Renal cysts sometimes occur as part of an inherited disease, such as polycystic kidney disease (see Plate 2-15), nephronophthisis/medullary cystic kidney disease (see Plate 2-18), tuberous sclerosis, or von Hippel-Lindau syndrome. In clinical practice, however, most renal cysts are sporadic and incidentally discovered during abdominal imaging performed for some other indication. Such cysts, known as “simple cysts,” are very common among adults over the age of 50 and rarely cause symptoms.
Although a majority of renal cysts are benign and require no treatment, a subset may contain renal cell carcinoma and require surgical extirpation. In an attempt to quantify the likelihood of malignancy, each cyst is graded according to the Bosniak system, which considers its appearance and enhancement characteristics on computed tomography (CT).
A Bosniak I cyst is a true “simple cyst” and is the most common type of cyst seen in general practice. It is surrounded by a hairline thin, smooth, nonenhancing wall that sharply demarcates it from the surrounding renal parenchyma. No internal septations are seen. Its ﬂuid contents appear homogeneous and nonenhancing, with the same density as water (-20 to 20 Hounsﬁeld units [HU]). Calciﬁcations and solid components are not seen. The risk of malignancy is near zero, and further evaluation is not required. Of note, a cyst seen on ultrasonography can also be classiﬁed as “simple” if it is anechoic, sharply deﬁned, and has an enhancing posterior wall, which indicates adequate transmission through the ﬂuid contents.
A Bosniak II cyst possesses a thin, smooth, nonen-hancing wall but may also possess a few hairline septa with very ﬁne or short areas of calciﬁcation. The septa may have “perceived contrast enhancement,” meaning there is the subjective perception of slight enhancement, which has been ascribed to the presence of contrast in the ﬁne capillaries that supply the septa. No enhancement, however, should be quantiﬁable. Also included in this category are nonenhancing cysts that are less than 3 cm in diameter and possess ﬂuid contents with a uniformly higher attentuation than water because of the presence of degenerated blood. Like class I cysts, class II cysts have a very low risk of malignancy and often do not require further follow-up.
A Bosniak type IIF cyst may have minimal smooth thickening of its external wall, as well as a greater number of internal septa. In addition, thick or nodular areas of calciﬁcation may also be seen. Nonetheless, no actual contrast enhancement should be seen in the wall, septum, or ﬂuid contents. Also included in this category are nonenhancing cysts greater than 3 cm in diameter that have uniformly hyperattenuating ﬂuid contents. The “F” is for “follow” because these lesions should be closely followed with regular CT imaging, which will reveal whether they are stable or progressive.
Bosniak III lesions have thickened, often calciﬁed, smooth or irregular walls and septa that possess measureable enhancement (>15 HU). About half of these cysts are malignant, and thus surgical extirpation is generally indicated.
Bosniak IV lesions possess the characteristics of category III lesions and, in addition, have enhancing soft tissue components that are adjacent to but independent of the wall or septa. The vast majority of these cysts are malignant, and thus surgical resection is always indicated.