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Imaging of Cystic Renal Lesions: Current Concepts and Approaches

Cystic lesions of the kidney represent a diverse group of fluid-filled masses ranging from completely benign simple cysts to complex, malignant tumors. Accurately identifying and categorizing these lesions is critical to avoid unnecessary surgeries for harmless cysts while ensuring timely intervention for renal cancers. This clinical evaluation relies heavily on advanced cross-sectional imaging modalities, including ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI). In this presentation, best-selling author Dr. Aryan will guide you into the world of radiology and the realm of knowledge we use to characterize the cystic lesions of the kidney.

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Imaging of Cystic Renal Lesions Presented by: Dr. Anish Dhakal Resident, MD Radiodiagnosis Kathmandu University School of Medical Sciences (KUSMS) 7th August, 2026
Cystic renal lesions The definition of “cystic mass” is a lesion with the solid portion occupying not more than one-fourth (<25%) of the volume Cystic renal masses constitute about 15% of all renal masses Bosniak classification system introduced in 1986 by Mortan Bosniak is a CT based classification system which helps robustly categorize cystic renal lesions and guide further management.
Renal cysts 1. Serous renal cysts: Most common cystic kidney disease and become more frequent with age. Single or multiple and vary in size. On ultrasound, they appear as round, anechoic cysts with a well-defined wall, posterior acoustic enhancement, and no doppler blood flow. On CT, they are well-defined, homogeneous, water-density lesions (0–20 HU) without calcification or contrast enhancement. On MRI, they show low signal on T1, high signal on T2, and no contrast enhancement. 2. Atypical cysts: Serous cysts complicated by hemorrhage, infection, or ischemia, making them difficult to distinguish from cystic tumors, infections, or other cystic kidney diseases.
Renal Cysts- Introduction Renal cysts usually represent notably dilated nephrons or collecting ducts. The term renal cystic disease refers to any disorder that results from the presence of multiple renal cysts. Imaging Modalities for Renal Cysts: 1. X-ray: Plain radiography and excretory urography are indirect imaging modalities and are relatively insensitive in detecting and characterizing renal cysts. Excretory urography may show the following imaging findings when renal cysts are present: Abnormality of the renal contour or adjacent fascial planes, displacement of bowel loops or splayed ribs Curvilinear or rim calcification in the cyst wall Filling defects in the opacified cortex during the nephrographic phase of urography Splaying and distortion of the opacified collecting system
2. USG: The modality where most renal cysts are detected incidentally. A simple cyst detected in USG needs no further imaging. Complex cystic lesions are imaged further. Gray-scale, real-time sonography is a noninvasive, multiplanar direct imaging modality, which requires no patient preparation, and unlike EU it shows the mass lesion itself, along with changes in the renal parenchyma and renal sinus. However, obesity and excessive bowel gas can obscure the kidneys. Gray-scale US is useful in characterizing simple cysts but is limited in evaluation of complex cysts Color Doppler can aid in assessing blood flow in renal cystic lesions, however with poor sensitivity. As Bosniak classification relies on enhancement, various studies have shown promising results using contrast-enhanced US in the evaluation of renal cystic lesions based on the Bosniak classification system.
3. CT Computed tomography is the main modality for imaging and characterizing cystic renal lesions. Multiphasic renal CT is obtained including unenhanced CT, corticomedullary phase, nephrographic phase, and excretory phase. The nephrographic phase is the most valuable for detecting mass lesions less than 3 cm which include clinically insignificant renal cysts and RCCs. A change of less than 10 HU from pre- to postcontrast images is considered typical of a benign cyst. Pseudoenhancement that is defined as an artefactual increase in HU values of >10 HU on contrast-enhanced CT can result from beam hardening artifact. Very rarely, cystic tumors may enhance less than 10 HU. Majority of pathologic lesions enhance more than 15 HU. An enhancement between 10 HU and 15 HU is considered suspicious but not diagnostic of a tumor. Also, if the lesion is smaller than twice the slice thickness used, then partial volume averaging may result in misinterpretation. Virtual monochromatic imaging on dual-energy computed tomography may completely eliminate pseudoenhancement in renal cysts larger than 1.5 cm.
4. MRI: Used for smaller cysts that may not be detectable or characterized with CT. Also used when radiation of CT is to be avoided. Multiplanar imaging capability and angiographic capabilities with excellent soft-tissue contrast resolution have made MRI a useful method for evaluating the kidneys High-field strengths, breathhold imaging, fat suppression techniques, and use of MR-contrast agents help in differentiating tumor from cysts. The multiplanar imaging capability of MRI makes it easier to assess polar lesions and coronal images make it simple for the urologist to visualize the relative position of a small lesion if wedge resection or partial nephrectomy is to be planned. Although the Bosniak classification has been described on CT, various researchers have proven that MR may be equivalent to CT in Bosniak classification of cystic lesions. Calcification is however better picked up on CT than on MR. Owing to increased contrast resolution, there is increased sensitivity of MR for picking up septations and wall thickening on T2-weighted images. This can result in false upgradation of the Bosniak category of the lesion. Other pitfalls include evaluation of enhancement in hemorrhagic and calcified lesions. In these situations, subtraction imaging may be used. Subtraction imaging technique can suffer from misregistration and additional noise. In few equivocal cases, MR can be problem solving. Postcontrast free breathing 3D fat-suppressed T1 gradient echo (T1 vibe fat sat) sequence allows better evaluation of enhancement which forms the main basis for Bosniak categorization of lesions. Diffusion-weighted imaging can also be of utility in differentiating cystic renal masses from simple cysts.
