Bosniak III lesions have thickened, often calcified, smooth or irregular walls and septa that possess measureable enhancement (>15 HU). About half of these cysts are malignant, and thus surgical extirpation is generally indicated.

Bosniak IV lesions possess the characteristics of category III lesions and, in addition, have enhancing soft tissue components that are adjacent to but independent of the wall or septa. The vast majority of these cysts are malignant, and thus surgical resection is always indicated.