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 (PKD) 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 interstitial 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, 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 "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".)
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]. They are benign, asymptomatic lesions that rarely require treatment. However, complex cysts may require follow-up imaging, biopsy, or surgical excision for diagnosis.
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.)
A 2019 version update of the Bosniak classification, endorsed by the Society of Abdominal Radiology, has been proposed [2]. 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 inter-reader 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. (See 'Bosniak classification of kidney cysts' below and 'Management by Bosniak category' below.)
BOSNIAK CLASSIFICATION OF KIDNEY CYSTS — The incidental finding of lesions in the kidney parenchyma is common with the increased use of imaging procedures such as ultrasonography and computed tomography (CT). Such lesions are usually simple benign kidney cysts, although complex kidney cysts or neoplasms may need to be excluded. If the lesion appears to be a simple kidney cyst, additional imaging is not needed. However, should the lesion appear to be other than a simple cyst, additional imaging with a dedicated CT scan without and with contrast enhancement should be performed [3]. (See 'Introduction' above.)
The Bosniak kidney cyst classification system was created to help diagnose and manage these lesions [3]. Based upon the morphologic and enhancement characteristics with unenhanced and contrast-enhanced CT scanning, cystic kidney masses are placed into one of five different categories (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 – Indeterminate 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) [3-5]. 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 [3,4,6,7].
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 the categories I, II, and IIF (typically benign) [3]. Such contrast enhancement should be at least 10 to 15 Hounsfield units higher on contrast-enhanced images when compared with unenhanced images.
MANAGEMENT BY BOSNIAK CATEGORY — The initial imaging study that detects such a cyst is commonly ultrasonography. If basic ultrasonography criteria for a benign simple kidney cyst are met, further study is not required. The following is a general approach to the management of patients with kidney cystic lesions based upon the Bosniak category [8,9].
Category I and II — Further evaluation of Bosniak category I and II lesions is generally not required, although a repeat ultrasonography may be done at 6 to 12 months in selected patients in order to confirm stability and a correct diagnosis. In addition, follow-up imaging is necessary if the clinician is unable to distinguish between a category II and IIF cyst.
Category IIF — The optimal approach to kidney cystic lesions with indeterminate findings on ultrasonography and computed tomography (CT) scanning is uncertain. These lesions are usually Bosniak category IIF or III, as defined above (see 'Bosniak classification of kidney cysts' above). However, some category II lesions will be included in this category based on the quality of the CT scan and the expertise of the radiologist; marked interobserver variation has also been noted when attempting to identify category II lesions that can be safely monitored [10].
With category IIF lesions, we recommend making every attempt to obtain prior studies for purposes of comparison. If such studies are not available, many radiologists would recommend an additional imaging study (typically good-quality contrast-enhanced magnetic resonance imaging [MRI]) for further characterization. If the radiologist is unable to clearly distinguish a category IIF from a III lesion, the lesion should be placed into the higher category. The approach to category III lesions is discussed below. (See 'Category III and IV' below.)
Most category IIF cysts require follow-up imaging (hence the "F") to document stability [3].
The absence of change supports benign disease, while progression suggests a neoplastic process, as 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 [11]. This strategy prevented unnecessary surgery in the remaining 40 patients (95 percent).
●The incidence of malignancy in category IIF cysts appears to be higher in lesions that require resection. In one study that included 62 patients with category IIF lesions, 16 lesions in 13 patients were resected for various reasons (six patients had a coexisting category IV lesion and/or a solid kidney mass, three had a history of primary kidney malignancy, two had an ipsilateral category III lesion, three patients requested definitive surgery, and two had recurrent cysts after aspiration and suspicion for malignancy by the urologist). Among 16 resected lesions, four (25 percent) were malignant [12]. Other risk factors for malignancy included a prior history of kidney malignancy, a coexisting stage IV lesion, and/or a solid kidney mass.
Category III and IV — The approach to category III and IV lesions varies among clinicians and depends upon patient factors.
Options available include continued active surveillance with periodic imaging, fine-needle biopsy and possible ablation, or surgery with partial nephrectomy, if feasible [3]. The decision as to which avenue to pursue generally depends upon the appearance of the lesion and the comorbidities and life expectancy of the patient. In general, among patients who are good surgical candidates, surgery is preferred. For patients who are not good surgical candidates or who elect surveillance, we obtain a CT or MRI at six months, with follow-up imaging with either a CT or ultrasound yearly [4,5,8,9,13-16].
MRI for additional evaluation and active surveillance — MRI is typically used to evaluate patients with indeterminate lesions [13-16]. 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 potential value of this approach was illustrated in a series of 37 patients with 55 complex cystic kidney lesions who underwent T1-weighted, T2-weighted, and gadolinium-enhanced MRI [14]. 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 four lesions with mural masses or nodules and 50 percent of four lesions with septa
The combination of mural irregularity and intense mural enhancement had the highest correlation with malignancy.
Serial MRI examinations at 3, 6, and 12 months are warranted in patients with indeterminate lesions on gadolinium-enhanced MRI. Further imaging is generally not necessary to follow up lesions that are stable at 12 months.