5. Angiography Prior to the advent of newer imaging modalities such as US, CT, and MRI, angiography was commonly used for evaluation of renal mass lesions. However, in the present era it is mainly indicated for interventional procedures of vascular mass lesions of the kidney. Actively bleeding tumors such as angiomyolipomas (AMLs) can be treated with selective angiography by embolization of the intrarenal supply of the tumor. 6. Biopsy Biopsy may not give a high yield from cystic lesions owing to paucity of cells within. It is however useful to biopsy or drain cystic lesions which are suspected to be abscesses and may be attempted in patients with suspected malignancy if they are poor surgical candidates.
Bosniak classification The Bosniak classification was described in 1986. This classification helps the radiologist to categorize each cystic renal mass as "nonsurgical" (ie, benign in category 1 and 2) or as "surgical" (ie, requiring surgery in category 3 and 4). After the original description, it became obvious that there were some category II cysts that were slightly more complicated than most category II lesions, but not complicated enough to place them in category III. For that reason, a category IIF (F for follow-up) was introduced in 1993.
Changes to this classification were made in 1990s and in 2005 and now five basic categories are recognized.
Reporting renal cysts in USG report: Major language recommendations On plain, non-contrast ultrasound, a radiologist should describe the cyst's size, location, wall thickness, septations, calcifications, and color Doppler vascularity, then categorize the lesion as either a simple cyst, a complicated cyst, or a complex/indeterminate mass. If the cyst fulfills all benign criteria, it should be labeled a simple cyst requiring no further action. If it exhibits mild deviations like a single thin septum, it can be reported as a likely benign complicated cyst suitable for routine follow-up. However, because a formal Bosniak classification cannot be assigned without contrast media to evaluate tissue perfusion, any lesion with high-risk features such as thick walls, multiple septations, nodular calcifications, or solid components must be reported as an indeterminate complex renal mass with an explicit recommendation for a contrast-enhanced CT or MRI to definitively establish the Bosniak score While the official Bosniak Version 2019 update formally incorporates only CT and MRI, major international organizations—such as the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB)—have established a validated CEUS-Bosniak classification system
Simple Cysts – Bosniak I Lesion Simple renal cysts may be solitary or multiple and are commonly bilateral. US is usually considered the most appropriate next investigation for further evaluation of a presumed renal cyst discovered on excretory urography. Such cysts may vary in size from less than 1 cm to 10 to 15 cm. USG features: Well defined anechoic lesion Classic features of a renal cyst include a smooth wall, an echo-free center, and posterior acoustic enhancement caused by increased through-transmission. Category I is a simple benign cyst with a thin well-defined wall ≤2 mm and no septa or calcifications. The wall may enhance. On CT they have fluid attenuation with HU values <20. On MRI they have uniform signal intensity similar to CSF.
The term milk of calcium (MOC) is given to dependent, sedimented calcification within a cystic structure or hollow organ. This sort of colloidal calcium suspension layering can occur in various regions like breast, gall bladder and kidneys Renal: Milk of calcium in renal cyst (most common)
Bosniak I cyst: CT features 1. Sharp margination and demarcation from surrounding renal parenchyma. 2. A smooth thin or imperceptible wall. 3. Homogeneous water-density content with an attenuation value of 0 to 20 HU. 4. No enhancement after IV administration of contrast medium If a renal mass fulfills these CT criteria, no further evaluation is necessary.
Simple Cysts – Bosniak I Lesion Simple renal cyst. Nonenhanced (A) and contrast-enhanced (B) CT scans show that the mass is round with a smooth margin and distinct margination from adjoining renal parenchyma. It has a homogeneous water density, does not enhance (pre- and postcontrast attenuation value = 7 HU), and has no discernible wall thickness.
CT Pitfalls: Small renal cysts may be volume averaged with normal renal tissue, causing spuriously high attenuation values, or falsely high attenuation values may be caused by streak artifacts. Remedy - US should be performed to determine whether the lesion is a cyst or a neoplasm. If US findings are indeterminate, dedicated renal CT with special attention to the lesion should be performed. If the mass is greater than 70 HU and homogenous, it’s highly likely to be benign (hemorrhagic or proteinaceous cyst)
MRI Features: Even a small simple renal cyst can be diagnosed on contrast-enhanced MRI if it fulfills the following criteria – 1. Sharp margination and demarcation from surrounding renal parenchyma. 2. A smooth thin wall. 3. Homogeneous contents with signal characteristics of water (such lesions have low signal intensity on T1-weighted spin echo images and high signal intensity on T2-weighted spin echo images). 4. No enhancement after IV injection of Gd-DTPA The use of Gd-enhanced MRI, particularly with gradient echo imaging techniques applied during breath hold, is helpful in evaluating renal cystic lesions in patients with contraindications to iodinated IV contrast medium (e.g., chronic renal failure, previous reactions to contrast media)
Simple Cysts – Bosniak I Lesion A, T1-weighted images shows a lesion (arrow) with lower signal intensity than renal parenchyma. B, On T2-weighted image, the cyst is hyperintense, indicating its fluid content. The cyst is well defined and has a smooth wall (arrow). C, Postcontrast gradient echo image obtained after IV injection of Gd-DTPA shows that the lesion (arrow) does not enhance.
