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 objective with simple cysts is to differentiate them from malignancy, abscess, or polycystic kidney disease. Cystic disease can develop in dialysis patients and has the potential to progress to malignancy. Ultrasound and CT scanning are recommended to evaluate these masses. Simple cysts must meet three sonographic criteria to be considered benign: (1) echo free, (2) sharply demarcated 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. Renal cell carcinoma will enhance but typically is of lower density than healthy parenchyma. Arteriography can also be used to evaluate a mass preoperatively. Renal cell carcinoma is hypervascular in 80%, hypovascular in 15%, and avascular in 5% of cases.
If a cyst has questionable imaging characteristics or is of uncertain significance, periodic reevaluation is recommended. Urologic consultation and surgical exploration may be considered. Benign cysts do not require any specific follow-up, though changes in clinical presentation should prompt repeat imaging.
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