System used to categorize renal cysts according to their malignant potential based on CT criteria. Class 1, simple cyst; class 2, cyst with benign calcification or hyperdensity or cyst with septations < 1 mm in width; classes 3 and 4, wall thickening, aggressive calcifications, abnormal density not fulfilling criteria of hyperdense cyst, nodular septations, multiloculated masses.