Bosniak Classification System for Cystic Renal Lesions

What is the Bosniak classification system for cystic renal lesions?

The Bosniak classification system utilizes specific CT or MR imaging features to help classify cystic renal lesions into those that are likely benign (and do not require surgical resection) from those that are likely malignant (and thus require surgical resection).

Bosniak Classification System for Cystic Renal Lesions

CATEGORYIMAGING FEATURESMANAGEMENT
I: simple cystFluid attenuation (CT)
Fluid signal intensity (MRI)
Thin smooth wall
No enhancement
100% benign
No further follow-up
II: minimally complex cystFine calcifications or a short segment of slightly thickened calcification (CT)
Few thin internal septations without enhancement
Homogeneously hyperattenuating (>20 HU) and nonenhancing but ≤ 3 cm and not fully intrarenal (CT)
Increased T1-weighted signal intensity or slightly decreased T2-weighted signal intensity relative to simple fluid but nonenhancing (MRI)
100% benign
No further follow-up
IIF: probable minimally complex cystSmall nodular calcifications (CT)
Multiple thin internal septations without enhancement
Minimal smooth thickening of wall or internal septations
Homogeneously hyperattenuating (>20 HU) and nonenhancing but either ≤ 3 cm and fully intrarenal or >3 cm (CT)
95% benign
Follow-up imaging
III: probable cystic renal neoplasmThick or irregular walls or internal septations with enhancement>50% malignant
Surgical resection
IV: cystic renal neoplasmSolid nodular components with enhancement>95% malignant
Surgical resection
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