| Bosniak Classification | Description | % Risk of Malignancy |
| I (Benign) | – Benign simple cyst – hairline thin wall – no septa, calcification, solid component – homogeneous attenuation (−10 to 20 HU) – no enhancement | 3.2 |
| II (No further evaluation needed) | Benign minimally complicated cyst – may a few hairline thin septa that may have “perceived”, not measurable enhancement. – Fine calcification or a segment of slightly thickened calcification may be present in the wall or septa. – Also, a well-marginated nonenhancing homogeneous mass < 3 cm with density above simple fluid attenuation (hyperdense cyst). | 6 |
| IIF (Needs follow-up) | Usually benign complicated renal cyst – multiple hairline thin septa or minimal smooth thickening of the wall or septa. – Wall or septa may contain thick and nodular calcification – may have “perceived” but not measurable enhancement. – Also, a well-marginated intrarenal nonenhancing mass > 3 cm with density above simple fluid. | 6.7 |
| III (Indeterminate. Surgical lesion.) | Indeterminate complicated cystic renal mass – thickened irregular walls or septa – measurable enhancement. | 55.1 |
| IV (Clearly malignant. Surgical lesion.) | Malignant cystic renal mass – enhancing soft tissue components (cystic renal cell carcinoma). | 91 |
Source: An update of the Bosniak renal cyst classification system (2005)
