This diagram depicts various renal cysts morphologies, listed in order by their potential for malignancy, using the Bosniak classification system. The cysts in the top row (1 and 2) do not need further evaluation or monitoring. The cysts in the bottom row (2F, 3 and 4) should be followed (the "F" in 2F means it requires "followup") and require further evaluation and management.
- type I: almost universally benign and appear as simple cysts on CT with extremely thin walls. To classify a renal cyst as type I, there must be no septa, calcifications, or solid components. These lesions do not enhance with IV contrast.
- type 2: almost universally benign, but unlike type I, they may contain thin septa. Thin or slightly thickened calcification may be noted within the cyst wall. Minimal but un-measurable enhancement of the septa or cyst wall may also be appreciated. High-attenuation cysts of less than 3 cm in diameter that do not enhance are considered type II.
- type 2F: (the “F” is for follow-up needed) cysts are more complex that simple type II cysts, but do not meet the criteria for a type III classification. These cysts may demonstrate an increased number of septa or minimal smooth thickening of the wall or septa. The wall and/or septa may contain calcifications. No part of these lesions should appreciably enhance. Non-enhancing high-attenuation cysts that are intrarenal and larger than 3 cm are also considered IIF.
- type 3: indeterminate with thick, nodular multiple septa or wall with measurable enhancement.
- type 4: clearly malignant. They appear similar to type III cysts, but also have enhancing mural components adjacent to, but separate from the wall or septum. (not depicted here).