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Simple cysts account for 65–70% of all renal masses. They are generally found at the outer cortex and contain fluid that is consistent with an ultrafiltrate of plasma. Most are found incidentally on ultrasonographic examination. Simple cysts are typically asymptomatic but can become infected.

The major diagnostic objective with simple cysts is to differentiate them from malignancy, abscess, or polycystic kidney disease. Renal cystic disease can develop in dialysis patients and has the potential to progress to malignancy. Ultrasound and CT scanning are the recommended procedures for evaluating these masses. Simple cysts must meet three sonographic criteria to be considered benign: (1) echo free, (2) sharply demarcated mass with smooth walls, and (3) an enhanced back wall (indicating good transmission through the cyst). Complex cysts can have thick walls, calcifications, solid components, and mixed echogenicity. On CT scan, simple cysts should have smooth thin walls that are sharply demarcated and should not enhance with contrast media. A renal cell carcinoma will enhance but typically is of lower density than the rest of the parenchyma. Arteriography can also be used to evaluate a mass preoperatively. A renal cell carcinoma is hypervascular in 80%, hypovascular in 15%, and avascular in 5% of cases.

If a cyst is benign, periodic reevaluation is the standard of care. If the lesion is not consistent with a simple cyst, urologic consultation and possible surgical exploration is recommended.

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Cramer  MT  et al. Cystic kidney disease: a primer. Adv Chronic Kidney Dis. 2015 Jul;22(4):297–305.
[PubMed: 26088074]  

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