SIMPLE CYSTS
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.
