Simple and complex kidney cysts in adults

INTRODUCTION — Kidney cysts result from genetic or nongenetic processes and occur in a variety of diseases in adults and children. The most common causes of radiologically evident kidney cysts in adults are simple kidney cysts, which will be discussed in this topic review. Autosomal dominant polycystic kidney disease (ADPKD) is discussed elsewhere. (See "Autosomal dominant polycystic kidney disease (ADPKD): Kidney manifestations" and "Autosomal dominant polycystic kidney disease (ADPKD): Treatment".)

Smaller cysts, usually <1 cm in diameter, also occur in medullary sponge kidney, autosomal recessive PKD, and autosomal dominant tubulointerstitial kidney disease (previously called medullary cystic kidney disease). (See "Medullary sponge kidney" and "Autosomal recessive polycystic kidney disease in children" and "Autosomal dominant tubulointerstitial kidney disease".)

Other unusual causes of kidney cysts in adults are von Hippel-Lindau disease, tuberous sclerosis complex, Fabry disease, and nephronophthisis. (See "Clinical features, diagnosis, and management of von Hippel-Lindau disease" and "Renal manifestations of tuberous sclerosis complex", section on 'Renal cysts' and "Fabry disease: Clinical features and diagnosis", section on 'When to suspect Fabry disease' and "Genetics and pathogenesis of nephronophthisis", section on 'Pathogenesis'.)

An overview of the kidney cyst classification system, our recommended approach to complex kidney cysts in adults, and a discussion of simple kidney cysts are presented in this topic review. The evaluation of a solid kidney mass is discussed separately. (See "Diagnostic approach, differential diagnosis, and management of a small renal mass".)

GENERAL PRINCIPLES

Types of kidney cysts — Kidney cysts are categorized as simple or complex. Simple kidney cysts are commonly observed in normal kidneys, with an increasing incidence as individuals age [1] (see 'Simple kidney cysts' below). They are benign, asymptomatic lesions that rarely require treatment. By contrast, complex cysts may require follow-up imaging, biopsy, or surgical excision for diagnosis and management.

The primary clinical concern is accurately distinguishing simple kidney cysts from complex kidney cysts that are associated with an increased risk of malignancy. This increase in risk is primarily seen in categories III and IV in the Bosniak classification. (See 'Bosniak classification of kidney cysts' below.)

Bosniak classification of kidney cysts — The Bosniak kidney cyst classification system was created to help diagnose and manage complex cysts [2]. In this schemata, simple cysts, which are at minimal risk for malignant transformation and do not require further evaluation, are classified as Bosniak category I. (See 'Management according to Bosniak category' below.)

The Bosniak classification of a kidney cyst is based upon specific morphologic and enhancement characteristics with unenhanced and intravenous (IV) contrast-enhanced computed tomography (CT). It should only be used for cystic lesions ≥1 cm in size since smaller cysts may appear to enhance due to volume averaging from adjacent normal kidney parenchyma. For cysts <1 cm, ultrasonography or magnetic resonance imaging (MRI) is recommended for evaluation.

The five Bosniak categories (I, II, IIF, III, and IV) are associated with a risk of malignancy that starts from near zero (Bosniak I; simple cysts) and rises to over 85 percent (Bosniak IV). The Bosniak categories are detailed below (table 1):

●Category I – A benign simple kidney cyst or multiple kidney cysts, each with a thin wall without septa, calcifications, or solid components. The cyst has the density of water and does not enhance. (See 'Simple kidney cysts' below.)

●Category II – Benign cystic lesions in which there may be a few thin septa and the wall or septa may contain fine calcifications or a short segment of slightly thickened calcification. This category also includes uniformly high-attenuation lesions that are <3 cm in diameter, well marginated, and nonenhancing (image 1).

●Category IIF – Cysts in this category are generally well marginated and are more complicated than category II cysts but less complicated than category III cysts. They may have multiple thin septa or minimal smooth thickening of the septa or wall, which may contain calcification that may also be thick and nodular. They have no measurable contrast enhancement. However, these lesions may have subjectively perceived enhancement of the septa or wall when the unenhanced and contrast-enhanced images are compared. This category also includes totally intrarenal, nonenhancing, high-attenuating lesions that are >3 cm in diameter. These cysts require follow-up to ascertain that they are not malignant.