Percutaneous needle biopsy, only if planning ablation — Preoperative needle biopsy has generally not been recommended for resectable kidney lesions because of concern about seeding of the peritoneum. However, biopsy is required prior to percutaneous cryoablation of kidney masses. The evaluation of solid kidney masses is discussed separately. (See "Diagnostic approach, differential diagnosis, and management of a small renal mass".)
The role of percutaneous needle biopsy in evaluating an indeterminate kidney mass or cyst is likely to grow since such masses are increasingly being identified with the widespread use of CT and MRI [17]. In patients with indeterminate imaging studies, percutaneous, image-guided kidney biopsy appears to be safe and can provide a diagnosis in up to 80 percent cases [5,10,17-19]. The following findings are illustrative:
●One report evaluated 28 patients with a complex cystic kidney mass on CT scan who underwent image-guided biopsy [5]. Seventeen patients (61 percent) had a malignant lesion (16 renal cell carcinoma, 1 lymphoma), and 11 had benign disease (six hemorrhagic cysts, three inflammatory cysts, one metanephric adenoma, and one cystic oncocytoma). The biopsy diagnosis was confirmed in all 17 patients who underwent surgical resection. There was no interval radiologic change at a minimum of one year in the 10 patients with benign lesions who did not undergo surgery.
●A second study evaluated 583 patients in whom image-guided biopsy was performed for kidney mass lesions that were classified as indeterminate by imaging (7.2 percent of kidney masses) [10]. In the 517 patients in whom follow-up was available, a diagnosis was established in 76 percent, with a malignant lesion found in 44 percent. A biopsy diagnosis could not be established with acceptable confidence in 21 percent, and a false diagnosis was made in 1.2 percent.
Partial nephrectomy — Partial nephrectomy is often performed, particularly in good surgical candidates with few comorbidities. (See "Diagnostic approach, differential diagnosis, and management of a small renal mass", section on 'Partial nephrectomy'.)
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 [20]. The cysts may be solitary or multiple and bilateral [6,20-22]. (See 'Types of kidney cysts' above.)
The evaluation of a solid kidney mass is discussed separately. (See "Diagnostic approach, differential diagnosis, and management of a small renal mass".)
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 computed tomography (CT) scanning [21,23-27].
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 [25]. 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 [27]. 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 [21]:
•15 to 29 years of age – 0 (males); 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 [28]. 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 [28].
Clinical features — Simple kidney cysts may be solitary or multiple and bilateral [6,20-22]. They typically produce no symptoms or signs. 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 [27]. An association with hypertension has also been reported in other studies [29-31], 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 [23].
Unless obstruction is present, simple kidney cysts do not compromise kidney function [32]. However, limited evidence suggests that younger patients with simple kidney cysts may have mildly reduced (but still normal) kidney function [33].
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 [34]. 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 renal parenchyma rarely causes angiotensin-dependent hypertension [35,36]. 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 [36]. 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 [37].
Diagnosis — Simple kidney cysts have characteristic changes on ultrasonography and CT. Since most simple kidney cysts are asymptomatic, the initial imaging study that detects such a cyst is commonly ultrasonography, which is usually performed for unrelated reasons. If ultrasonography criteria for a benign simple kidney cyst are met, additional imaging is not required. (See 'Ultrasonography as initial test' below.)
A CT scan should be performed if the ultrasonogram is equivocal, if calcifications or septae are seen, or if multiple kidney cysts are clustered in a pattern that could mask an underlying carcinoma. Based upon findings on CT scanning, the lesion is classified into different categories. (See 'Bosniak classification of kidney cysts' above.)
Ultrasonography as initial test — 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 [6,38]:
●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 [6,20,38]. The accuracy of ultrasonography in evaluating kidney masses was illustrated in a retrospective study that assessed 260 lesions detected by intravenous (IV) urography in 242 patients [20]. The ultrasonographic diagnosis was confirmed by cyst puncture, surgery, or autopsy: 168 were benign 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 benign cyst.
Perform CT scan if ultrasound is equivocal — A CT scan should be performed if the ultrasonogram is equivocal, if calcifications or septae are seen, or if multiple cysts are clustered in a pattern that could mask an underlying carcinoma. 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 (table 1) [3,4,6,7,38]:
●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.
The findings suggestive of malignancy are described above and include thickened, irregular or smooth walls or septa and enhancement after contrast injection (image 2) [3]. Malignant lesions are most likely to occur in category III and category IV kidney cysts. (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 renal parenchyma. When this occurs or is suspected, ultrasonography or magnetic resonance imaging (MRI) is recommended to confirm the nature of the cyst.
Diagnosis of complications — Clinical, radiographic, and other findings may confirm the presence of one of the rare complications associated with simple kidney cysts. These include infection, hypertension, and hemorrhage.
