What is the bosniak classification?
In 1986 Bosniak proposed a classification to characterize cystic kidney masses detected by CT scan as “nonsurgical” (i.e., benign) or “surgical” (i.e., requiring surgery). In his original classification, there were four categories:
• Category I: simple benign cysts (fluid-filled, no perceptible wall)
• Category II: benign cystic lesions that are minimally complicated (mural calcifications, few thin septations)
• Category IIF—added in 1993 (F = follow-up): more numerous thin septations, slight cyst wall thickening, totally intrarenal, nonenhancing, high-density lesions (i.e., hyperdense cysts)
• Category III: more complicated cystic lesions (calcifications, thickened or numerous septations, enhancement of the septations, mural nodules, thickened, irregular wall)
• Category IV: lesions that are clearly malignant cystic carcinomas (mural nodules with vascularization, enhancement of solid components)
Categories I and II are considered nonsurgical, whereas categories III and IV are surgical. The risk that a category III or IV lesion is malignant is approximately 50% (range, 25% to 100%). The risk of malignancy in a Bosniak IIF cystic lesion is approximately 5%.
