[Cystic renal lesions]

Cystic renal lesions are most often simple or complicated cysts, which can be seen solitary or as part of cystic renal disease. The minority of these lesions are benign or malignant cystic tumors. The classification of cystic renal masses by Bosniak (category l - IV) based on specific ultrasound and CT features is very useful for the characterization of the lesion and for the therapeutic decision. The main objective of this classification is to differentiate nonsurgical (category II) from surgical cystic masses (category III/IV). Ultrasound is the first modality of choice in the diagnostic work-up of cystic renal masses, because an accurate and economically reasonable diagnosis of the frequent simple cyst can be made by maintaining rigid ultrasound criteria of the Bosniak classification. If a complicated cyst or a cystic tumor is suspected a three phasic contrast-enhanced CT of the kidneys should be performed. MRI is superior to CT in the characterization of complex cystic masses.