Genitourinary Radiology > Kidneys > Masses > Cystic > Renal Cysts

Renal Cysts

Simple renal cysts are present in over 50% of the population older than 50 years, thought to be caused by obstructed tubules or ducts. Most are asymptomatic, though a large cyst can cause discomfort and hypertension. A ruptured cysts may result in hematuria; an infected cyst results in fever.  Both infected cysts and hemorrhagic cysts are non-simple by imaging (Bosniak II or greater).

Ultrasound shows a sharp interface between cyst and adjacent renal parenchyma. The lesion is round or oval, anechoic (black or without echoes), has an imperceptibly thin wall and demonstrates increased sound through transmission.

On CT, lesions show a sharp interface with adjacent renal parenchyma, are water density (<20HU) and show no contrast enhancement of wall or cyst contents after IV contrast administration. Renal cysts were classified by Bosniak (a uro-radiologist) according to their imaging characteristics on CT:


Simple Cyst
Septated, minimal calcium described as “egg shell” thin, high-density cysts (> 20HU), non-enhancing
Multiloculated, hemorrhagic, dense calcifications; nonenhancing solid component
Renal-sparing surgery
Marginal irregularity, enhancing solid component
Radical Nephrectomy

Same patient. Contrast enhanced CT scan through right kidney upper pole (1) shows a simple cyst (C) which is <20HU. Ultrasound of right upper pole cyst (2) showing classic features of a simple cyst. MRI with gadolinium (3) and on T2 weighted axial images show no enhancement in this cyst and high signal fluid (C), respectively.

T1 weighted and T2 weighted axial MRI images through the kidneys shows bilateral renal cysts (C).  The cyst on the right appears simple (i.e., it follows the signal of simple fluid on all sequences) while the cyst on the left (arrows) shows high signal on T1 and T2 weighted images suggesting it is more complex perhaps from hemorrhage or from proteinaceous contents.  This cyst is a Bosniak II cyst.

T2 weighted axial MRI images through the right kidney shows a septated mass.  The mass is predominantly cystic (contains high signal fluid on T2 weighted images).  These septations (arrows) showed minimal enhancement on the gadolinium enhanced portion of the MRI study consistent with Bosniak III cyst.  These lesions are usually removed because of a high likelihood of malignancy.

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T1 weighted gadolinium enhanced coronal MRI image and axial T2 weighted MRI image through the right kidney shows a markedly complex cystic renal mass (red arrows).  Enhancement of a solid portion within the mass (yellow arrow) seen on the gadolinium enhanced images confirms this is quite likely to be a cystic renal cell carcinoma (Bosniak IV cyst).  This lesion was surgically removed.

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Bosniak I lesions follow the imaging characteristics of simple fluid with imperceptibly thin walls and no septations, calcifications or enhancement. On CT, simple fluid possesses an attenuation at or around 0 HU. On sonography, simple fluid is anechoic with increased through transmission.

Bosniak II lesions contain hair-thin septations, faint calcifications, and/or non-simple fluid. Again, no contrast enhancement should be present. CT demonstrates a faint calcification not seen on MR (blue arrow). Ultrasound clearly depicts the septum.

Bosniak III lesions may demonstrate wall or septal thickening, bulky calcifications, or septal enhancement. Unenhanced CT demonstrates a lobulated cystic lesion, with an irregularly calcified septum and the hint of other septa.

The hallmark of Bosniak IV lesions is enhancement. Any enhancement other than that of thin walls or septa places a lesion in this category. CT and MRI correlate well to demonstrate an enhancing cystic and solid right renal lesion, pathologically proven to be a clear cell renal cell carcinoma.

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