Class I lesions are simple, benign cysts. The characteristics of a simple cyst include a well-defined and homogeneous mass with a thin, imperceptible wall. These lesions do not enhance and have the attenuation value of water density. If these criteria are met, no further workup is necessary.
Class II lesions are minimally complicated cysts with features that cause concern. Internal septations that are smooth, thin and do not have thickened elements may be considered benign. A thin, peripheral rim of calcification in the cyst wall or septa may also be considered benign. These lesions do not show enhancement, have smooth, sharp margins, and are homogeneous. No further workup is necessary.
Class IIF lesions are more suspicious cysts. These lesions are minimally complicated cysts but may be hyperdense, may contain more calcium in the wall, or may have thicker internal septations. Follow up scanning is recommended.
Class III lesions are more complicated cystic structures that often show irregular and thickened septa, wall thickening, solid non-enhancing mural nodules, or irregular calcifications. This category also includes multiloculated cysts. These lesions require surgical exploration.
Class IV lesions show non-uniform wall thickening, have irregular margins, and/or contain solid components that enhance on CT. These lesions are clearly malignant and total nephrectomy is warranted.
Key Points:
Class I: Simple cysts, 100% benign
Class II: Minimally complicated cyst, 100% benign
Class IIF: Minimally complicated cyst with suspicious features, follow up scan recommended, most likely benign
Class III: Moderately complicated or multiloculated cyst, 50% malignant, surgery recommended
Class IV: Cleary malignant lesion, surgery recommended