●Category III – Cystic masses that have thickened irregular or smooth walls or septa. Measurable enhancement is present. Approximately 40 to 60 percent are malignant (cystic renal cell carcinoma and multiloculated cystic renal cell carcinoma) [2-4]. The remaining lesions are benign and include hemorrhagic cysts, chronic infected cysts, and multiloculated cystic nephroma.

●Category IV – Category IV lesions have all the characteristics of category III cysts, plus they contain enhancing soft-tissue components that are adjacent to and independent of the wall or septum (image 2). Approximately 85 to 100 percent of category IV lesions have been reported to be malignant in various studies [2,3,5,6].

The presence of measurable contrast enhancement of the lesion is the most important characteristic separating categories III and IV (typically associated with malignancy in 40 to 100 percent of cases) from categories I, II, and IIF (typically benign) [2]. Such contrast enhancement should be at least 10 to 15 Hounsfield units higher on contrast-enhanced images when compared with unenhanced images.

A 2019 version update of the Bosniak classification, endorsed by the Society of Abdominal Radiology, has been proposed [7]. This update includes quantitative definitions of the qualitative terms (eg, "thin" changed to "≤2 mm," "minimally thickened" changed to "3 mm") and descriptive modifications (eg, homogenous or heterogenous by Hounsfield units) to improve the specificity of higher-risk categories. The goal of this 2019 update was to improve the interreader agreement and cross-study consistency, and to decrease the proportion of benign masses that are unnecessarily resected. However, additional validation studies using this 2019 update are needed before it can be adopted into routine clinical practice. We continue to use the original Bosniak classification as discussed below until such validation studies become available.

SIMPLE KIDNEY CYSTS — Simple kidney cysts are frequently observed in normal kidneys. They are the most common kidney masses, accounting for approximately 65 to 70 percent of cases [8]. The cysts may be solitary or multiple and bilateral [5,8-10].

The evaluation of solid kidney masses and other noncystic kidney lesions is discussed separately. (See "Diagnostic approach, differential diagnosis, and management of a small renal mass" and "Evaluation of the incidental kidney lesion".)

Epidemiology — The prevalence of simple kidney cysts varies with the population studied and the imaging modality utilized. These cysts occur most often in patients >50 years of age and are more common in males than females, as determined from postmortem examination, kidney ultrasonography, and/or abdominal CT scanning [9,11-15].

The following findings are illustrative:

●A Japanese study of 14,314 individuals undergoing multiphasic health screening programs found at least one kidney cyst on ultrasonography in 12 percent [13]. Cyst formation was twice as likely in males compared with females, and a sevenfold increase in prevalence was noted from the fourth to the eighth decade of life (5 to 36 percent). The majority of cysts increased in size over time, with the mean rate of growth being higher in patients <50 years of age (3.9 versus 1.8 mm per year in older patients).

●In a series of 1948 potential kidney donors, a cortical, medullary, or parapelvic cyst ≥5 mm was detected by CT in 12, 14, and 2.8 percent of individuals, respectively [15]. The prevalence of cortical or medullary cysts was higher among donors aged 50 to 75 years compared with those aged 18 to 49 years (33 versus 12 for cortical cysts and 17 versus 12 percent for medullary cysts).

●In a report of 729 patients who underwent ultrasonography for reasons unrelated to the urinary tract, the incidence of at least one kidney cyst according to age and sex was [9]:

•15 to 29 years of age – 0 (males) and 0 (females)

•30 to 49 years – 1.9 and 1.4 percent

•50 to 69 years – 15 and 7 percent

•≥70 years – 32 and 15 percent

•Bilateral cysts were much less common and were rare in subjects <50 years of age

Histopathology — Simple kidney cysts are oval or round in shape and of varying size, ranging from <1 to >10 cm [16]. The lining of the cyst is a single epithelial cell layer without kidney tissue elements. There is a clear to straw-colored fluid that resembles a plasma ultrafiltrate [16].