Bosniak II Lesion Category II lesions are minimally complicated cysts that also usually do not require surgery. This category includes: 1. Cysts with 1–3 thin septa, that is, septa ≤2 mm in thickness on CT or MRI. The septa or wall may enhance and have calcification of any type 2. Homogeneous nonenhancing cysts with >20 HU attenuation at renal mass protocol CT; may have calcification of any type 3. Homogeneous lesions of attenuation 21–30 HU at portal venous phase CT or homogeneous lesions of attenuation −9 to 20 HU or ≥70 HU on noncontrast CT 4. Homogeneous low-attenuation lesions that are too small to characterize 5. Homogeneously markedly T2 hyperintense (similar to cerebrospinal fluid) cysts or markedly T1 hyperintense (approximately 2.5 times normal renal parenchymal signal intensity) cysts on noncontrast MRI 6. Cystic lesions with any number of thin (≤2 mm) nonenhancing septa on MRI These cysts are benign and do not require treatment or radiological follow-up.
Bosniak II Lesion Minimally calcified benign cyst (arrow), category II lesion. Contrast-enhanced CT scan reveals fine linear calcification in the cyst wall and septa. Lesion shows a thin wall and no focal nodules or areas of enhancement. Fluid in the cyst measured 12 HU.
High Density Cysts These lesions have attenuation values ranging from 40 to 100 HU on nonenhanced CT scans and are usually caused by high protein content or previous hemorrhage Most are benign and can simply be monitored with serial imaging, provided the following CT criteria are fulfilled – 1. The lesion must be perfectly smooth, round, sharply marginated, and homogeneous. 2. The lesion should not enhance with IV contrast medium. 3. At least one fourth of the lesion’s circumference should extend outside the kidney so that the smoothness of some of the wall can be evaluated. 4. The lesion should be less than 3 cm in diameter.
Although most lesions meeting these criteria are benign hemorrhagic cysts, cystic RCC may rarely show similar characteristics on CT. High- attenuation renal cysts and RCCs may have a similar appearance on nonenhanced CT images. Jonisch and colleagues demonstrated that a homogeneous renal mass with an attenuation of 70 HU or higher on nonenhanced CT has a greater than 99.9% chance of representing a high-attenuation renal cyst rather than an RCC. Hemorrhagic cysts appear on MRI as sharply marginated, smooth, round, homogeneous lesions. They have high signal intensity on both T1- and T2- weighted sequences, and they do not enhance after IV administration of Gd- DTPA. In addition, hemorrhagic cysts often show fluid-iron levels on MRI, probably because of dependent settling of methemoglobin-containing sediment.
Bosniak II Lesion Hyperdense benign renal cyst, category II lesion. Nonenhanced CT scan (A) shows a high-density (92 HU) mass (arrow) located anteriorly in the kidney. On contrast-enhanced CT scan (B), the lesion (arrow) did not enhance; it is homogeneous and reveals a smooth margin where it projects beyond the renal outline.
Bosniak II Lesion Hemorrhagic cyst in upper pole of left kidney, category II lesion. A, Contrast-enhanced CT scan shows a well- defined mass (arrow) in the upper pole of the left kidney. The lesion had a precontrast and postcontrast attenuation value of 45 HU. B, Gradient echo MRI shows fluid-iron level in the cyst, with high-intensity methemoglobin sediment located posteriorly (arrow) and lower intensity cyst fluid located anteriorly. C, T2-weighted spin echo image reveals that anteriorly located cyst fluid is hyperintense, whereas dependent sediment (arrow) has a lower signal intensity.
Bosniak IIF cysts This category (“F” for follow-up) was introduced as a modification of the original Bosniak classification. It includes lesions that lie intermediately between categories II and III. They are slightly more complex than category II but do not fulfill the criteria for category III. The following cysts are included: 1. Cystic lesions with a smooth minimally thickened (3 mm) enhancing wall, or smooth minimal thickening (3 mm) of one or more enhancing septa, or many (≥4) smooth thin enhancing septa. 2. Cystic masses that are heterogeneously hyperintense at unenhanced fat- saturated T1-weighted imaging are also included.  Neither the septa nor the wall of these lesions show measurable enhancement.  These lesions are considered indeterminate and may advance to category III with time and therefore need follow-up at 6 months.
Bosniak IIF Lesion Category IIF lesion. A 4.2-cm cystic lesion at the lower pole of the left kidney with enhancing (perceived enhancement, not measurable) minimally thick wall (arrow) and multiple septa (arrowhead). Although the size is not a criterion in Bosniak classification, it is believed by various researchers that small-sized lesions (<1.5 cm) in a patient with no risk factor for RCC can be considered benign.
Bosniak IIF Lesion Benign renal cyst, category IIF lesion. Lesion (arrow) shows thick irregular mural calcification, has an attenuation value of 12 HU on a nonenhanced CT scan (A), and shows no enhancement after contrast administration (B). This lesion requires a follow-up.
On follow up of IIF cysts Progression in these cysts is generally a strong indicator of malignancy. The development of enhancing soft-tissue component(s) should upgrade the cystic mass and result in surgical treatment. Malignancy rates for category IIF cysts of up to 5% have been reported There are no definitive studies determining duration or frequency of follow-up for Bosniak IIF cysts. ACR white paper recommends first semiannual and then annual follow-up for a minimum of 5 years to allow for morphologic change to be identified
Bosniak III Lesion Lesions in category III are indeterminate complicated cystic lesions. The presence of nodular and thick calcifications in walls or septa, which was considered a criterion for inclusion in category III according to the original Bosniak classification, is no longer a criterion. The modified version of the Bosniak system stresses the presence of abnormal enhancement of the walls or septa for inclusion in category III. The wall or septa of these lesions are thick or irregular and most show measurable enhancement. Category III includes: Cystic masses that are more complicated and contain enhancing thick irregular walls and/or septa are included here. Cystic masses with one or more enhancing thick (≥4 mm width) walls or septa, or enhancing irregular (displaying ≤3 mm obtusely marginated convex protrusion/s) on the walls or septa. Approximately 50% of the Bosniak III masses are malignant (ranging from 25% to 100%) Definitive treatment of lesions in this category is surgery
Bosniak III Lesion Bosniak III lesion. (a) Axial nonenhanced CT image shows a low-attenuating (18-HU) mass arising from the left kidney. (b) Axial contrast-enhanced CT image shows multiple measurably enhancing (45 to 75 HU) septations in the mass (arrow). The lesion was resected and was found to be cystic renal cell carcinoma.
