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Before we get into the actual categories of kidney cysts there are two general classes.  The first class is the simple kidney cyst and is the most common and carries no risk of turning into renal cancer.  The other class of kidney cyst is know as the complex cyst which has many forms and has the potential to become cancerous and needs to be monitored.  The Bosniak System contains 5 different categories of kidney cysts that help physicians determine your risk of renal cancer.  The categories are listed below.


    Category I – This is a simple benign cyst with a thin wall and is fluid filled with no calcifications.
    Category II – Most of these are benign cysts but have calcifications in the wall of the vesicle and the cysts are 3cm or greater.
    Category IIF – These cysts have a thicker wall more calcifications and the edges are well marginated.
    Category III – These cysts have even thicker walls that are irregular.
    Category IV – These cysts are irregular in shape and contain soft tissue components


Category II and III renal cysts have an 18 to 33% change of cystic cancer.  Category IV  cysts are thought to have a 92% chance of cancer.  If you have a high risk of cancer, your cysts will be monitored and if growth continues you may have to have the kidney removed.


The most common form of kidney cyst, the simple form, can be observed with a radiological imaging tests for example ultrasound or a CT scan.  These cysts simply look like round fluid filled vesicles with nothing else contained within them.  At the age of 50, your chance of having a cyst on your kidney runs about 50 percent.  It is also possible to have a simple cyst that is filled with blood.  These are called hyper-dense cysts and these are benign as well.  More often than not a simple kidney cyst is found when a doctor is looking for something else on a scan.  They don't usually cause any symptoms however in rare cases the benign cyst may cause some back pain.  This cyst can be removed if necessary by aspiration and sclerosis by using some form of radiological guidance with ultrasound or CT scan.  With radiological guidance a needle can be placed into the cyst and the fluid removed (aspirated) and then a hardening chemical can be dropped on the flat cyst to cause it to scar or sclerose.  This usually takes care of the problem and your pain should go away.
     
The complex kidney cyst is a suspicious cyst and may become cancerous.  This type of cyst is easily spotted due to its irregular vesicle shape and inclusions (solid stuff inside other than fluid).  The more inclusions found the greater the probability of cancer.  These cysts will contain blood vessels as cancer cells induce the growth of blood vessels within their mass.

At this time, it is a good idea to distinguish general renal cysts from Polycystic Renal Disease.  The cysts of Polycystic Renal disease are usually found on the bottom of the kidney and tend to obstruct the urinary tract and because of that are filled with urine instead of body fluids and may contain some infection.  Polycystic Kidney disease is a genetic disease and runs in families whereas simple kidney cysts are not believed to be so and do not run in families.  Polycystic Kidney Disease may generate hematuria (blood in the urine), hypertension (high blood pressure), inflammation and possible kidney failure.  So don't get these two kidney cysts confused.  If you know your family's medical history, then you'll have some idea of what is going on and can convey that to your physician.