• Renal cysts associated with congenital disorders
• Tuberous sclerosis
• Von Hippel-Lindau syndrome
• Medullary sponge kidney
• Uremic Medullary cystic disease
• Medullary necrosis
• Pyelogenic cyst
Medulla + Cortex
• Autosomal recessive polycystic kidney disease (infantile and childhood)
• Autosomal dominant polycystic kidney disease (adult)
• Multicystic dysplastic kidney
• Acquired cystic kidney disease
• Miscellaneous lesions
• Infection (pyogenic abscess, TB, hydatid cyst)
• Haematoma
• Lithium nephropathy
Extraparenchymal cysts
• Parapelvic cyst
• Perinephric cyst
Figure 5.1 Classification of non-neoplastic renal cystic diseases.
5.3 Cystic lesions affecting renal cortex
Simple renal cyst
A simple renal cyst is the commonest benign cystic lesion of the kidney.
Clinical features
• Usually asymptomatic and associated with normal renal function.
• Commonly seen as an incidental finding in middle aged and elderly patients during cross sectional imaging of the abdomen.
• They are rare in children and young adults under the age of 30. A cyst in a child must be carefully differentiated from a cystic Wilms’ tumor.
• May present as an abdominal mass causing some flank discomfort.
• Loin pain may occur due to cyst wall distension or spontaneous intracystic bleeding.
• Rarely may cause hematuria or polycythemia.
• They uncommonly become infected or traumatized, a large cyst may obstruct the collect- ing system or cause hypertension.
• Simple renal cysts can be associated with hereditary disorders such as tuberous sclerosis or Von Hippel Lindau syndrome.
Pathology
• Simple renal cysts are acquired lesions that probably arise from obstructed ducts or tubules.
• They Occur mainly in the renal cortex but occasionally seen in the renal medulla.
• They are usually unilocular, often multiple and vary in size. A few thin septa within the cyst are occasionally seen.
• They have thin fibrous wall and are lined by flattened epithelium.
• The cyst contains serous fluid and does not communicate with the collecting system.
Imaging
• Simple renal cysts are easily diagnosed by US, CT or MRI.
• The findings in intravenous urography are often non-specific.
• Renal angiography is rarely required these days to diagnose renal cysts with availability of modern cross sectional imaging techniques.
• Percutaneous needle aspiration under imaging guidance for cytological, bacteriological and biochemical analysis of the aspirate may occasionally be required in difficult cases to exclude the possibility of malignancy or infection.
Ultrasound
• Ultrasound represents the most cost efficient modality to confirm the presence of a simple cyst. When all the criteria for a benign simple cyst are present, further evaluation is not indicated.
◦ Typical features of a simple cyst (Fig. 5.2):
a rounded homogeneous echolucent mass
a sharp interface with the surrounding renal parenchyma
acoustic enhancement posterior to the lesion.
A few thin septa may occasionally be seen within the lesion.
◦ Tissue harmonic imaging improves the characterization of renal cysts.
◦ Atypical features such as high echo content within the lesion, thick irregular wall or septa should raise the possibility of a neoplastic lesion and should be evaluated further by MRI or CT with contrast enhancement.
◦ Bleeding in a simple cyst would produce internal echoes and these may be mobile. A decrease in the posterior acoustic enhancement may be observed.
◦ A simple renal cyst is avascular on color or power Doppler US scanning.
◦ It does not show enhancement after intravenous injection of an US contrast agent.
78 CH 5 NON-NEOPLASTIC RENAL CYSTIC LESIONS
Figure 5.2 A large cyst at the upper pole of the right kidney shows the typical sonographic features of a simple cyst.
CT
• CT of the kidneys performed before and after the administration of intravenous contrast is used for characterizing renal lesions when US has been indeterminate or suspicious of a neoplastic lesion.
• It is extremely important to determine the presence or absence of contrast enhancement, to distinguish benign cysts from neoplasms. Typically, greater than 10 Hounsfield unit increase in density after contrast enhancement is only seen in neoplastic processes.
• A simple renal cyst at plain CT
◦ Presents as a well defined lesion of water density (slightly lower in density in compar- ison to adjacent renal cortex).
◦ Thin wall calcification is occasionally seen but more often encountered in neoplastic lesions.
◦ May occasionally presents as a homogeneously high density well-defined lesion (Fig. 5.3a). This is due to bleeding within the cyst. A high density benign cyst does not show enhancement after contrast medium injection.
• Post contrast, scanning(nephrographic phase)
◦ Well defined uniform water density.
◦ The lesion is often in the cortex.
◦ No septation, solid elements or enhancement.
◦ Thin septa without contrast enhancement may occasionally be seen.
MRI
• Renal MRI can be used as an alternative to CT.
(a) (b)
(c) (d)
(e)
Figure 5.3 (a) Unenhanced CT shows a high density cyst in the left kidney (arrow) due to hemorrhage. (b) The same hemorrhagic cyst (arrow) at T1 MRI shows high signals due to presence of methemoglobin. (c) The hemorrhagic cyst (arrow) at T1 MRI with fat saturation shows obvious high signals. (d) The hemorrhagic cyst (arrow) at T2 MRI shows low signals due to presence of intracellular methemoglobin. (e) The hemorrhagic cyst (arrow) at T1 MRI, fat saturation after gadolium contrast injection shows no enhancement.
• A simple renal cyst will be of low signal intensity on T1, and very high signal intensity on T2-weighted images.
• It appears as a homogeneous rounded mass with a thin wall and sharp interface with the surrounding normal renal parenchyma.
80 CH 5 NON-NEOPLASTIC RENAL CYSTIC LESIONS
• Appearance of the hemorrhagic cyst varies according to the time lapsed between the onset hemorrhage and the MRI examination.
◦ Hemorrhage of less than 24 hours may cause low signal in T1 and high signal in T2 imaging due to intracellular oxyhemoglobin.
◦ Hemorrhage between 1–3 days old may cause low signal in T1 and T2 imaging due to intracellular deoxyhemoglobin.
◦ Hemorrhage between 3–7 days may cause high signal in T1 and low signal T2 imaging due to intracellular methemoglobin (Fig. 5.3b, c, d).
◦ Hemorrhage between 7–14 days old may cause high signal in T1 and T2 imaging due to extracellular methemoglobin.
◦ Hemorrhage of more than 14 days old may cause low signal in T1- and T2 imaging due to extracellular hemosiderin.
• No enhancement is seen in the wall or septa of a simple renal cyst on T1-weighted imaging after intravenous injection of extracellular gadolinium based contrast medium (Fig. 5.3e).