Bosniak III Lesion Multilocular cystic renal cell carcinoma, category III lesion. A contrast-enhanced CT scan reveals a well- defined, mainly fluid- containing mass in the right kidney. Enhancing septa are seen through- out the lesion. A right nephrectomy was performed.
Bosniak IV Lesion A Bosniak IV cyst contains enhancing nodular soft- tissue components in their septa or their wall and is considered malignant until proven otherwise. A small percentage of these lesions have a benign pathology (eg. MEST). Bosniak IV cystic masses have an enhancing nodule. The “nodule” is defined as a focal enhancing convex protrusion in the wall or septa that has acute margins (of any size) or a focal enhancing convex protrusion ≥4 mm that has obtuse margins with the wall or septa. An enhancing convex protrusion with obtuse margins that is less than or equal to 3 mm is not considered as a nodule, and is a feature of a Bosniak III mass. Malignancy rates in this category are approximately 80% (range 67–100%). Definitive treatment of these lesions is surgery. Imaging surveillance is considered an acceptable alternative management approach in patients with a short life expectancy or comorbidities, especially because these lesions tend to be less aggressive than ccRCCs
Bosniak IV Lesion Bosniak IV lesion. (a) Axial nonenhanced CT image shows a low-attenuating (22-HU) cystic lesion arising from the left kidney. (b) Axial contrast-enhanced CT image shows an enhancing (114-HU) peripheral nodule (arrow) with subtle enhancing septations. The lesion was resected and was found to be cystic renal cell carcinoma.
I. MEST These complex cystic and solid masses are characterized histologically by the presence of stromal elements that resemble ovarian stroma, as well as an epithelial component consisting of cysts with an epithelial lining. MEST is most frequently encountered in perimenopausal women (female-to-male ratio, 11:1), with an average age at presentation of 56 years . The use of oral estrogen or OCPs is frequently reported in patients with MEST. The imaging appearance is nonspecific, and MEST cannot be confidently differentiated from cystic renal cell carcinoma at imaging. MEST manifests at imaging as – 1. Expansile multiloculated cystic mass that may herniate into the renal pelvis. The intervening septa show variable contrast enhancement, and an enhancing mural nodule or mural calcifications may be seen. Delayed enhancement is often noted. 2. Intralesional hemorrhage may be seen and is suggested by the presence of hyperattenuating components at CT or hyperintense foci at T1-weighted MR imaging.
MEST Axial (a) and coronal (b) contrast-enhanced CT images show a cystic and solid mass in the left kidney, with numerous enhancing septations. The mass extends into the renal pelvis (arrow in b) and was confirmed to be MEST at histologic analysis.
II. Cystic Nephroma  Cystic nephroma is a rare benign tumor that typically occurs in boys younger than 4 years or women aged 40–60 years.  Children often present with a painless abdominal mass, while adults are more likely to present with abdominal pain or hematuria but may be asymptomatic.  On cross-sectional images, cystic nephroma typically appears as 1. A well-circumscribed, encapsulated, multicystic mass with variably enhancing septa. When compared with simple fluid, the contents of the mass may have similar or slightly higher attenuation at CT and higher signal intensity at T1-weighted MR imaging. 2. The lesions often herniate into the renal pelvis or proximal ureter. 3. Because of their fibrous composition, the septa and capsule of the lesion are hypointense on both T1- and T2-weighted MR images. Nonetheless, cystic nephroma may mimic a variety of benign and malignant renal lesions, including complex renal cysts and cystic renal cell carcinoma, along with MEST and localized cystic disease. Because it is challenging to distinguish cystic nephroma from cystic renal cell carcinoma, many cases are treated surgically.
Cystic Nephroma Multilocular cystic nephroma. A, Transverse sonogram demonstrates a multiseptated, exophytic, complex cystic mass with noncommunicating locules Accordingly, the recent WHO classification considers ACN to be a part of the MEST family of tumors. These lesions predominantly occur in middle-aged females. Pathologically, ACNs are more cystic, whereas MESTs have a greater solid ovarian like stromal component
Cystic nephroma. Coronal T2-weighted MR image (a) and axial contrast-enhanced fat-saturated T1-weighted gradient-recalled- echo MR image (b) show a multiseptated cystic mass arising from the upper pole of the right kidney and extending into the renal pelvis. In a, variable signal intensity in the contents of the cyst locules represents areas of simple and complex fluid, and signal hypointensity in the surrounding capsule and septa of the lesion reflects their fibrous content. Enhancement of the thin septa is seen in b.
III. AML with epithelial cysts (AMLEC) This cystic variant of AML is rare and usually seen in women. There are no specific ultrasonography features. They may present as a Bosniak category 4 mass, mimicking a cystic RCC. The solid part is usually hyperattenuating on CT, homogeneously enhancing, and hypointense on T2WI, which are diagnostic clues. A signal drop on CS-MRI may sometimes occur in this type of AML.
