Simple renal cysts

Clinical
Simple renal cysts are present in about 10% of the population. They are not common in children but increase in size and number with age. Their pathogenesis is unknown. Most simple cysts are solitary, unilocular, and located in the cortex, with the more central ones known as parapelvic cysts. Perinephric cysts are those located beneath the renal capsule. The terms parapelvic and perinephric cyst are useful to describe a cyst’s location.Exoparenchymal cysts are larger and have a greater tendency to
increase in size than endoparenchymal cysts. Most simple cysts are detected incidentally. Some larger ones are associated with hematuria or flank pain.
Imaging
A simple cyst should have a sharp, well-defined margin at its interface with normal renal parenchyma. Simple cysts do not have thick walls or prominent septa. Likewise, no associated soft tissue mass should be present.Many of these cysts are defined by US, but any suggestion of an associated solid component or wall thickening requires CT, or, in some instances a biopsy. Some benign cysts have thin, linear calcifications corresponding to the cyst wall or septa. Any mottled or amorphous calcifications should be viewed with suspicion. Septa in a benign cyst should be uniformly thin. Computed tomography attenuation of a simple cyst should be that of water. A density
greater than 10HU suggests a complex cyst. Higher densities are seen with hemorrhage into a simple cyst. Infection results in a thickened wall. Compared to an unenhanced density, the attenuation coefficient of a cyst should increase by no more than 10HU during the corticomedullary and parenchymal phases. Any greater enhancement or nodularity should raise suspicion for a malignancy.However, a caveat is in order: a renal cyst attenuation increase up to about 10HU postcontrast is often due to an artifact from surrounding parenchymal enhancement. With gray-scale US a simple cyst should show an anechoic lumen, well-defined walls, acoustic enhancement posterior to the cyst, and no associated nodularity or wall thickening. Thin septa may be identified, findings often not seen with CT.
The differential diagnosis for a “simple cyst” detected by gray-scale US includes an obstructed calyx, calyceal diverticulum, papillary necrosis, an aneurysm, and a vascular tumor. A pseudoaneurysm mimics a cyst on gray-scale US, and Doppler US is necessary to differentiate between them; Doppler US shows no flow, thus excluding a vascular lesion (except a thrombosed one). Similar to other nonflowing fluid of water density, a simple cyst is hypointense or almost has a signal void on T1- and is hyperintense on T2-weighted MRI. It does not enhance postcontrast. The cyst wall should be imperceptible; otherwise a more complex cyst etiology should be suggested.
Therapy
A high recurrence rate is common after aspiration of a simple cyst. Typical therapy consists of percutaneous injection of a sclerosing agent, with repeat injection if a cyst recurs.Use of 95% to 98% alcohol is effective and leads to complete regression of most cysts, except those >10 cm in diameter. One technique consists of catheter insertion under US control, cyst drainage and
sampling for bacteriologic and cytologic study, cyst opacification with contrast, instillation of roughly half the cyst volume of ethanol, and clamping for 20 minutes. Some authors prefer two sequential sclerosing agent injections over a 48-hour period. Local infiltration or intravascular injection results in lysis of cell membrane, protein denaturation, local vascular occlusion, and cell death.
Laparoscopic decortication of simple renal cysts is effective therapy for pain relief. Such therapy must be limited to Bosniak categories I and II cysts with their low risk for cancer. In an occasional hypertensive patient, drainage of a simple cyst corrects hypertension; rather than ascribe hypertension directly to the
cyst, it is more likely that the cyst had compressed
and narrowed an adjacent renal artery.
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