Clinical features — Simple kidney cysts may be solitary or multiple and bilateral [5,8-10]. They typically produce no symptoms or signs and as such are generally detected incidentally during radiographic studies performed for other reasons. The clinical significance of simple kidney cysts is not completely clear. Most nephrologists have believed simple kidney cysts to be of little significance. However, in a study cited above of 1948 healthy potential kidney donors, after adjusting for age and sex, cysts ≥5 mm were associated with higher albumin excretion, hypertension, and hyperfiltration [15]. An association with hypertension has also been reported in other studies [17-19] but not in a series of over 1000 patients in whom there was no increase in the incidence of hypertension, flank pain, hematuria, or proteinuria when compared with matched controls without cysts [11].

Unless obstruction is present, simple kidney cysts do not compromise kidney function [20]. However, limited evidence suggests that younger patients with simple kidney cysts may have mildly reduced (but still normal) kidney function [21].

Simple kidney cysts are rarely associated with rupture, hematuria, pain, abdominal mass, infection, and/or hypertension:

●Rupture of a simple kidney cyst can cause hematuria and/or flank pain. In one of the largest series of 25 patients with simple kidney cyst rupture, 21 had hematuria, and 17 had flank pain [22]. The cyst rupture was spontaneous in 21 patients and due to blunt abdominal or iatrogenic (during a procedure) trauma in four patients. Communication with the collecting system spontaneously closed in 11 patients and persisted in two patients.

●Infection of a simple kidney cyst is a rare cause of kidney abscess. The clinical features of an infected simple kidney cyst are the same as those of kidney abscess, which are discussed elsewhere. (See "Renal and perinephric abscess", section on 'Clinical manifestations'.)

●Compression of the surrounding kidney parenchyma rarely causes angiotensin-dependent hypertension [23,24]. In a retrospective review of 22 cases in the literature, removal or cyst aspiration cured or lowered the blood pressure in 15 and 2 patients, respectively [24]. The drop in blood pressure was closely related to reduced activity of the renin-angiotensin system in the involved kidney, which harbored a large cyst in all cases.

Despite this observation, hypertension in patients with simple kidney cysts is most commonly due to factors independent of the cyst, such as essential hypertension. This is particularly true in older patients, in whom both simple kidney cysts and hypertension are common.

●Simple kidney cysts rarely cause calyceal or renal pelvic obstruction [25].

Diagnosis of simple cysts — Simple kidney cysts have characteristic features on ultrasonography, CT, and MRI. Since most simple kidney cysts are asymptomatic, the initial imaging study that detects such a cyst is usually performed for unrelated reasons. If radiographic criteria for a benign simple kidney cyst are met, additional follow-up is not required.

Ultrasonography — There are three major criteria for a single simple kidney cyst on ultrasonography, which, if present, allow the cyst to be differentiated from a carcinoma or abscess [5,26]:

●The mass is round and sharply demarcated with smooth walls.

●There are no echoes (anechoic) within the mass.

●There is a strong posterior wall echo, indicating good transmission through the cyst and enhanced transmission beyond the cyst.

Further evaluation is not necessary if all of these criteria are satisfied, since the likelihood of malignancy is very low [5,8,26].

Additional imaging should be performed if the ultrasonogram findings are not consistent with a simple cyst (eg, calcifications or septa are seen), if the study quality does not permit adequate evaluation of all criteria for a simple cyst, or if multiple kidney cysts are clustered in a pattern that could mask an underlying carcinoma. Abdominal CT obtained before and after the (intravenous) IV administration of radiocontrast media can classify the lesion into different categories that determine subsequent management. (See 'Bosniak classification of kidney cysts' above.)

The accuracy of ultrasonography in evaluating simple kidney cysts was illustrated in a retrospective study that assessed 260 lesions detected by IV urography in 242 patients [8]. The ultrasonographic diagnosis was confirmed by cyst puncture, surgery, or autopsy: 168 were "benign cysts" (ie, simple cysts), and all were diagnosed correctly by ultrasonography. The remaining 92 lesions were renal carcinomas, 90 of which were diagnosed correctly by ultrasonography. Retrospective analysis revealed that the two missed cancers did not meet all three ultrasonographic criteria for a simple cyst.

CT scan — To accurately characterize a cyst with CT scan, the CT images should be ≤5 mm in thickness and obtained before and after the IV administration of radiocontrast media.

