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Table 1 The Bosniak classification of renal cystic masses

From: Diagnosis and treatment of cystic renal cell carcinoma

Bosniak category Features Work-up
I A simple benign cyst with a hairline-thin wall that does not contain septa, calcification, or solid components. It has the same density as water and does not enhance with contrast medium. Benign
II A benign cyst that may contain a few hairline-thin septa. Fine calcification may be present in the wall or septa. Uniformly high-attenuation lesions <3 cm in size, with sharp margins but without enhancement. Benign
IIF These cysts may contain more hairline-thin septa. Minimal enhancement of a hairline-thin septum or wall can be seen. There may be minimal thickening of the septa or wall. The cyst may contain calcification, which may be nodular and thick, but there is no contrast enhancement. There are no enhancing soft-tissue elements. This category also includes totally intrarenal, non-enhancing, highattenuation renal lesions ≥3 cm in size. These lesions are generally well-marginated. Follow-up. A small proportion are malignant.
III These lesions are indeterminate cystic masses that have thickened irregular walls or septa in which enhancement can be seen. Surgery or follow-up. Over 50% of the lesions are malignant.
IV These lesions are clearly malignant cystic lesions that contain enhancing soft-tissue components. Surgical therapy recommended. Mostly malignant tumor.