Kidney cysts common, but rarely signal cancer

By Dr. Brian Irwin

Our kidneys are amazing organs. These bean-shaped structures are highly efficient at filtering our blood of the many waste products it carries. They do this by pumping blood through an intricate system of fine blood vessels that run side-by-side with microscopic hollow tubes. Both the blood vessels and the hollow tubes are permeable, meaning water and its contents can cross from one into the other and vice versa. As the blood vessels course next to the tubules, some of their contents drift or are pumped into the tubule. By the time the tubule ends, its full of waste products and extra elements and vitamins, whereas the blood is returned to a rich, purified product.Sometimes these tubules can dilate and develop out-pouchings. These out-pouchings can grow in size, become filled with greater amounts of fluid and develop into cysts. Most of the time these cysts are discovered accidentally when a patient undergoes a CT scan of the abdomen for a totally different reason. Interestingly, the large majority of cysts cause no symptoms. An astounding 50 percent of people older than age 40 have kidney cysts, also known as renal cysts, so they are certainly a condition worth understanding.There are many causes of renal cysts. While the mechanical cause is, as previously mentioned, dilation of a tubule, the reasons for that dilation are variable. While the exact trigger is unknown, its thought that the majority of patients develop cysts by microscopic occlusion of a tubule, perhaps from crystal formation. Other patients develop multiple cysts due to genetically-driven changes within the tubules cells proper, something known as Polycystic Kidney Disease. Other conditions like kidney failure that requires dialysis, recurrent kidney stones, infection and even cancer can cause renal cysts, although these are much less common causes.There is a system for classifying cysts. The purpose of this system is to differentiate between cysts that are totally benign (not cancerous) and can be ignored and ones that may represent something more serious, like cancer. Considering these cysts are first characterized on CT scan or ultrasound, its challenging to definitively be sure a cyst is benign without surgically removing it and examining it under the microscope. Since 50 percent of the population has cysts, the great majority of which are of no concern, we certainly wouldnt want to remove chunks out of the kidneys of half our population. As such, were stuck with imaging studies to evaluate who needs a biopsy and who doesnt.Simple cysts are the most common and most benign cysts. These are encased by a single layer of cells, and when examined by CT or ultrasound they appear round, water-filled and are without any debris or solid material. Assuming no symptoms are present, these cysts are best ignored, as theyll almost never cause any problems. Complicated cysts look like simple cysts on imaging studies but have thin separating walls, debris, stones or calcium within them or on the inner surface of the cyst. Depending on the degree of complication, these may be managed with serial CT scans or in some cases even removal and biopsy.So if our screening is so effective at telling the difference between benign and cancerous cysts, why dont we screen everybody for kidney cancer? One study actually did evaluate the utility of screening the general population and detected cancer in 0.2 percent of those screened. While a small number, considering the study evaluated almost 6,000 patients, this figure detected cancer in 12 people!The biggest problem with screening for kidney cancer is the same problem all screening programs face: false positive test results. A false positive is a reported test result that looks like cancer but turns out to be benign. Obviously false negative (when the test failed to detect cancer) results are also an issue; however, they are in any test, be it lab or X-ray. Even mammography has come under scrutiny in a study this month that suggested the harm from unnecessary biopsies may actually outweigh its cancer-detection benefits. Out of thousands of kidney cysts seen during screening in studies, about a third had some mild characteristics that might, but probably dont, look like cancer. f all of those patients were then taken for surgery, the number would be astounding. Even though surgery is very safe, it carries obvious risks, and by operating on a thousand people in order to detect a dozen cases of cancer we would very likely be doing more harm than good.Its easy to be concerned with the possible presence of cancer in our bodies. Its even easy to become obsessed with, and driven crazy by, the fear that it is smoldering, undetected and spreading silently. Regardless, modern medicine has not yet found a way to detect all kinds of cancer safely without inflicting harm on the greater population. What we know for sure is kidney cysts are common, kidney cancer is rare and our imaging studies and guidelines are effective.Theres a fair chance that if you end up with an abdominal CT in your adult lifetime, regardless of the reason for the study, you will be told you have kidney cysts. If this is the case, discuss it with your health-care provider. The odds are certainly on your side that your cyst is nothing more than a bag of water youll never have to think about again.Dr. Brian Irwin is a family physician at Tamworth Family Medicine and Ossipee Family Medicine, divisions of Huggins Hospital.