A simple kidney cyst is considered to be present if the following findings are seen on CT scan [2,3,5,6,26]:

●The cyst is sharply demarcated from the surrounding parenchyma and has a smooth, thin wall.

●Fluid within the cyst is homogeneous, with a density of <20 Hounsfield units (similar to water). However, higher values may be seen with a benign proteinaceous or hemorrhagic cyst.

●There is no enhancement following the administration of radiocontrast media, indicating the presence of an avascular lesion. Enhancement implies vascularity and is strongly suggestive of a malignant lesion, even in the absence of irregular margins.

Simple cysts diagnosed by CT require no further evaluation. Complex cysts are assigned to different categories based on specific CT findings; these categories are associated with varying risks of malignancy and inform further management. (See 'Bosniak classification of kidney cysts' above.)

Caution should be exercised when determining enhancement since small cysts (<1 cm in diameter) may appear to enhance due to volume averaging from adjacent normal kidney parenchyma. When this occurs or is suspected, ultrasonography or MRI is recommended to confirm the nature of the cyst.

MRI — The morphologic features that denote a simple cyst on MRI are the same as those for CT: a simple cyst should have well-defined walls, no mural irregularities, and fluid in the cavity. Additionally, a simple kidney cyst on MRI is hypointense on T1-weighted images, demonstrates no postcontrast enhancement, is strongly hyperintense (bright) on T2-weighted images (unless some hemorrhagic debris is present which is slightly darker), and shows increased signal with no restricted diffusion on diffusion weighted imaging.

MRI can sometimes show fine septa not apparent on CT; thus, the presence of septa on MRI does not necessarily preclude characterization of a cyst as simple if all other radiographic criteria for a simple cyst are met.

Differential diagnosis — The major concern with simple kidney cysts is differentiating them from more serious disorders, such as autosomal dominant polycystic kidney disease (ADPKD) and renal cell carcinoma.

●Polycystic kidney disease – The ability to distinguish simple kidney cysts from ADPKD is easy in patients with overt polycystic disease. Affected patients may present with a positive family history, gross hematuria, cyst infection, flank pain and/or kidney function impairment, and large kidneys with multiple bilateral cysts on ultrasonography or CT scanning (image 3).

Establishing the diagnosis of ADPKD is often more difficult in patients with less severe disease or in those who undergo screening for the disorder. These issues are discussed in detail separately. (See "Autosomal dominant polycystic kidney disease (ADPKD) in adults: Epidemiology, clinical presentation, and diagnosis".)

●Localized cystic disease – Localized cystic disease of the kidney is an uncommon, benign condition that can be confused with PKD or simple kidney cysts. In a study of 18 patients without a family history of PKD, the age at diagnosis ranged from 24 to 83 (average 54) years [10]. Imaging studies revealed multiple cysts of various sizes, separated by normal or atrophic parenchyma in one kidney. Localized cystic disease is neither bilateral nor progressive, in contrast to PKD, and is often symptomatic (hematuria, flank pain, abdominal mass), in contrast to simple kidney cysts [10].

●Malignancy – In contrast to radiographic characteristics of a simple cyst (see 'Diagnosis of simple cysts' above), the following findings on CT scan (or MRI) are highly suggestive of a neoplastic lesion (image 2) [3,5,6,26]:

•Thickened, irregular walls

•Thickened or enhanced septa within the mass

•Enhancement of the lesion after IV contrast

•A multilocular mass

As mentioned above, kidney lesions that definitively meet radiographic criteria for simple cysts have minimal risk of malignancy or malignant transformation and require no further evaluation.

In general, cystic lesions that are unable to be adequately characterized by one imaging modality will require additional imaging. The management of complex cysts depends on their appearance and is discussed below. (See 'Management according to Bosniak category' below.)

Management of complications — The vast majority of simple kidney cysts are asymptomatic and require no treatment. However, uncommon complications include the following:

●Pain – Because flank pain due to an uninfected, nonbleeding cyst is so rare, a thorough evaluation for other causes of pain should be performed before attributing pain to a simple cyst. Pain associated with simple kidney cysts is treated with acetaminophen or, if kidney function is normal, a nonsteroidal antiinflammatory drug for three to five days. In patients with persistent pain, options include percutaneous aspiration, often with a sclerosing agent [27-29], or laparoscopic unroofing of the cyst [30]. Cyst sizes in patients who require intervention generally range from 5 to 20 cm [27,30].

