Table 2.

The Bosniak renal cyst classification system

Category: Criteria	Management
IA: Benign simple cyst with a hairline thin wall that does not contain septa, calcifications, or solid components; it has water attenuation and does not enhance	No intervention
IIA: Benign cystic lesion that may contain a few hairline thin septa, in which perceived (not measurable) enhancement may be appreciated; fine calcification or a short segment of slightly thickened calcification may be present in the wall or septa; uniformly high-attenuating lesions (3 cm) that are sharply marginated and do not enhance are included in this group	No intervention
IIB: Cysts may contain multiple hairline thin septa; perceived follow-up (not measurable) enhancement of a hairline thin smooth septum or wall can be identified; there may be minimal thickening of wall or septa, which may contain calcification that may be thick and nodular, but no measurable contrast enhancement is present; there are no enhancing soft tissue components; totally intrarenal nonenhancing high-attenuating renal lesions (3 cm) are also included in this category; generally well marginated	Repeated imaging
III: Cystic masses with thickened irregular or smooth walls or septa, in which measurable enhancement is present; these masses need surgical intervention in most cases, because neoplasm cannot be excluded; this category includes complicated hemorrhagic or infected cysts, multilocular cystic nephroma, and cystic neoplasms	Surgical for histology or resection
IV: Clearly malignant cystic masses that can have all of the criteria of category III but also contain distinct enhancing soft tissue components independent of the wall or septa	Surgical resection

Adopted from reference 27, with permission.