A cyst on my kidney? What simple and complex cysts mean

Dr. Joan J. Morales Lappot, MD
3 min read · Leer en español

In brief"They found a cyst on my kidney" is a very common reason for a visit. Most of the time, it is good news hiding behind a scary word.

Simple cysts are common

A simple kidney cyst is a thin-walled, fluid-filled pocket. They become more common with age: a large share of people over 50 have at least one. Simple cysts are not cancer, do not turn into cancer and usually need no treatment or follow-up.

The Bosniak system

Radiologists sort cysts using the Bosniak classification, based on how they look on CT or MRI:

• Category I and II: simple or minimally complex, almost always benign
• Category IIF: a little more complex; "F" means follow-up imaging
• Category III and IV: more complex, with a higher chance of cancer; usually seen by a urologist

When it is something else

Many cysts in both kidneys, especially with a family history, may point to polycystic kidney disease, which is different from scattered simple cysts. Large cysts that cause pain or block urine flow can be drained or treated, but that is uncommon.