Not all masses discovered in the kidney are cancerous tumours. Renal cysts represent another type of mass that is commonly found in the kidney and, depending on the type of cyst, are usually benign. Indeed, about half of all patients aged 50 years and older have such cysts. 

Renal cysts are classified differently based on their characteristics and are graded on a scale of I to IV.

• Clear renal cysts (I) found by routine ultrasound are almost always benign, regardless of size
• Complex renal cysts (II, III, IV) discovered by routine ultrasound can be benign or malignant kidney cancer and usually require further testing and follow-up. 
• Polycystic kidney disease is a condition where multiple fluid-filled cysts develop in the kidneys causing them to enlarge and appear bumpy. It is usually discovered by ultrasound or CT scan. Polycystic kidney disease may also be associated with certain pre-existing factors such as high blood pressure and urinary tract and kidney infections.