●Infection – In the appropriate clinical setting (eg, fever and flank pain), newly developed internal septations or layering material within an otherwise simple kidney cyst may suggest the presence of infection. However, imaging studies are generally unreliable for diagnosing or excluding infection within a cyst. The approach to diagnosis and management of infected simple kidney cysts is similar to the approach used for patients with ADPKD and an infected cyst. (See "Autosomal dominant polycystic kidney disease (ADPKD): Evaluation and management of complicated urinary tract infections", section on 'Diagnosis' and "Autosomal dominant polycystic kidney disease (ADPKD): Evaluation and management of complicated urinary tract infections", section on 'Management'.)

●Hypertension – Hypertension in patients with simple kidney cysts is most commonly independent of the cyst since both simple kidney cysts and hypertension frequently occur in older individuals. However, compression of the kidney parenchyma by a large simple cyst may lead to excess renin secretion and subsequent hypertension. This phenomena may be suspected in patients with clinical features suggestive of secondary hypertension, such as an acute rise in blood pressure over a previously stable value or persistent hypokalemia that cannot be otherwise explained (table 2). (See "Evaluation of secondary hypertension".)

The management of hypertension due to a simple kidney cyst is similar to that of other forms of angiotensin-dependent hypertension. (See "Renin-angiotensin system inhibition in the treatment of hypertension".)

●Hemorrhage – Bleeding in a simple kidney cyst can cause flank pain and hematuria and is diagnosed by the characteristic imaging finding of high-attenuation, nonenhancing internal debris. Usually, bleeding into a simple cyst is self-limited and requires no intervention.

COMPLEX KIDNEY CYSTS — By definition, complex cysts do not meet radiographic criteria for simple cysts (see 'Diagnosis of simple cysts' above). In contrast to simple cysts, some complex cysts may be malignant and therefore require further evaluation. The management of complex cysts is discussed here; the approach to solid kidney masses and other incidentally discovered noncystic kidney lesions is discussed elsewhere. (See "Diagnostic approach, differential diagnosis, and management of a small renal mass" and "Evaluation of the incidental kidney lesion".)

Management according to Bosniak category — The following is a general approach to the management of patients with kidney cystic lesions based upon the Bosniak category (algorithm 1 and table 1) [31,32]. Bosniak category I cysts are simple cysts that do not require further evaluation; only complex cysts (ie, Bosniak categories II, IIF, III, and IV) are discussed below.

Category II — Further evaluation of Bosniak category II lesions (see 'Bosniak classification of kidney cysts' above) is generally not required. If the cyst is presumed category II but there are concerns about image quality (eg, motion artifact, larger body habitus), repeat ultrasonography may be done at 6 to 12 months to confirm stability and the correct diagnosis. If the radiologist cannot distinguish between a category II and IIF cyst despite optimal imaging, the cyst should be managed as IIF.

Category IIF — Newly discovered category IIF cysts (see 'Bosniak classification of kidney cysts' above) require follow-up imaging (hence the "F") to document stability [2]. However, there are no clear data to guide the optimal frequency or total duration of follow-up imaging; most experts would advocate documenting radiographic stability for at least five years. If the radiologist is unable to clearly distinguish a category IIF from a III lesion, the lesion should be managed as a category III lesion. (See 'Categories III and IV' below.)

For category IIF lesions, we make every attempt to obtain prior radiographic studies of the same modality for purposes of comparison. Our approach to category IIF cystic lesions is as follows:

●Patients without prior radiographic studies – For patients without prior radiographic studies, we obtain additional imaging with a contrast-enhanced MRI for further characterization of the complex cyst. MRI may place a category IIF cyst into a higher Bosniak category [33], which would alter management.

•If the MRI confirms that the cyst is category IIF, we obtain follow-up CT (with and without intravenous [IV] contrast) in six months and then either a CT or ultrasound yearly until radiographic stability is documented over at least a five-year period. Management of a category IIF cyst that evolves into a higher Bosniak category on surveillance CT is discussed below (see 'Categories III and IV' below). Radiographic evolution of a IIF cyst on follow-up ultrasound requires further characterization by CT or contrast-enhanced MRI.