Malignant Bosniak IIF, III, IV lesions I. RCC with cystic changes RCC can undergo cystic changes, more commonly seen with ccRCC. These constitute less than 5% of all RCCs. It demonstrates a male predominance (male-to-female ratio of 2:1), mostly occurring in the sixth to seventh decades. ccRCC with cystic changes is a multiloculated cystic mass with an irregularly thickened wall and areas of enhancing solid nodularity. It is important to differentiate this from RCC with necrosis which typically has a poorer prognosis
II. Multilocular cystic renal neoplasm of low malignant potential  Multilocular cystic renal cell neoplasm is an uncommon neoplasm.  These tumors are seen in middle-aged adults with no gender predilection.  They are almost entirely cystic, containing serous fluid. The septa are lined by single or few layers of clear cells without any expansile component.  On imaging, they are classified ranging from a Bosniak IIF to Bosniak IV lesion, with the higher Bosniak categories corresponding to an increased degree of vascularized fibrosis within the lesion.  It is a low-grade tumor with an excellent prognosis and no reported cases of metastasis. A well-defined rounded cystic lesion measuring 4.5 x 5.0 x 4.5 cm is noted at the mid pole of the right kidney. The lesion is hypointense on T1W and hyperintense on T2WI, has relatively thick wall and internal septations, and shows no diffusion restriction. Post-contrast scan shows enhancement of the wall and the septae which is equal to the renal parenchyma. No solid component, or hemorrhagic contents are seen in it (Bosniak 3 lesion)
III. Tubulocystic RCC Tubulocystic RCC is another rare tumor. It is also termed low-grade collecting duct carcinoma and Bellini duct carcinoma, and has overlapping pathologic features with pRCC. Tubulocystic RCCs occur mostly in men (85% men vs 15% women) in the sixth decade. On imaging, the lesions have increased echogenicity and may appear solid on USG, cystic to indeterminate attenuation on CT, and cystic on MRI. This pattern of increased echogenicity/solid appearance (due to microcysts) on USG and cystic on CT/MRI indicate the probable diagnosis. Prognosis is excellent with metastases being rare.
Summary Bosniak algorithm
Renal sinus cysts (Peripelvic Cysts) Originate within the sinus itself and have a lymphatic origin Are asymptomatic cysts that arise from the lymphatics of the renal hilum. They usually are multiple, small, and bilateral. Their appearance is often difficult to distinguish from hydronephrosis. However, the distinction is readily apparent on contrast-enhanced excretory phase CT images, where the high-attenuation contrast agent–filled collecting system can easily be distinguished from fluid-attenuation renal sinus cysts. Benign extra parenchymal cysts located in renal sinus. Uni/multilocular and usually bilateral. These cysts are surrounded by renal sinus fat. Cysts of Renal Pelvis. Coronal excretory phase maximum intensity projection CT image of the abdomen shows the contrast material– filled collecting system surrounded by multiple cysts in the renal pelvis.
Cysts of Renal Pelvis Cysts involving the renal pelvis are classified as parapelvic or renal sinus cysts according to their site of origin. Parapelvic Cysts Are cortical cysts that extend into the renal sinus fat. Histologically, they are lined with a single epithelial layer. They usually are single and unilateral but may be multiple. Although most are asymptomatic, parapelvic cysts may cause hydronephrosis if they compress the collecting system. At US, CT, and MR imaging, parapelvic cysts are simple cysts that extend into the renal sinus fat. Rarely, parapelvic cysts have complex features at imaging.
Differentials of a T2 dark renal cyst (1) Lipid poor AML A small percentage of AMLs are lipid poor in the general population. About 30% of their AMLs in tuberous sclerosis are lipid poor. (2) Hemorrhagic cyst These will likely be T1 bright (3) Papillary subtype RCC Clear cells (the most common sub-type) are T2 hyperintense Both clear cell and papillary will enhance, but the clear cell enhances more avidly (equal to cortex on cortico-medullary phase).
Focal Cystic Renal Disease 1. Mixed Epithelial and Stromal Tumor (MEST). 2. Cystic Nephroma. 3. Pyelocalyceal Diverticulum.
Pyelocalyceal Diverticulum Pyelocalyceal diverticulum is a urine-filled cavity within the renal cortex that communicates with the renal collecting system. There are two types of pyelocalyceal diverticulum – 1. Type 1, the more common type, communicates with a minor calyx. 2. Type 2 connects with a major calyx or the renal pelvis. Pyelocalyceal diverticulum is relatively rare and is identified at 2.1–4.5 per 1000 intravenous urographic evaluations. The etiology of pyelocalyceal diverticulum is controversial; however, most cases are thought to be congenital. The most common theory is that pyelocalyceal diverticulum results from failure of regression of the third- and fourth-generation ureteric buds. A smaller percentage of cases of pyelocalyceal diverticulum may be acquired, resulting from obstructing stones or infection.
Fifty percent of cases of pyelocalyceal diverticulum are complicated by calculi and milk of calcium. A simple pyelocalyceal diverticulum without stones has an appearance similar to that of a simple cyst at ultrasonography (US), MR imaging, and nonenhanced or nephrographic phase contrast-enhanced CT. In the excretory phase of imaging, the structure fills with contrast material because of communication with the renal collecting system, and layering of contrast material is seen within the cyst like structure. Complications of pyelocalyceal diverticulum include hemorrhage, infection, and cyst rupture. Malignancy in a pyelocalyceal diverticulum is exceedingly rare, with only four cases reported in the literature Pyelocalyceal diverticulum. (a) Axial contrast-enhanced nephrographic phase CT image shows calcifications in a cystic renal structure. (b) Axial contrast-enhanced excretory phase CT image shows layering of excreted contrast material in the cystic structure, a finding that proves communication with the collecting system.
