Conventional US has high accuracy in differentiating simple cysts from minimally complex cysts. On baseline US, a simple cyst is defined by the presence of a linear thin wall and anechoic content without septa, calcifications, or solid components. When a simple renal cyst is detected, no further imaging examinations are necessary. However, up to 8% of renal cysts may show a complex pattern defined by the presence of increasing intracystic echogenicity, at least one intracystic septum, wall thickening or solid nodules. In these cases, a baseline US is not reliable enough to differentiate between complex benign cysts and the 10% of renal cell carcinomas (RCC) that may appear as complex cystic lesions. In these cases, further examinations with contrast agents such as CT, MRI or CEUS are required for characterization of the lesion. The administration of a contrast agent is essential since the enhancement of solid components is the most specific sign suggesting malignancy Fig. 1
The Bosniak classification correlates the malignant potential of the cysts depending on their features. It was initially based on CT findings providing criteria for deciding whether a complex cyst should be surgically explored. The Bosniak system classifies complex cysts into five groups (I, II, IIf, III, IV) with increasing probability of malignancy.
Bosniak categories I and II are benign and characterized by no enhancement or some transient enhancement of very thin septa and do not require further examination or intervention. Conversely, surgery is recommended for categories III and IV, because they have a medium to high risk of malignancy. Category IIF is characterized by thin walls or septa with continuous or prolonged enhancement and needs a follow-up at six-monthly intervals for 1 year and yearly follow-up for 3– 5 years afterward Fig. 2
This classification was first introduced in 1986 and has undergone some adaptations, being the last update from 2019. In this last one, MRI has been formally incorporated. Regarding CEUS, its role has not been fully established in this new version of the Bosniak classification. However, some studies have demonstrated the accuracy of CEUS to be similar compared to that of CT in the characterization of complex renal cysts Fig. 3
In comparison to CT and MRI, CEUS has some advantages and disadvantages.
CEUS’ advantages:
- Use of no nephrotoxic contrast agents.
- Real-time evaluation help to differentiate confluent septa from a solid mass.
- Lack of radiation which is very useful in the follow-up of cysts.
- Very low tax of hypersensitivity and adverse events of US contrast agents.
- High sensitivity in revealing even the tiny capillaries that feed hair-line septa. Some studies have reported a higher accuracy of CEUS than CT in the detection of wall and septa microvasculature in complex cysts.
CEUS’ disadvantages:
- Lack of visualization of cysts in obese patients or deep kidneys.
- Operator-dependent.
- Large wall calcifications may hamper the visualization of possible deeper enhancing nodules or intracystic septa due to back shadowing of the calcifications.
OUR EXPERIENCE WITH CEUS:
In our routine clinical practice, we use CEUS to characterize complex cysts found on conventional US, but also to further investigate indeterminate masses found on CT or MRI.
We use the standard dose of 2.4 mL of US contrast agent approved in Europe for radiological clinical purposes (Sonovue, Bracco), which is composed of sulfur hexafluoride microbubbles stabilized with a phospholipid shell, plus a 10 mL of flush of a saline solution.
Enhancement timings are similar to those utilised for renal-specific CT imaging.
The cortical phase typically begins 10 to 15 seconds after injection and lasts 20 to 40 seconds, followed by a slower medullary phase lasting 45 to 120 seconds.
Due to its sensitivity to detect microvascularization of the cysts’ septa and walls, CEUS can be used to classify renal cysts depending on the probability of malignancy of the Bosniak classification, distinguishing renal cysts in the same 5 Bosniak categories Fig. 4 . In addition to that, features of every group can be compared to the typical enhancement and morphological features described on CT since CEUS and CT reveal similar findings in most complex cysts.
It is very important to distinguish categories II F and III, because surgery is recommended for III and IV categories, whereas category II F means follow-up.
Bosniak I:
A bosniak I cyst is benign and requires no follow-up since it has a 0% probability of malignancy.
It has a well-defined, thin («2 mm) smooth wall with homogeneous simple fluid without septa or calcifications. The wall may enhance.
Bosniak II:
A Bosniak II renal cyst with also an estimated 0% probability of malignancy has well-defined, thin («2 mm), smooth walls. They can be cystic masses with thin («2 mm) and few (1–3) septa. Septa and wall may enhance and may have calcification of any type. Fig. 5
Bosniak II F:
A Bosniak II F cyst, where ‘‘f’’ indicates need for follow-up imaging, and with an estimated 5% probability of malignancy, is a renal cyst with a smooth minimally thickened (3 mm) enhancing wall, or smooth minimal thickening (3 mm) of one or more enhancing septa, or many (»4) smooth thin («2 mm) enhancing septa. Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10
Bosniak III:
A Bosniak III cyst has an estimated 50%–70% probability of malignancy. It has one or more enhancing thick (»4 mm width) or enhancing irregular walls or septa. No enhancing solid masses are detected.
Fig. 11 Fig. 12 Fig. 13
Bosniak IV:
A Bosniak IV cyst, with an estimated 95%–100% probability of malignancy, is a clear malignant cystic mass with the presence of soft-tissue enhancing mass independent of the wall or septa. Fig. 14