Benign renal cysts are common in the general population and are detected on CT in up to 50% of patients older than 50 years.
Most renal cysts represent benign renal cortical lesions and do not require any follow-up imaging. However,
as renal cysts become more complex,
it becomes increasingly difficult to make the distinction between benign and malignant lesions,
and also the probability of malignancy increases.
Approximately 10–15% of all renal cell carcinomas can appear as complex cystic lesions on imaging studies [1].
Bosniak classified renal cysts into four main categories [2] and one more sub-category [Fig.1],
on the basis of imaging appearance,s in an attempt to predict the risk of malignancy.
Category I.—Category I lesions are simple benign cysts showing homogeneity,
water content,
with no wall thickening,
calcification,
or enhancement and have sharp distinct borders from surrounding renal parenchyma.
Category II.— This group consists of benign lesions that are either hyperdense benign cysts with all the features of category I cysts except for homogeneously high attenuation,
or consists of cystic lesions with one or two thin ( ≤1 mm thick) septations or thin,
fine calcification in their walls or septa A category II lesion to be considered benign must be 3 cm or less in diameter,
have one quarter of its wall extending outside the kidney so the wall can be assessed,
and be nonenhancing after contrast material is administered.
Category IIF.— This is a group not well defined by Bosniak but consists of lesions that do not have the clearly benign characteristics of category II,
but do not fall into category III either.
These lesions are minimally complicated cysts with some suspicious features that deserve follow-up to detect any change in character.
Category III.—Category III consists of true indeterminate cystic masses that need surgical evaluation,
although many prove to be benign.
They may show uniform wall thickening,
nodularity,
thick or irregular peripheral calcification,
or a multilocular nature with multiple enhancing septa.
Hyperdense lesions that do not fulfill category II criteria are included in this group (>1mm thick septations).
Category IV.—These are lesions with a non-uniform or enhancing thick wall,
enhancing or large nodules in the wall,
or clearly solid components in the cystic lesion.
Enhancement was considered present when lesion components increased by at least 10HU.]
Furthermore,
the Bosniak classification system advocates treatment for each category.
Categories I and II were considered leave-alone lesions,
with follow-up imaging recommended for category IIF lesions.
Surgical resection was recommended for category III and IV lesions [3,4]. In series,
the reported incidence of malignancy was 0% for category II,
5% for category IIF [5],
25-45% for category III,
and 100% for category IV lesions.
The range of probability in category III is a reflection of the qualitative parameters of evaluation,
different levels of experience of the observers and suboptimal images [6].
Sometimes the distinction between categories IIF and III lesions can be difficult.
In those cases there is a tendency to upgrade category II lesions to category III when any uncertainty exists and Bosniak,
himself,
has advocated placing borderline II–III lesions,
especially hyperdense cysts,
into category III [7].
It is extremely important to place a renal cyst into the right category because that will lead to the proper management; IIF=benign cyst that needs follow up,
III= probably malignant cyst that immediately indicates renal excision.
![](https://epos.myesr.org/posterimage/esr/ecr2012/108978/media/402021?maxheight=300&maxwidth=300)
Fig. 1: 1.Category I lesions: In Category I, are simple benign cysts with thin walls, no septa, or calcification and contains fluid with attenuation of water. Category II lesions: In category II lesions, are benign cysts with hairline-thin septa, fine calcification in the walls or septa. Category IIF (follow up)lesions: In category IIF lesions, are more complex cysts which may contain an increased number of septa and amount of calcification, which may be thicker or nodular.A change of less than 10HU from pre- to post-contrast images is usually considered typical of a benign cyst. Cysts in that category need follow up imaging for a period of 2 years. If there is stability in the findings then the cyst is considered to be benign. Category III lesions: In category III lesions, are cysts with thick, irregular walls or septa, and may contain either small or large amounts of calcification and wall enhancement. Category IV lesions: In category IV lesions, are malignant cystic masses which look like the ones in category III but they have also enhancing soft-tissue components independently from the wall or the septum.
One-hundred-twenty-six patients underwent MDCT at our department,
during the period 2006-2010,
and were diagnosed having cysts or cyst’s like renal tumors.
The cysts were either incidentally detected during abdominal sonography,
CT scan or during clinical diagnostic workup including sonography and CT of the kidneys and urinary track for micro or macroscopic hematuria.
The cystic renal masses were evaluated with a MDCT scanner (16 and 4-Slice CT).
Unenhanced images (5mm slice thickness) were acquired first and then images during the corticomedullary (3 mm slice thickness) and the nephrographic phase after contrast media administration (150mL nonionic iodinated contrast agent at a rate of 2-3mL/s).
The images were used to perform three dimensional reconstructions and were evaluated for lesion size,
the presence of calcification,
septations,
nodules,
the wall thickness,
the density,
the enhancement and were placed in a category according to the Bosniak system classification.
Renal units that had cystic lesions in more than one category were placed in the greater category.
The renal cysts categorized Bosniak III and IV underwent nephrectomy and the images were then compared with the histological findings.