CT and MR imaging of cystic renal lesions

The most widely used system to classify cystic renal lesions was introduced by Bosniak in 1984 and revised in 1997 [14, 15]. This system was originally developed on CT findings, but it can be also used at MRI [11, 16, 17].

Renal cysts are divided into five categories on the basis of imaging appearance (Table 1, Fig. 2). Each Bosniak category reflects the likelihood of cystic RCC that ranges from I (simple cyst) to IV (cystic tumors). Category I, II, and, IIF cysts are nonsurgical, while categories III and IV are surgical.

Imaging findings include attenuation/signal intensity, size, presence of calcifications, septa and enhancing nodularity. Among these, enhancing nodularity is considered the most important predictor of malignancy [18]. At CT, enhancement requires an increase of attenuation of at least 15–20 HU from unenhanced to the contrast enhanced images [18]. A 10–15 HU change in attenuation can be due to incorrect placement of the region of interest, patient motion, or beam hardening from adjacent enhancing renal parenchyma (the so called “pseudoenhancement”) [19]. To overcome this problem, it has been suggested to use dual-energy CT, where true unenhanced images can be replaced by virtual unenhanced images [20]. Iodine quantification and iodine-related attenuation are used to differentiate nonenhancing cysts from enhancing solid masses [20].

In equivocal cases, another option is to use subtraction MRI to assess the presence or absence of enhancement [21].

Septa are defined as dividing wall within a renal cyst and are best appreciated at MRI than at CT. When present, they can be classified as thin, minimally thickened, or grossly thickened and irregular, and as enhancing or non-enhancing.

Calcifications are usually easy to recognize at CT but may be unapparent at MRI. Despite the importance in predicting the malignancy of solid renal masses, calcifications have limited utility in the Bosniak classification since they can be found in the wall or septa of both benign and malignant cysts [22]. Similarly, the size does not reliably predict the benignity or malignity of a renal cyst. Indeed, larger cysts can be benign and small ones can be malignant.

Cystic RCC is relatively rare and comprises approximately 3–15 % of all cases of RCCs. It is found more commonly in younger age and in females compared with solid RCC [33]. The cystic appearance can be related to their inherent architecture or secondary to cystic degeneration and extensive necrosis [34]. Clear cell type RCC is the most common subtype, followed by papillary and chromophobe RCC. Clear cell type RCC can show a dominant cystic component or can arise in a simple cyst [35]. Multilocular cystic RCC of low malignant potential is a rare variant of clear cell type RCC with no reported recurrence or metastasis. This tumor is composed exclusively by cysts with low-grade tumor cell [36] and shows a variable imaging appearance, which ranges from category IIF to category IV renal cysts [35]. Papillary RCC can appear as a cyst with hemorrhagic or necrotic content and a thick pseudocapsule [35]. Cystic renal RCCs have a more favorable prognosis of all subtypes of RCC: they have a low Fuhrman grade, grow slowly, and rarely metastasize or recur [37].

Renal metastases are not uncommon, with reported frequencies ranging from 7 to 20% at post-mortem studies [38‐41]. The most common primary malignancies are the lung, breast, gastrointestinal tract, and melanoma. CT and MR imaging diagnosis is less frequent because post-mortem studies included microscopic lesions, which are beyond CT resolution [42, 43].

Renal metastases can show a solid or cystic appearance. The differentiation of renal metastasis from RCC on the basis of CT and MR findings alone may be impossible [42‐44]. However, some features are likely to be distinctive: renal metastases are frequently multiple, bilateral and small [42, 43].

The MESTs area heterogeneous group of rare renal tumors occurring predominantly in perimenopausal women (female-to-male ratio, 11:1). The MEST appears as a well-marginated lesion with a variable proportion of solid and cystic components [45]. Septa and nodules can show heterogeneous and delayed enhancement [45]. MEST can show an exophytic growth or herniate into the renal pelvis [45]. Adult cystic nephroma is now classified within MEST family due to similar histologic and epidemiologic findings [36]. This tumor appears as an encapsulated lesion, with cysts of variable size, and thin, variably enhancing, septa [46]. Calcifications are peripheral and curvilinear [46]. Solid components are typically absent [46]. At MRI, the capsule and septa can show hypointensity on both T1- and T2-weighted images due to the fibrous composition. Since imaging features are non-specific, differentiation between MEST and cystic RCC requires pathologic examination.

