The Bosniak classification of renal cysts

Abstract

Renal cysts are a common, usually incidental finding on routine radiological studies, including ultrasound (US), computed tomography (CT), and magnetic resonance (MR). In general, renal cysts may be classified as simple or complicated. Simple cysts are benign and do not require any follow-up or treatment. Most of the cysts belong to this category. Complicated cysts however include a wide range of cystic kidney lesions, from completely benign to definitely malignant. Bosniak classification is a useful and applicable method for renal cyst evaluation. It was developed from clinical experience with the intent that morphologic features of a cyst could serve as a predictor for reliable categorization and differentiation of “nonsurgical” and “surgical” lesions. Although other imaging modalities like MR and US are used for diagnosing renal cysts, CT is the imaging method of choice. Standard CT protocol includes a noncontrast and postcontrast arterial and/or parenhymal phases. Size, location within the kidney, shape, and density of the cyst, the number and morphology of the septa and/or calcification, and postcontrast opacification are evaluated. US is most commonly used as a “screening” method, appropriate for frequent examinations, while MR is the initial method of choice for young patients, those with renal insufficiency and hypersensitivity to iodinated contrast agents, and is proven helpful when CT findings are equivocal.