•If the MRI places the category IIF cyst into a higher Bosniak category, the cyst should be managed as category III or IV [33]. (See 'Categories III and IV' below.)

●Patients with prior radiographic studies – Our approach in these patients is as follows:

•For patients with prior radiographic studies documenting stability of the Bosniak IIF cyst for five years or longer, no further evaluation is required.

•For patients whose prior radiographic studies have documented stability of the IIF cyst for less than five years, follow-up imaging is indicated until radiographic stability is documented over at least a five-year period. For most patients, repeat imaging with CT (with and without IV contrast) or ultrasound can be obtained in one year and then annually. However, for patients with prior studies performed only within the last six months, the next surveillance imaging study should be scheduled so that potential radiographic changes can be assessed over an approximate six-month period; subsequent imaging can then be obtained annually. Management of a category IIF cyst that evolves into a higher Bosniak category on surveillance CT is discussed below (see 'Categories III and IV' below). Radiographic evolution of a IIF cyst on follow-up ultrasound requires further characterization by CT or contrast-enhanced MRI.

•If comparison with prior imaging demonstrates radiographic progression rather than stability, further characterization of the IIF cyst should be pursued with contrast-enhanced MRI.

For category IIF cysts, the absence of radiographic change over time supports benign disease, while progression suggests a neoplastic process. This is illustrated by the following findings:

●In a report of 42 patients with category IIF lesions who had follow-up CT examinations for two or more years (average 5.8 years), two lesions became more complex with thicker septa; both were found to be cystic neoplasms at surgery [34]. This strategy prevented unnecessary surgery in the remaining 40 patients (95 percent).

●In one study that included 62 patients with 69 category IIF complex cysts, radiographic follow-up demonstrated progression in nine of the complex cysts; eight of these cysts were resected, and four (50 percent) were malignant [35].

Category III with inconclusive enhancement — Some patients have complex cysts with features of Bosniak category III lesions but inconclusive or even negative CT contrast enhancement. Because of the importance of CT contrast enhancement in assigning malignancy risk to complex cysts (see 'Bosniak classification of kidney cysts' above), additional imaging of these lesions is generally pursued with contrast-enhanced MRI. Contrast enhancement on MRI confirms that the cyst should be managed as a Bosniak III lesion, whereas lack of contrast uptake permits a downgrade to Bosniak IIF. (See 'Category IIF' above and 'Categories III and IV' below.)

Categories III and IV — Bosniak III and IV complex cysts have an increased risk of malignancy (see 'Bosniak classification of kidney cysts' above). An experienced urologist should be involved with the management of category III or IV cystic lesions, and the approach may vary according to the general health and comorbidities of the patient, the individualized risk of malignancy, the efficacy and risk of possible interventions, and patient preference.

Management options include surgical resection (eg, partial nephrectomy), thermal ablation (cryotherapy or radiofrequency ablation), and, in selected patients, active surveillance. For patients managed with surveillance, we obtain a CT (with and without contrast) or MRI at six months, with follow-up imaging with either a CT or ultrasound yearly [3,4,31-33,36-38]. The optimal duration of follow-up imaging is unclear; some experts would advocate documenting radiographic stability for at least five years. For Bosniak III and IV cysts <4 cm, the approach to management is the same as that for solid kidney masses of the same size and is discussed in detail elsewhere. (See "Diagnostic approach, differential diagnosis, and management of a small renal mass", section on 'Management options'.)

Percutaneous needle biopsy is required prior to thermal ablation of a cystic kidney lesion to provide a pathologic diagnosis and to assist in guiding subsequent surveillance. However, the role of needle biopsy in other settings is controversial. We generally do not recommend preoperative needle biopsy for cystic lesions because of concern about seeding the peritoneum and sampling error. However, some clinicians use percutaneous needle biopsy to identify nonmalignant Bosniak III cystic lesions that do not require surgical resection.

Complex cysts without Bosniak classification — The Bosniak kidney cyst classification system should not be used for cysts less than 1 cm in size and cannot be used for cysts observed on CT performed without the use of IV iodinated contrast. Our approaches in patients with complex cysts <1 cm and in patients who may have contraindications to IV CT contrast are below. (See 'Bosniak classification of kidney cysts' above.)

