Last week, I got an email from a residency friend asking me what I would do with a patient who had a Bosniak II renal cyst. Since “I don’t know” no longer seemed a satisfactory answer for someone who specializes in kidneys, I looked it up, returned her email, then decided to review the radiologic classification of renal cysts on the blog, for anyone else who would like a refresher. An excellent review by Glickman et al can be found in the journal Radiology.
Renal cysts are masses filled with fluid. They are extremely common: some studies have estimated their incidence in the general public at around 50%. Most are discovered incidentally, since they rarely cause symptoms. Flank pain and hematuria are occasionally seen. Radiologic features of renal cysts convey important information about their malignant potential—our main concern as physicians. Does the wall of the cyst appear thin and simple, or does it have a thickened lining? Is the fluid in the cyst attenuated? How heterogeneous does the cyst appear? Are there septae or enhancing soft tissue components? In 1986, a classification scheme was published by Bosniak that stratified malignant risk based on characteristics of the cyst on CT scan. The Bosniak classification remains a useful guide for management of incidentally-discovered renal cysts. It should be emphasized, however, that evaluation of cysts must take into account a patient’s demographics and medical history: a diagnosis cannot be made by imaging alone.
Bosniak Classification of Renal Cysts
Bosniak I- benign, simple cysts, with thin walls. Note that size is not a factor in classifying cysts in this category. Bosniak I cysts are always benign, and can be ignored. These cysts are not attenuated, and tend to have Hounsfield units of 0-20.
Bosniak II- benign, minimally complicated cysts; may have hairline-thin septa with some perceived enhancement. There may be fine calcification or areas of calcified thickening along the wall or septa. Fine calcification or a short segment of slightly thickened calcification may be present in the wall or septa. Homogeneous hyperattenuating cysts also belong in this category—high Hounsfield units usually signify proteinaceous fluid. Bosniak II cysts are also felt to have non-malignant potential, and can be ignored by the physician.
Bosniak IIF- these are slightly more complicated that class II cysts, and so warrant observation. IIF cyts may contain multiple, hairline-thin septa that demonstrate perceived (but not measurable) enhancement. There may be minimal smooth thickening of the wall or septa. Calcification that is more irregular and thicker than in Bosniak II cysts may be present. Soft tissue elements are absent. Hyperattenuating cysts otherwise classified as category II cysts, but are > 3cm and completely within the renal parenchyma are included in class IIF. The general recommendation for observation of these cysts is to reimage with CT or MR in six months, then yearly for a minimum of five years. If they remain stable, it can be assumed that they are benign, and no further follow-up is needed.
Bosniak III- the malignant potential of class III masses is indeterminate on imaging; therefore, surgical removal is recommended. These cysts have thickened walls or septa, and display enhancement. They include multilocular cysts (in which the walls have fibrous lining), hemorrhagic or infected cysts, multilocular cystic nephroma (which conatin blastemal elements), or cystic renal cell carcinoma. It is estimated that 50% are malignant (studies have shown malignancy rates ranging from 31% to 100%).
Bosniak IV- share features of Bosniak III, plus enhancing soft-tissue components adjacent to or separate from the wall or septa. They are almost always malignant. Management is surgical.
The Bottom Line: radiologic followup is needed only for class IIF lesions. The others are either ignored or referred for surgical management secondary to the high rates of malignancy.