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Renal cyst: simple or complex?

The diagnosis of a simple benign renal cyst on ultrasound requires the presence of all the following findings: a well-defined, roundish, anechoic structure, imperceptible near wall and thin echogenic far wall, and increased through transmission manifested by acoustic enhancement. Acoustic enhancement refers to the hyperechoic or bright area relative to surrounding tissues, distal to structures that are excellent transmitters of sound waves. This artefact is not restricted to cysts and can be seen with any fluid containing space such as a blood vessel or urinary bladder. Any lesion that does not meet criteria for a simple cyst is considered a complex cyst and may be characterized by findings such as irregular thickened walls, septations, internal echoes, and calcifications.

What is the utility of sonography in Bosniak classification of cysts?

The Bosniak classification system helps in the diagnosis and management of renal cysts. As the detection of neovascularization in malignant lesions, indicated by contrast enhancement (on CT) of solid components, septa or walls, is an essential part of the classification, ultrasound cannot be used instead of CT. However, it is known that ultrasound may demonstrate internal septa better than CT and MRI. Therefore, it has been suggested that simple and minimally complex (Bosniak I and II) cysts may be followed with sonography alone.

Bosniak classification illustration: Case courtesy of Dr Matt Skalski, Radiopaedia.org, rID: 20989
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