Cysts <1 cm — In general, cysts <1 cm cannot be classified using the Bosniak classification system (see 'Bosniak classification of kidney cysts' above). Complex cysts <1 cm in size are usually benign. There are no clear data to guide the optimal frequency or total duration of follow-up imaging for small cysts, and practice patterns vary considerably. For most patients, it would be reasonable to obtain follow-up imaging with ultrasound in six months and then yearly until radiographic stability is documented over at least a five-year period. Growth of the complex cyst to ≥1 cm during surveillance imaging allows contrast-enhanced CT characterization and assignment to a specific Bosniak category. For the minority of patients at especially high risk of renal cell carcinoma, such as those with a prior history of the disease or those with a genetic predisposition (eg, von Hippel-Lindau disease), we obtain contrast-enhanced MRI, which can effectively characterize small cysts.

Cysts in patients who cannot receive IV iodinated contrast — For patients who have severe allergic reactions to intravenous (IV) iodinated contrast or whose kidney function impairment represents a relative contraindication to the administration of IV iodinated contrast, MRI and/or ultrasound are used to evaluate complex cysts. (See "Patient evaluation prior to oral or iodinated intravenous contrast for computed tomography", section on 'Preventing recurrent allergic-like reactions' and "Prevention of contrast-induced acute kidney injury associated with computed tomography", section on 'Prevention among high-risk patients'.)

Contrast-enhanced MRI was previously avoided in patients with an estimated glomerular filtration rate <30 mL/min per 1.73 m² due to concern about nephrogenic systemic fibrosis. However, with the use of next-generation gadolinium-based MRI contrast agents (table 3), which prevent the release of gadolinium from chelating agents, nephrogenic systemic fibrosis is rare [39]. (See "Patient evaluation before gadolinium contrast administration for magnetic resonance imaging", section on 'Approach to preventing nephrogenic systemic fibrosis' and "Nephrogenic systemic fibrosis/nephrogenic fibrosing dermopathy in advanced kidney disease", section on 'Prevention'.)

Our approach to patients with complex cysts who cannot receive IV iodinated contrast with CT depends on whether gadolinium can be administered. Many but not all centers allow patients with severe chronic kidney disease or end-stage kidney disease to proceed with dedicated MRI of the kidneys.

●For patients who can receive gadolinium, we obtain contrast-enhanced MRI. Patients with findings that suggest malignancy should be managed the same as patients with Bosniak III or IV lesions (see 'Categories III and IV' above). For patients with MRI findings that are indeterminate, follow-up imaging with MRI should be performed to assess radiographic stability. Although there are no clear data to guide the optimal frequency or total duration of follow-up, a repeat MRI in six months and then either MRI or ultrasound annually for at least five years is a reasonable surveillance strategy. Radiographic evolution of the cyst on follow-up ultrasound requires further characterization by MRI.

●For patients who cannot receive gadolinium, consultation with a multidisciplinary team including a radiologist and urologist is warranted. Options vary and can range from surveillance imaging with multiple modalities (eg, noncontrast MRI and ultrasound) to surgical resection in some patients.

Although MRI is typically used as a complement to contrast-enhanced CT in selected patients with Bosniak IIF and III complex kidney cysts (see 'Category IIF' above and 'Category III with inconclusive enhancement' above), MRI is an effective stand-alone modality for determining the risk of malignancy associated with cystic kidney lesions. In addition to signal characteristics, MRI evaluates the same morphologic findings as described for CT scanning: wall thickening, nodularity, septa, and enhancement. One difference is that MRI does not detect calcification. (See 'Bosniak classification of kidney cysts' above.)

MRI is most useful for characterizing the internal contents of cysts, such as hemorrhage or mucin, and is more sensitive than both ultrasonography and CT scanning in showing enhancement of internal septations.

The value of MRI was illustrated in a series of 37 patients with 55 complex cystic kidney lesions who underwent T1-weighted, T2-weighted, and gadolinium-enhanced MRI [37]. The following frequencies of malignancy were noted with specific radiographic abnormalities:

●22 percent of 37 lesions that contained fluid of heterogeneous signal intensity

●44 percent of 32 lesions with intense mural (ie, cyst wall) enhancement

●63 percent of 16 lesions with mural irregularity

●71 percent of 14 lesions with a thick wall (>2 mm)

●75 percent of 4 lesions with mural masses or nodules

●50 percent of 4 lesions with septa

The combination of mural irregularity and intense mural enhancement had the highest correlation with malignancy.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Chronic kidney disease in adults".)