Pyelocalyceal Diverticulum Ruptured pyelocalyceal diverticulum in a woman who presented with left flank pain. (a) Axial contrast-enhanced CT image shows a cystic lesion in the left kidney. (b, c) Axial contrast-enhanced delayed phase CT images show filling of the cystic structure with excreted contrast material, a finding that confirms connection with the collecting system. Excreted contrast material tracks from this structure into a retroperitoneal fluid collection (arrow in c), a finding that confirms a urinoma from a ruptured pyelocalyceal diverticulum.
Acquired Multifocal Cystic Renal Disease 1. Glomerulocystic kidney disease. 2. Lithium induced nephrotoxicity. 3. Acquired cystic kidney disease.
Glomerulocystic kidney disease Glomerulocystic kidney disease (GCKD) is a rare form of cystic renal disease characterized histologically by uniform cystic dilatation of the Bowman capsule. These cysts are located predominantly in the subcapsular area of the renal cortex and spare the tubular portions of the nephron. The cortical and subcapsular distribution helps distinguish GCKD from other cystic renal diseases, such as ARPKD, ADPKD, MCDK, and cystic renal dysplasia. Most often, GCKD occurs in neonates and young children. In many instances, mild renal failure with accompanying infantile congenital malformations is seen. Although an autosomal dominant inheritance pattern has been demonstrated in families with GCKD, several sporadic cases have also been reported.
GCKD The predominant imaging finding is – 1. Small renal cysts with a primarily cortical subcapsular distribution. 2. The kidneys may be normal in size or hypoplastic. The cysts may be difficult to define at US, and cortical hyperechogenicity may be seen instead of the typical anechoic appearance of simple cysts. Because patients typically present with symptoms of renal failure, nonenhanced CT is typically performed but is of limited value in establishing the diagnosis. Nonenhanced T2- weighted MR imaging is the preferred imaging modality. Coronal T2-weighted MR image shows numerous hyperintense lesions predominantly in the subcapsular portions of the renal cortex, a finding indicative of simple cysts. This distribution is a classic finding in GCKD.
Lithium induced nephrotoxicity Nephrotoxicity due to long-term lithium therapy is a well-described entity and is divided into three main categories – 1. Acute intoxication. 2. Nephrogenic diabetes insipidus – m/c and reversible with cessation of drug. 3. Chronic renal disease. CT and MR imaging findings of lithium induced nephrotoxicity typically include- 4. Normal- sized kidneys with abundant and uniformly distributed renal microcysts. 5. Microcysts have been reported in 33%–62% of patients. These cysts are found in the cortex and medulla and typically are 1–2 mm in diameter.  At CT, the cysts appear as multiple tiny hypoattenuating lesions. MR imaging best depicts the number and size of cystic lesions.  Cysts are best appreciated on T2-weighted MR images, which will depict multiple hyperintense foci.  The D/Ds includes ADPKD, GCKD, medullary cystic kidney disease, and acquired cystic kidney disease. These entities can usually be differentiated from lithium-induced nephrotoxicity on the basis of clinical history as well as the size and distribution of cysts at imaging.
Axial contrast-enhanced CT image shows multiple hypoattenuating lesions scattered throughout the cortex and medulla of both kidneys. The findings are consistent with microcysts. Coronal T2-weighted MR image shows multiple tiny hyperintense lesions uniformly distributed throughout both kidneys. The findings are consistent with microcysts.
Acquired cystic kidney disease Acquired cystic kidney disease refers to the development of renal cysts in patients with end- stage renal disease who do not already have a heritable cystic renal disease. Risk factors include male sex and increased duration of time on dialysis. The male-to-female ratio is 3:1. Acquired cystic kidney disease is thought to be multifactorial in cause. The initiating factor is thought to be compensatory hypertrophy of functional nephrons in response to adjacent tissue destruction. Renal tubular obstruction and expansion caused by interstitial fibrosis, epithelial hyperplasia, and increased fluid secretion are thought to lead to cyst development. There is no correlation with race, cause of renal failure, or method of dialysis (ie, hemodialysis or peritoneal dialysis). Acquired cystic kidney disease. Sagittal sonogram demonstrates an echogenic small kidney containing multiple cysts. A small amount of intraperitoneal dialysate fluid is seen.
It is postulated that with progressive destruction of functioning renal tissue, cystogenic substances accumulate and cause hypertrophy and hyperplasia of epithelial cells. The tubular epithelium proliferate and cystic dilatation of tubules occurs. Subsequently, there is a continuum of changes from single-layered epithelial cysts to multilayered cysts to RCCs. Approximately 8%–13% of patients with end-stage renal disease have acquired cystic kidney disease before they undergo dialysis. After 10 years of dialysis, the incidence is believed to approach 100%. Complications include cyst hemorrhage, ureteral stones, and the development of renal cell carcinoma. Renal malignancy will develop in 3%–7% of patients with acquired cystic kidney disease. Risk factors include male sex and an increased length of time on dialysis. The imaging features are small echogenic kidneys with multiple cysts in kidneys (in USG) and similar in CT/MRI. Acquired cystic renal disease. (a) Coronal T2-weighted MR image shows no renal lesions. (b) Coronal contrast-enhanced CT image obtained in the same patient 6 years later, after 5 years of dialysis, shows numerous small cysts throughout both kidneys.
