Clinical Cases Renal Cyst

Renal Cyst

A cyst is defined as a closed sac containing fluid or semisolid material that develops abnormally in the body. Two broad categories of renal cyst exist:

Simple cyst- Very commonly found (up to 27% of people older than 50 have asymptomatic simple cysts), very low risk of being or becoming cancerous. These cysts are spherical in shape and filled with fluid. The lining of the cyst is thin and without irregularities, and there are few if any septa inside the cyst.
1. Complex cyst- An irregularly shaped cyst that is suspicious for cancer. These cysts are often irregularly shaped and contain both a thick outer wall and thick septa, they are often calcified, and they often receive their own blood supply.
   

   Diagnosis

   Renal cysts are rarely symptomatic, though if large enough they can cause flank pain. Most renal cysts are found incidentally when imaging the abdomen for problems unrelated to the kidneys. Because renal cysts are so often discovered on abdominal CT scans, a system for grading renal cysts based on their CT appearance was developed. This system is known as the Bosniak classification, and it divides cysts into 4 categories based on their likelihood of being or becoming malignant. One of the many criteria used in the Bosniak system is whether the cyst becomes enhanced when a contrast dye is injected into the patient's vasculature just prior to CT imaging. If the cyst does become enhanced, this indicates that the cyst has its own vasculature and so is more likely a malignant cyst. The 4 categories are defined as follows:

   Category I - A benign, simple cyst with very thin wall. Does not contain septa, calcifications, or solid material. The cyst has a density similar to that of water and it does not become enhanced after contrast dye is injected.
   1. Category II - A benign cyst that contains a few thin septa. Fine calcifications exist in the wall or septa. Also includes uniformly attenuated (whiter than water on CT) lesions less than 3 cm that have sharp edges and do not enhance with contrast dye (do not have their own vasculature).
   2. Category IIF - Wall defined cyst with sharp edges and multiple septa that may or may not show thickening or enhancement when contrast is given. There are no enhancing soft tissue components. Calcifications may be thick and nodular. Also includes uniformly attenuated (whiter than water on CT) lesions greater than 3 cm that have sharp edges and do not enhance with contrast dye.
   3. Category III - An indeterminate cystic mass with thickened, irregular walls and septa that enhance when contrast is given.
   4. Category IV - Malignant cystic masses that have all the characteristics of category III as well as soft tissue components that enhance when contrast is given.

      These CT images show some of the different types of renal cyst. The upper right image shows a cyst with multiple, thickened septa with an indeterminate/ fuzzy cyst wall, indicating at least a category III cyst. The bottom images (left image is axial, right image is coronal, both are with an injected contrast dye) show a benign, simple cyst with well-defined edges, no septa, and no enhancement with the contrast. The fluid inside the cyst is similar in density to water, indicating this is a category I cyst.

      PKD is a genetically inherited disease characterized by the presence of multiple simple cysts in the kidneys (and possibly the liver, pancreas, and more rarely the heart and brain). Signs and symptoms of PKD include abdominal discomfort, hematuria, urinary tract infection, abdominal mass, and elevated serum creatinine. These signs and symptoms may not always be present, and diagnosis can be obtained through incidental discovery of hypertension or incidental finding of cystic kidneys on imaging studies. As cysts become larger and more numerous, they begin to crowd out the healthy kidney tissue, resulting in renal insufficiency and eventually kidney failure.

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      Gross inspection of kidneys taken from a patient with severe PKD.
      Image courtesy of Steven Fruitsmaak
      

      The CT scan above is a coronal scan taken after a contrast dye was given to the patient. The inferior vena cava (IVC) can be seen as the large vessel that moves behind the liver. Follow the IVC down to find the renal veins, which will then lead you to the severely enlarged, polycystic kidneys. Notice how the cysts themselves do not enhance like the vasculature or renal soft tissue. This is because the cysts in PKD are simple, benign cysts (category I). The cysts are pathological via their mechanical disruption of the kidneys (crowding out healthy tissue), and are not themselves malignant.

      Renal Cyst on CT

      The movie file below is the postmortem CT scan of cadaver 33515. Notice from time 69-71 at H11 that a fluid filled sac (fluid similar in density to water) can be seen protruding from the right kidney (remember your right and left orientation on CT scans- we are looking at the cross section as though we are standing at the patient's feet). The cyst has a well-defined, thin wall and does not contain septa. Because this is a postmortem CT scan no contrast could be given, so we do not know whether the cyst enhances with contrast. Based on the cyst's other characteristics, though, we can safely classify this as a category I cyst (simple and benign).