Renal abscess is an uncommon entity that usually results from a complication of untreated or inadequately treated acute pyelonephritis or ascending urinary tract infection. More rarely, it results from hematogenous spread from an extra-urinary source of infection (e.g., diverticulitis, pancreatitis). Patients may present with signs and symptoms of infection. Renal abscess can appear as a complex renal cyst with inhomogeneous areas of fluid attenuation/intensity and a thick and irregular wall that shows a little enhancement on excretory phase (Fig. 10). Because of the presence of viscous pus, the fluid component shows a characteristic strong and heterogeneous diffusion restriction on diffusion-weighted imaging, which favors the diagnosis of renal abscess over that of RCC [47]. Renal parenchyma around the abscess can show low density/intensity on early phases and delayed enhancement [48, 49]. Fat stranding is often found adjacent to the renal abscess [50]. Gas can be rarely present within the lesion and strongly suggests abscess formation. When imaging findings, clinical history and laboratory tests do not permit a confident differentiation between renal abscess and cystic RCC; biopsy/drainage should be performed to obtain the correct diagnosis.

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disorder and occurs in approximately one of 500 live births [51]. Mutations in one of the two genes encoding plasma membrane—spanning polycystin 1 and polycystin 2 (PKD1 and PKD2)—are responsible of the disease. It is characterized by progressive development and growth in size of simple renal cysts, leading to symmetric enlargement of the kidneys and chronic renal failure by late middle-age [52, 53] (Fig. 11). Cysts have variable dimension (from few millimeters to several centimeters) and are diffusely distributed through the kidneys. Cyst complications include hemorrhage, pyogenic infection, and, more rarely, rupture. The risk for RCC is not increased in comparison with the general population except in patients on dialysis [54]. The added risk of malignancy in dialysis patients is probably related to the effects of coexistent acquired cystic renal disease [54].

Hepatic cysts are the most common extra-renal manifestations of ADPKD and show variable number, size, location, and distribution [51, 54]. Polycystic liver disease is uncommon and leads to hepatomegaly [51, 54]. More rare hepatic complications include congenital hepatic fibrosis and segmental dilatation of biliary tract [54].

The other extra-renal manifestations of ADPKD include cysts in other organs such as pancreas and non-cystic abnormalities such as cardiac valvulopathies and intracranial aneurysms [51]. Imaging plays a crucial role in the identification of ADPKD in high-risk individuals (those with a positive family history). The diagnosis of ADPKD requires at least three renal cysts (unilateral or bilateral) in high-risk patients 15–39 years of age, at least two cysts in each kidney in high-risk patients 40–59 years of age, and several bilateral renal cysts in high-risk patients 60 years of age or older [55]. Since renal enlargement correlates with a decline of renal function, estimation of renal volume can predict the risk for renal failure [53].

Acquired cystic kidney disease (ACKD) is a consequence of sustained uremia in patient with end-stage renal disease [52]. The disease is found in 8–13% patients with end-stage renal disease and in approximately 50% patients on dialysis. The disease is multifactorial. It is the progressive destruction of renal functioning nephrons with compensatory hypertrophy of remaining renal parenchyma, obstruction of renal tubules by interstitial fibrosis or oxalate crystals, and cyst formation [52]. Kidneys are atrophic and contain multiple cysts with variable size (from few millimeters to several centimeters) and imaging appearance (Fig. 12). Since renal cysts are extremely common in the adult population, the diagnosis of ACKD requires the presence of three or more cysts in each kidney, in conjunction to end-stage renal disease, and no history of hereditable renal disease [56]. Cyst hemorrhage is a common complication and can cause hematuria, whereas cyst rupture, perinephric hematoma, and retroperitoneal hemorrhage are less frequent [52]. Development of RCC in the wall of the cyst is the most serious complication of ACKD, with a higher rate in comparison to the general population [55].

The most common tumor type in patients with ACKD is acquired cystic disease (ACD)-associated RCC, followed by papillary and clear cell type RCC [36]. ACD-associated RCC has unique morphologic features and is found exclusively in patients with end-stage renal disease and ACKD [36].

Long-term lithium therapy is a well-known cause of nephrotoxicity in the form of polyuria-polydipsia syndrome (diabetes insipidus) and chronic renal insufficiency [57]. Characteristic imaging findings include normal or slightly decreased size of kidneys with multiple, uniformly, and symmetrically distributed microcysts [58]. Microcysts measure 1–2 mm in diameter and are located in both cortex and medulla [58].

Localized cystic renal disease is a rare, nonhereditary, form of cystic renal disease, which manifests as a conglomeration of multiple simple cysts of variable size [59] (Fig. 13). In contrast to ACKD and ADPKD, localized cystic renal disease is typically unilateral and not progressive. The disease usually involves only a portion of the kidney with a polar predilection [59]. Entire renal involvement is rare [58]. The contralateral kidney is normal. The presence of interposed normal renal parenchyma and the absence of a capsule help to differentiate localized cystic renal disease from cystic nephroma and multiloculated cystic RCC [58]. Cystic involvement of other organs is typically absent [58].