●Infection – In the appropriate clinical setting, newly developed internal septations or layering material within an otherwise simple kidney cyst may suggest the presence of infection. However, imaging studies are unreliable for diagnosing or excluding infection within a cyst. The clinical features of an infected simple kidney cyst (such as fever and weight loss) are the same as those of renal abscess, which are discussed separately. Percutaneous aspirate with culture and Gram stain may be diagnostic. (See "Renal and perinephric abscess".)
●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. Hypertension due at least in part to a simple kidney cyst 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".)
●Hemorrhage – Bleeding in a simple kidney cyst is diagnosed by the characteristic imaging finding of high-attenuation, nonenhancing internal debris.
Differential diagnosis — The major concern with simple kidney cysts is differentiating them from more serious disorders, such as polycystic kidney disease (PKD), and solid masses, such as a renal carcinoma or abscess. Ultrasonography, CT, and/or MRI can be used to help establish the correct diagnosis. (See 'Diagnosis' above.)
●Polycystic kidney disease – The ability to distinguish simple kidney cysts from PKD, which most often has autosomal dominant inheritance, 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 PKD 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 [22]. 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 [22].
●Malignancy – As mentioned above, CT scanning should be performed if ultrasonography is equivocal, if calcifications or septae are seen, or if multiple cysts are clustered in a pattern that could mask an underlying carcinoma.
The following findings on CT scan (or MRI) are highly suggestive of a neoplastic lesion (image 2 and table 1) [4,6,7,38]:
•Thickened, irregular walls
•Thickened or enhanced septae within the mass
•Enhancement of the lesion after intravenous contrast
•A multilocular mass
In some cases, however, CT scanning is equivocal, being suggestive but not diagnostic of simple kidney cysts. (See 'Category IIF' above.)
●Indeterminate lesions – The optimal approach to kidney cystic lesions with indeterminate findings on ultrasonography and CT scanning is uncertain. (See 'Category IIF' above.)
Treatment — The vast majority of simple kidney cysts require no treatment. Therapy is rarely required for symptoms, signs, and/or complications.
●Pain – 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 [39-41], or laparoscopic unroofing of the cyst [42]. Cyst sizes in patients who require intervention generally range from 5 to 20 cm [39,42].
●Infection – Infected cysts are frequently refractory to antimicrobial therapy since many drugs do not readily penetrate the cyst [43]. Issues surrounding the treatment of infected simple kidney cysts are similar to those associated with the treatment of infected cysts in patients with PKD. (See "Autosomal dominant polycystic kidney disease (ADPKD): Evaluation and management of complicated urinary tract infections".)
●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".)
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
●Kidney cysts result from genetic or nongenetic processes and occur in a variety of diseases in adults and children. (See 'Introduction' above.)
●To help diagnose and manage simple and complex kidney cysts, the Bosniak kidney cyst classification system was created. Based upon morphologic and enhancement characteristics with unenhanced and contrast-enhanced computed tomography (CT) scanning, cystic kidney masses are placed into one of five different categories (table 1). (See 'Bosniak classification of kidney cysts' above.)
●The evaluation of kidney cystic lesions is based upon initial classification according to the Bosniak system:
•Further evaluation of Bosniak category I and II lesions is not required. If the clinician is unable to distinguish between a category II and IIF cyst, follow-up imaging should be performed. (See 'Category I and II' above.)
•For category IIF lesions, we recommend making every attempt to obtain prior studies for purposes of comparison. If such studies are unavailable, we recommend an additional imaging study, such as a contrast-enhanced magnetic resonance imaging (MRI), for further characterization. If the radiologist is unable to clearly distinguish a category IIF from a III lesion, the lesion should be treated like a category III lesion. (See 'Category IIF' above.)
•For category III and IV lesions, the approach varies among clinicians and depends upon patient factors. Options available include continued active surveillance with periodic imaging, fine-needle biopsy and possible ablation, or surgery with partial nephrectomy, if feasible. The decision between these options depends upon the appearance of the lesion and the comorbidities and life expectancy of the patient. In general, among patients who are good surgical candidates, surgery is preferred. For patients who are not good surgical candidates or who elect surveillance, we obtain a CT or MRI at six months, with follow-up imaging with either a CT or ultrasound yearly. (See 'Category III and IV' above.)
●Simple kidney cysts are observed frequently in normal kidneys. They are the most common kidney masses, accounting for roughly 65 to 70 percent of cases. The prevalence of simple kidney cysts varies with the population studied and the imaging modality utilized. These cysts most often occur in patients >50 years of age. (See 'Epidemiology' above.)
●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.)
●Simple kidney cysts have characteristic changes on ultrasonography. If basic ultrasonography criteria for a benign simple kidney cyst are met, further study is not required. A CT scan should be performed if the ultrasonogram is equivocal, if calcifications or septae are seen, or if multiple cysts are clustered in a pattern that could mask an underlying carcinoma. (See 'Diagnosis' above.)
●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.)
●The vast majority of simple kidney cysts require no treatment. Therapy may rarely be required for symptoms, signs, and/or complications. (See 'Treatment' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges William M Bennett, MD, who contributed to earlier versions of this topic review.
