Renal cysts are fluid-filled sacs that can develop on or in the kidneys. While most are benign, a small percentage can be cancerous or become malignant. Understanding their characteristics is important for assessing cancer risk.
What Are Renal Cysts?
Renal cysts are typically small, round sacs with thin walls filled with fluid. They are common, especially as people age, often discovered incidentally during imaging tests for other conditions because they rarely cause symptoms.
Their exact reason for formation is not fully understood, but one theory suggests they develop when tiny kidney tubes swell and fill with fluid. This can be due to age-related changes or, in some cases, genetic conditions like Polycystic Kidney Disease. Most simple renal cysts do not affect kidney function and rarely lead to complications.
Understanding Different Types of Renal Cysts
Renal cysts are categorized into “simple” and “complex” based on their appearance on imaging. Simple cysts are the most common type and are almost always benign. They appear as round, fluid-filled sacs with thin, smooth walls and no internal divisions or solid components. Simple cysts typically do not require follow-up or treatment unless they become very large or cause symptoms.
In contrast, complex renal cysts possess features that raise suspicion for malignancy, necessitating further evaluation. These characteristics can include thicker walls, internal divisions (septations), calcifications, or solid components within the cyst. Complex cysts are less common than simple cysts and their intricate structure indicates a higher potential for being cancerous.
Assessing Cancer Risk: The Bosniak Classification
Radiologists use the Bosniak classification system to standardize cancer risk assessment in renal cysts. This system categorizes cysts into five groups (I, II, IIF, III, and IV) based on imaging characteristics, particularly on contrast-enhanced CT scans. This classification guides monitoring and treatment decisions.
Bosniak Category I cysts are simple, benign, with thin walls and no septa, calcifications, or solid components, carrying a near-zero malignancy risk. Category II cysts are also benign but may have a few thin septa or fine calcifications, with no measurable enhancement after contrast. Category IIF cysts are minimally complex, showing slightly thickened septations or walls, or thicker calcifications, and require follow-up imaging due to a small, approximately 5%, risk of malignancy.
Category III cysts are indeterminate, having thickened, irregular walls or septa that show measurable enhancement. Their malignancy risk is approximately 50%. Category IV cysts are highly suspicious for malignancy, featuring clear solid components that enhance after contrast, with a cancer probability of about 90%.
When a Cyst is Suspected or Diagnosed as Cancerous
When a renal cyst is Bosniak Category III or IV, further evaluation is recommended due to increased suspicion of malignancy. This may involve additional imaging or a biopsy to determine the nature of the cells. Surgical removal is frequently recommended for these cysts.
If a cyst is confirmed cancerous, treatment options depend on the type and stage of kidney cancer. Surgical removal of the tumor is a common approach. Active surveillance may be considered for very small tumors, while other treatments like ablation may be options. The goal is to manage the cancer effectively while preserving kidney function where possible.