Multicystic dysplastic kidney MCDK is a nonheritable developmental disorder in which the kidney is replaced by non-functioning noncommunicating cysts. It occurs in approximately one of 4300 live births. Most common form of cystic disease in infants. The imaging appearance of MCDK varies by age. Often it is diagnosed in utero. 1. In children, it is most commonly assessed with US and appears as a large cluster of noncommunicating cysts that replaces the renal parenchyma. 2. In some cases, the cysts may replace only a portion of the renal parenchyma. In children diagnosed with MCDK, it is important to assess the contralateral renal collecting system because there is vesicoureteral reflux in 5%–43% of cases. Also associated with renal artery hypoplasia. Two types of MDK may be shown by imaging, pelvoinfundibular atresia and the hydronephrotic type.In the classic type, pelvoinfundibular atresia, no discernible renal pelvis is demonstrated on imaging. The kidney may be small, normal in size, or enlarged and contains multiple variably-sized noncommunicating renal cysts . No perfusion of the affected kidney occurs on renal scintigraphy, and contrast-enhanced CT shows no evidence of contrast excretion by the affected kidney. The hydronephrotic form of MDK is characterized by dilatation of the renal pelvis and calyces. Differentiation of this type of MDK from simple hydronephrosis is based on the imaging demonstration of parenchymal cysts that do not communicate with the collecting system.
The proposed theory of this dysplastic kidney is failure of the ureteric bud to induce development of metanephric blastema. The ureteric bud may be absent or may not integrate with metanephric blastema owing to urinary tract obstruction. It may also be picked up on antenatal USG Multicystic dysplastic kidney. Small, malformed kidney containing multiple cysts.
In most cases, the dysplastic kidney will completely regress by adulthood. However, in few of cases, a small residual mass may be seen in the renal fossa. Typical imaging features are – 1. A central region of soft tissue; peripheral noncommunicating cysts. 2. Absence or severe atrophy of the ipsilateral ureter, renal collecting system, and renal vasculature. Occasionally, the cysts may be calcified and identifiable at radiography. There may be compensatory hypertrophy of the contralateral kidney. Coronal T2-weighted MR images show a small collection of hyperintense cysts in the left renal fossa (arrows in a), a finding that represents MCDK. Compensatory hypertrophy of the otherwise normal right kidney is seen in b.
A few patients present in adulthood. The kidney regresses in size and presents as “renal agenesis” or a small central soft tissue surrounded by peripheral cysts which could be confused with cystic renal neoplasm. The findings that help to reach the correct diagnosis are nonfunctioning kidney with peripheral cysts around central soft tissue along with atrophic pelvicalyceal system, ureter, and renal vessels. With time, the affected kidney involutes and is typically not a source of clinical concern. The contralateral kidney may show compensatory hypertrophy. There may also be a segmental form involving the upper pole of a duplex kidney and this may be associated with ureterocele of the lower moiety.
Localized cystic renal disease Localized cystic renal disease is a rare condition in which a cluster of tightly spaced, variably sized cysts replaces a portion of or the entire renal parenchyma of one kidney. Normal renal parenchyma is seen between the cysts. The remaining renal parenchyma in the affected kidney as well as in the contralateral kidney may contain a few scattered cysts. Also known as segmental renal cystic disease and unilateral renal cystic disease. It is a benign, nonprogressive condition. Multiple cysts are seen in one kidney with no cysts in the contralateral kidney or any other organ. Most patients are asymptomatic. Some may present with abdominal pain, hematuria, or hypertension but with intact renal function. At imaging, localized cystic renal disease manifests as – 1. A conglomeration of cysts in one kidney. 2. The presence of intervening normally enhancing renal parenchyma and the absence of a capsule around the cysts help distinguish from cystic nephroma and multiloculated cystic RCC. However, the intervening renal parenchyma may be stretched and attenuated, which makes differentiation of thick septations from intervening parenchyma difficult. Remaining renal parenchyma and contralateral kidney are generally normal but can show a few scattered cysts.
Localized Cystic Renal Disease Localized cystic renal disease. Axial (a) and coronal (b) T2-weighted MR images show a cluster of nonenhancing cysts replacing the middle of the right kidney. No capsule is seen around the cysts, and normal renal parenchyma is seen between the cysts.
Renal Infections appearing as cystic lesions 1. Renal Abscess 2. Renal Aspergillosis 3. Renal Echinococcosis
Renal abscess Renal abscesses are typically seen as a complication of acute pyelonephritis. They often occur as the result of untreated or incompletely treated pyelonephritis or ascending urinary tract infections and hematogenous seeding in patients with bacteremia. Immunocompromised, pregnant, and diabetic patients have an increased risk for abscess formation. At radiology, a renal abscess may mimic a renal cyst; however, differentiation can typically be made on the basis of the patient’s clinical scenario. 1. US most commonly demonstrates an anechoic to hypoechoic complex cystic mass with posterior acoustic enhancement. Internal echoes, septations, and loculations may also be seen at US. 2. CT and MR imaging are more sensitive modalities for detecting a renal abscess. Contrast- enhanced CT demonstrates a well-defined, hypoattenuating, round mass, often with a thick rim or halo. Perinephric extension of the abscess may be seen. 3. MR imaging can also be used to evaluate renal abscesses, particularly in cases where it is important to limit ionizing radiation exposure. MR imaging typically shows a heterogeneous thick-rimmed lesion that is hypointense on T1-weighted images and hyperintense on T2-weighted images, with restricted diffusion. The presence of gas will result in blooming artifact on gradient- recalled-echo MR images.
