I Have a Bleeping Cyst on My Kidney

This post is the first in a planned series entitled “I Have a Bleeping (What) on My (Where),” a series focused on incidental findings. An incidental finding does not cause symptoms and is discovered during an imaging study performed for some other reason. For example, a CT of the abdomen and pelvis identifies acute appendicitis as the reason for abdominal pain, but a cyst is also noted on one of the kidneys.

I’m glad you asked, because I will tell you what happens next when we encounter these incidental findings. Obviously the approach differs depending on the specific finding, and today we will discuss kidney cysts.

What is a kidney cyst?

A kidney (or renal) cyst is a fluid-filled structure with a thin, almost imperceptible wall consisting of cells (epithelial cells, for you busy bodies). Picture a water balloon stretched to the max.

The most common type of renal cyst is called a simple cyst, which forms sporadically with age in many individuals. An estimated 50% of individuals have at least one simple cyst by age 50.

The mystery of…cyst-ery

I love a good mystery, but I will be honest: the puzzle of why and how simple renal cysts form is no page turner. It starts with our kidneys, the Brita filters of the human body. They largely consist of microscopic tubes which filter the blood to form and concentrate urine; blood flows through the kidneys, magic happens, and urine is produced. Current experts on simple cyst formation postulate that when the tiny tubes in the kidneys are blocked or damaged in some way, they collect fluid, grow in size, and form cysts.

The end. What a thriller!

In addition to the sporadic formation of simple cysts just described, dozens of other genetic and acquired conditions can result in renal cysts. Examples of these conditions include autosomal dominant polycystic kidney disease (ADPKD), a genetic condition, and acquired cystic kidney disease (ACKD), which occurs in patients with kidney failure.

The take home point so far is this: if you are going to get a renal cyst, you want one of these simple cysts. If a cyst is characterized as simple, there is nothing to worry about (with the rare exception of a very large cyst that causes symptoms, and may be surgically removed or drained).

Smile, cyst, it’s time for your close up

You are reading a radiology website, so it’s time we discuss how to image these mischievous renal cysts. When a radiologist identifies a renal cyst, it is most critical to first determine whether or not it can be classified as a simple cyst. What exactly do I mean by a simple cyst? A simple cyst needs to exhibit a few characteristics:

• Round or oval in shape
• Thin, almost imperceptible wall
• Internal fluid that is free of debris or blood
• No solid component
• No septations (internal divisions)

These characteristics can be demonstrated well by 3 imaging modalities: ultrasound, CT, and MRI. Renal cysts that meet all of these criteria are considered benign simple cysts, and need no further follow up.

Could it be cancer?

Up until this point we have been describing simple renal cysts. Alas, other types of renal lesions can appear similar to simple cysts and must be differentiated on imaging. When a radiologist notes one or more of the following characteristics, red flags are raised that it might be something more concerning. Notice these contradict the definition of a simple cyst above.

• Solid component
• Thickened wall and/or thickened septations
• Calcification
• Internal complexity or debris

Cysts with this appearance are described as complex cysts or cystic renal masses, depending if they are more cystic or solid. A precise distinction between these two categories is less important than the fact that they do not represent simple cysts. But what do they represent, and why are they “something more concerning?”

Many of these cystic lesions that cannot be classified as simple cysts fall into the category of complex cysts. A complex cyst can form when a simple cyst bleeds internally (hemorrhagic cyst) or when its fluid contents become filled with debris (proteinaceous cyst). While hemorrhagic and proteinaceous cysts are not worrisome in and of themselves, they can sometimes be difficult to differentiate from tumors. More on how we deal with this below.

More rarely we encounter a cystic lesion that turns out to be something more serious: renal abscess or renal tumor (including cancer).  With the former, an infection in the kidney can form a thick-walled complex fluid collection called an abscess. A cystic lesion that has solid nodular components or thickened septations is usually described as a cystic renal mass, and most of these represent renal tumors, which can be benign (not cancer) or malignant (cancer).

OK, you found something concerning. Now what?

As discussed above, if the radiologist finds a simple cyst, there is nothing to worry about and no follow up is necessary.

When radiologists suspect a complex cyst (hemorrhagic or proteinaceous), we have a few ways to confirm it.  If intravenous contrast—dye administered through the veins—was given for a CT or MRI, a complex cyst should not enhance, i.e., it should not light up after the dye is given. Lack of enhancement tells us the complex cyst does not contain any solid components, and is not worrisome for a tumor. Complex cysts can also be followed on imaging over time to ensure they do not change or grow. Sometimes it may be helpful to look at these cysts with a different imaging modality, e.g., get a follow-up ultrasound when the complex cyst was initially discovered on CT. Once diagnosed as a complex cyst, usually no further follow up is necessary.

Renal abscesses and tumors require intervention. A renal abscess is usually drained, either by placing a long thin tube through the skin surface into the abscess, or via surgery. Renal tumors represent a large and variable group of lesions, and a complete discussion of their treatment is too complex to address here, but many are removed surgically.

Thus concludes our whirlwind tour of renal cysts. Certainly more can be said regarding the symptoms, diagnosis, and treatment of these renal lesions, but I hope you find this brief summary of renal cyst imaging useful. Please let me know!