<unk> s are classified by malignant risk using the Bosniak classification system. The system was created by Morton Bosniak (1929–2016), a faculty member at the New York University Langone Medical Center in New York City. The Bosniak classification categorizes<unk> s into five groups. Category I :Benign simple cyst with thin wall without septa, calcifications, or solid components, and has a density of 0–20 Hounsfield units (HU) (about equal to that of water). In such cases, a CT scan without intravenous contrast is enough for classification. Still, if a contrast CT is performed, a category I cyst should not show significant enhancement, which can be regarded as an increase of less than 10HU. Category II :Benign cyst with a few thin septa, which may contain fine calcifications or a small segment of mildly thickened calcification. This includes homogenous, high-attenuation (60–70 Hounsfield units) lesions less than 3 cm with sharp margins but without enhancement. Hyperdense cysts must be exophytic with at least 75 percent of its wall outside the kidney to allow for appropriate assessment of margins, otherwise they are categorized as IIF. A Bosniak category IIF cyst. This one is 3 cm wide, with calcifications within its wall, seen as very radiodense (white in this presentation) areas in its margins. There is also a septation which is calcified. Yet, the cyst does not show enhancement (uptake of contrast). Category IIF This category includes<unk> s with multiple thin septa, a septum thicker than hairline, slightly thick wall, or with calcification, which may be thick. It also includes intrarenal cysts larger than if: * there is no contrast enhancement (otherwise category III). * there is high attenuation or there is a maximum 25% of their walls visible outside the kidney (otherwise category II). Category IIF cysts have a 5–10% risk of being kidney cancer, and therefore follow-up is recommended. However, there is no consensus recommendation on the appropriate interval of follow up. Category III :Indeterminate cystic masses with thickened irregular septa with enhancement. 50 percent of these lesions are ultimately found to be malignant. Category IV :Malignant cystic masses with all the characteristics of category III lesions but also with enhancing soft tissue components independent of but adjacent to the septa. 100 percent of these lesions are malignant. Bosniak category I II IIF III IV Attenuation 0–20 HU 60–70 Hounsfield units Walls Thin and smooth Small and fine calcifications Nodular or irregular calcifications Thick, heterogeneous. Gross calcifications with enhancement Solid components No Yes
Renal cyst
Renal medullary fibroma
Ectopic kidney
Glomerulonephrosis
Cystic nephroma
Renal oncocytoma
Hydronephrosis
Page kidney