SUMMARY AND RECOMMENDATIONS

●Types of kidney cysts – Kidney cysts are categorized as simple or complex. Simple kidney cysts are benign, asymptomatic lesions that rarely require treatment. By contrast, complex cysts may require follow-up imaging, biopsy, or surgical excision for diagnosis and management. (See 'Types of kidney cysts' above.)

●Bosniak classification of kidney cysts – The Bosniak kidney cyst classification system was created to help diagnose and manage complex cysts. The Bosniak classification of a kidney cyst is based upon specific morphologic and enhancement characteristics with unenhanced and intravenous (IV) contrast-enhanced CT and should only be used for cysts ≥1 cm in size. The five Bosniak categories (I, II, IIF, III, and IV) are associated with a risk of malignancy that starts from near zero (Bosniak I; simple cysts) and rises to over 85 percent (Bosniak IV) (table 1). (See 'Bosniak classification of kidney cysts' above.)

●Simple kidney cysts – Simple kidney cysts have characteristic features on ultrasonography, CT, and MRI. If radiographic criteria for a benign simple kidney cyst are met, additional follow-up is not required. (See 'Diagnosis of simple cysts' above.)

•Epidemiology – Simple kidney cysts are commonly observed in normal kidneys, with an increasing incidence as individuals age. They are the most common kidney lesions, accounting for roughly 65 to 70 percent of cases. (See 'Epidemiology' above.)

•Clinical features – Simple kidney cysts may be solitary or multiple and bilateral. They typically produce no symptoms or signs. Rarely, however, they can be associated with rupture (hemorrhage), hematuria, pain, abdominal mass, infection, and/or hypertension. (See 'Clinical features' above.)

•Differential diagnosis – The major concern with simple kidney cysts is differentiating them from more serious disorders, such as polycystic kidney disease (PKD), complex cysts, and solid masses (such as a renal carcinoma or abscess). (See 'Differential diagnosis' above.)

•Treatment – The vast majority of simple kidney cysts require no treatment. Therapy may rarely be required for symptoms, signs, and/or complications. (See 'Management of complications' above.)

●Complex kidney cysts – In contrast to simple cysts, some complex cysts may be malignant. The management of kidney cystic lesions is based upon initial classification according to the Bosniak system. Bosniak category I cysts are simple cysts that do not require further evaluation. Our approach is outlined below (algorithm 1):

•Category II – Further evaluation of Bosniak category II lesions is generally not required. If the radiologist cannot distinguish between a category II and IIF cyst, the cyst should be managed as IIF. (See 'Category II' above.)

•Category IIF – Category IIF cysts generally require follow-up imaging (hence the "F") to document stability over time. For patients without prior radiographic studies available for comparison, we obtain additional imaging with contrast-enhanced MRI; MRI may place a category IIF cyst into a higher Bosniak category. If the radiologist is unable to clearly distinguish a category IIF from a III lesion, the lesion should be managed as a category III lesion. (See 'Category IIF' above.)

•Categories III and IV – Bosniak III and IV complex cysts have an increased risk of malignancy. An experienced urologist should be involved with the management of category III or IV cystic lesions. Management options include surgical resection (eg, partial nephrectomy), thermal ablation (cryotherapy or radiofrequency ablation), and, in selected patients, active surveillance. (See 'Categories III and IV' above.)

•Complex cysts without Bosniak classification – The Bosniak classification system should not be used for cysts less than 1 cm in size and requires the use of IV iodinated contrast. For patients with small cysts, ultrasound follow-up may be reasonable. Contrast-enhanced MRI can be used as the sole imaging modality for patients unable to tolerate IV iodinated contrast. For patients who cannot receive gadolinium, options vary and can range from surveillance imaging with multiple modalities (eg, noncontrast MRI and ultrasound) to surgical resection in some patients. (See 'Complex cysts without Bosniak classification' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges William M Bennett, MD, who contributed to earlier versions of this topic review.