seen in patients with polycystic kidney disease, have a higher attenuation than surrounding renal parenchyma on non-contrast-enhanced CT scans. Following intravenous (IV) administration of contrast material, these lesions appear either isodense or hypodense when compared to the renal parenchyma, and can be considered benign if all of the following criteria are met: a) the lesion is smooth, round, sharply marginated and homogeneous, b) the lesion does not enhance with IV administration of contrast material and does not change in configuration, and c) the lesion is 3 cm or less in size. Lesions over 3 cm in size may still be benign, though US should be used to characterize it as a fluid-filled cyst. Lesions meeting all of the above criteria do not require surgical exploration or removal but should be monitored carefully through CT surveillance.

Bosniak MA. The current radiological approach to renal cysts. Radiology. 1986 Jan;158(1):1-10.

Additional Review:

Bosniak, MA. The Bosniak Renal Cyst Classification: 25 Years Later. Radiology.  2012 Mar;262(3):781-5.

Israel GM, Bosniak MA. How I do it: evaluating renal masses. Radiology. 2005 Aug;236(2):441-50.

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