The Bosniak classification of renal cysts has been a has been a relatively consistent way of distinguishing benign renal cystic disease from renal cell carcinoma (or other cancers) since 1986. This system uses radiologic findings from CT or ultrasound to classify renal "cysts" into four categories.
Israel & Bosniak - Click for HyperLink
Table of Classes - Click for HyperLink

• Class 1:
» Homogeneous water attenuation
» No internal architecture
» No enhancement
The first category includes only simple cysts that are homogeneous, have well defined margins, are -10 to +20 Hounsfield units in density (like that of water), do not contain septa, calcifications, nor solid components; and, they do not enhance with contrast. This is the most common category, occurring in at least 50% of the population over 50 years of age. This is also the most benign category and requires little if any follow-up.

• Class 2:
The second category includes simple cysts that may have additional radiologic findings. Findings that will increase the classification from Class 1 to Class 2 include:
» the presence of thin hyaline septa
» mild calcification in the cyst wall or in the thin septa
» uniform hyperdensity

These have < 5% chance of malignancy and often are not provided follow up.

There has been much controversy in the literature about inter-observer variability when it comes to the distinguishing between class 2 and class 3 cysts. This forced the classification system to give rise to a new category so as to more accurately describe the range between Class 2 and Class 3 cysts.

• Class 2F (meaning "needs follow-up"):
» multiple thin septa that may have mild or moderate nodular calcification
» mild thickening of the wall or septa
» minimal "perceived enhancement" of cyst wall or septa
» non-enhancing hyperdense intrarenal cysts > 3cm

Class 2F cysts may require 6 month follow-up exams over 24 months.

All Class 2 and 2F cysts may NOT have "measurable enhancement" with contrast and should be generally well marginated.

• Class 3:
The third category includes complex cysts that have irregular, thickened walls and/or septa. These cysts may have extensive calcification and will enhance with contrast. However, these cysts should be relatively homogeneous in density and should not contain solid components.

These masses need surgical intervention in most cases, because they may be:
» MLCN - Multiloculated Cystic Nephroma
» Hemorrhagic cysts
» Infected cysts

• Class 4:
The fourth and last Bosniak category can include all of the previously stated components - but show definite enhancing soft-tissue regions, appear heterogeneous in density, and have various solid or hypercalcified inclusions - indicating possible necrosis. In previous studies, this class has always been shown to indicate cancer on follow-up pathological report - and they should be removed.

Many small retrospective studies have shown the Bosniak classification system to be quite useful in making a clinical decision between choosing surgical therapy versus clinical follow-up with serial CT scans.

Its only criticism comes from individuals who feel that the radiologic inter-observer variability is too large and is a major diagnostic concern.

eMedicine - Click for HyperLink

CTISUS - Click for HyperLink

Radiology - Click for HyperLink

AJR - Click for HyperLink