Kidneys are part of the urinary system and take care of filtering toxic impurities out of the blood. As people grew older, there are chances of formation of kidney cyst. These cysts are also alternatively called renal cysts. There are two types; simple and complex kidney cysts. Simple cysts are spherical sacs, which might appear anywhere inside the kidneys. These thin-walled sacs are filled with liquid or blood. There is a 50 percent risk of simple kidney cyst occurring in persons over the age of 50 years. Older the person, greater is the risk of having it. There is no risk of simple cyst causing kidney cancer. Causes One may inherit kidney cyst congenitally. On the other hand, they can be acquired after the birth, due to problems in the kidney tubule. One of the causes of renal cyst is intestinal parasite infection, such as by tapeworm. Kidney cysts are most commonly seen in patients undergoing dialysis. The patients who are undergoing peritoneal dialysis and hemodialysis are especially, at the risk of formation of multiple and bilateral kidney cysts. The patient may suffer pain as the growing cyst stretches its wall. Some of these cysts grows to be larger than the kidney itself and puts pressure on it and obstruct the drainage from it. In addition, they also cause pressure on the other organs in the vicinity such as stomach and intestine. Sometimes they burst, bleed and become an easy target for infection. Complex renal cysts manifest in many disorders and diseases. These incidences include polycystic kidney disease, medullary sponge kidney and medullary cystic disorder. They also appear in the diseases such as cysts of the renal sinus (or peripelvic lymphangiectasia) and acquired cystic kidney disease. They are also found in patients with chronic kidney failure and multilocular cysts, which is also referred to as multilocular cystic nephroma. These cysts may also be observed in disorders such as multicystic dysplastic kidney and pyelocalyceal cysts. Few other disorders, which are characterized by the presence of these cysts are nephronophthisis and von Hippel-Lindau disease. Complex Renal Cyst and Cancer Simple kidney cysts differ from the complex ones in regularity of shape, cyst lining and internal structure. The complex renal cyst has irregular cyst lining and may show irregular shape and internal structure. The internal walls may divide it into many parts. These walls or the septum inside the complex renal cyst can be thin or thick and coarse. The presence of multiple internal walls give rise to the possibility that the cyst may develop into kidney cancer. Calcification of the cyst is another characteristic of these cysts. A radiology imaging test such as CT scan or ultrasound, can detect the presence of calcium inside the kidney cyst. These tests may also reveal that a part of the cyst is well supplied with blood. Presence of mass inside the complex renal cyst increases the risk of acquiring kidney cancer. It is a warning that there is some problem and you need to consult a competent urologist about treatment as soon as possible. Your urologist may use Bosniak classification method to judge the risk of a renal cyst transforming into the cancer of kidney. The method depends on the age of molecular biology, appearance of the cyst in the CT scans with contrast, ultrasonography or MRI. According to this method, Bosniak category I, II, III and IV complex renal cysts have 2%, 18%, 33% and 92% chances, respectively, of being linked with kidney cancer. The Bosniak IV complex renal cysts are usually related to kidney cancer. Categories of Complex Renal Cysts According to Bosniak

• Category I: Describes simple benign cyst devoid of septa or solid mass. It has hairline thin walls and it shows no signs of calcification. It has the density of water and does not absorb the contrast material and shows no enhancement.
• Category II: Images shows a benign cyst which may or may not harbor multiple calcified hairline thin septa. Presence of well marginated lesions in the cyst that cannot be enhanced with the use of contrast material. These lesions are smaller than 3 cm and indicates abnormal structural changes. The wall of the cyst may also show calcification.
• Category III: Describes cyst that may have multiple hairline thin septa. Some of the septa, along with the wall of the cell may show thickening. Images may also show thick and nodular calcification, however, there is no contrast enhancement which shows there are no soft tissues inside the cyst. It also shows non-enhancing and well marginated renal lesions (3 cm or less).
• Category IV: Images shows the lesions in the form of cystic masses. Thickened wall of the cyst and septa.
• Category V: Describes lesions which may turn out to be cystic lesions in the form of enhanceable soft tissues.

Generally, simple as well as complex kidney cysts are caused by the dilatation of any part of the nephrons or tubules which make the kidneys or a collecting duct of the kidney. They can lead to the chronic kidney pain and are known to cause renal atrophy. In most of the cases of kidney cysts, the patient is not aware of their presence, however, those undergoing dialysis may experience flank pain, kidney infection and hematuria i.e., blood in urine which should warn them enough to seek medical advice.