Renal abscess in a patient with a 1-week history of fever, chills, and left flank pain who had recently been treated for pyelonephritis. Coronal contrast-enhanced CT image shows a hypoattenuating mass with a thickened enhancing rim in the left kidney, a finding consistent with a renal abscess.
Renal aspergillosis Renal aspergillosis is an uncommon renal infection that manifests as a complex cystic renal mass. Renal aspergillosis occurs in immunocompromised patients, including patients with diabetes or human immunodeficiency virus infection and those undergoing corticosteroid therapy. Renal aspergillosis is acquired in one of three ways – 1. Hematogenous dissemination – m/c. 2. Ascending infection. 3. Aspergillus casts in the renal pelvis. The radiologic appearance of renal aspergillosis mimics that of a complex renal cyst or abscess. 4. CT demonstrates a hypoattenuating mass with a thick enhancing wall and internal septations. The surrounding renal parenchyma may demonstrate features of focal pyelonephritis and delayed enhancement. The diagnosis of renal aspergillosis is typically made after urinalysis or aspiration of the lesion.
Renal aspergillosis Renal Aspergillosis in a patient with AIDS who presented with flank pain and fever. (a) Axial contrast-enhanced CT image shows a large unilocular cystic mass arising from the left kidney, with thick rim enhancement. (b) Axial contrast-enhanced CT image obtained 4 months later shows progression to a multilocular, complex, inflammatory mass that extends into the perinephric space.
Renal Echinococcosis Echinococcosis is a zoonosis caused by the larvae of the Echinococcus tapeworm. Humans can be an intermediate host when infected through contact with the definitive host or through contaminated water or food. Liver serves as the first line of defense and is the m/c involved organ (75%). The lungs are the second m/c site of involvement (15%). However, hematogenous dissemination can occur to almost any anatomic location and may rarely involve the kidneys. Renal echinococcosis is seen in 2%–3% of patients with echinococcal disease. Renal hydatid cysts typically are unilateral single lesions that are found in the upper or lower poles of the kidney. Imaging findings vary according to the stage of cyst growth. 1. Renal hydatid cysts initially appear as unilocular cystic masses. 2. In the intermediate stage, the cysts appear as a multilocular cystic mass because of the development of multiple daughter cysts .These cystic masses demonstrate a thick enhancing wall. Internal septations, when present, typically demonstrate enhancement. 3. With quiescence or death of the parasite, the lesion becomes calcified.
Renal Echinococcosis Renal echinococcal (hydatid) disease. (a) Axial nonenhanced CT image shows a complex cystic mass arising from the right kidney. (b) Axial contrast-enhanced CT image shows the thick enhancing wall of the mass, with enhancing internal septations.
Heritable Multi­ focal Cystic Renal Disease 1. Autosomal Dominant Polycystic Kidney Disease (ADPKD) 2. Von Hippel Lindau Disease (VHL) 3. Tuberous Sclerosis (TS)
ADPKD ADPKD is a heritable cystic renal disease that causes cysts along the nephron. The disease occurs in approximately one of 500–1000 live births and is the most common heritable renal disorder. There are two types of ADPKD. Mutation in the PDK1 gene, in the short arm of chromosome 16 – m/c, 85%. Mutation in the PDK2 gene, in the long arm of chromosome 4. and 3. PKD-3. The PKD2 mutation results in a milder form of disease than the PKD1 mutation and also has a later age of onset. Age of presentation is 44.8 years in PKD-1 and 69.1 years in PKD-2. Most patients are born with normal kidneys. Only 2% patients present before 15 years of age. Most affected individuals present in the fourth or fifth decade with flank pain, palpable masses, hypertension, hematuria, urinary tract infection, proteinuria, or polycythemia. Progressive azotemia ensues as renal parenchyma is destroyed and renal failure occurs in most patients by the age of 60 years. Complications include superimposed infection, cyst hemorrhage, uric acid stones, and hypertension.
The radiologic appearance of ADPKD includes enlarged kidneys with multiple cysts in bilateral kidneys. Extrarenal manifestations may also be seen and most commonly include hepatic cysts. Cysts are less commonly seen in the pancreas, spleen, ovaries, and testes. Patients are at an increased risk for cardiac valve abnormalities (26%) and intracranial aneurysms (10%–15%). Renal complications of ADPKD include cyst hemorrhage, cyst infection, cyst rupture, and nephrolithiasis. About 50% of individuals with ADPKD will develop end-stage renal disease. Unlike in some other congenital cystic kidney diseases, there is no increased risk for renal cell carcinoma unless the patient is undergoing prolonged dialysis. Genetic testing identifies only 70% of individuals with ADPKD. Sunburst or streak nephrogram may be seen in IVU
Imaging plays an important role in diagnosis, detection of complications, and follow-up. Conventional radiograph may reveal bilateral enlarged kidneys with lobulated margins causing displacement of the surrounding structures, such as colonic flexures. Calcification may be curvilinear or amorphous and the presence of renal calculi often signifies superimposed infection. On intravenous urography, in nephrogram phase numerous, smoothly marginated radiolucencies throughout the cortex and medulla giving "Swiss cheese nephrogram "Spider leg pyelogram" is also described, since a stretched-out and attenuate, stretched and crescentic calyces is seen as a result of the mass effect caused by renal cysts.
Ultrasound is the modality of choice for primary evaluation of most patients with suspected ADPKD. On US, cysts of varying sizes are noted in enlarged kidneys, which may result in a lobulated outline. However, the reniform shape is maintained. Similar cysts may be seen in liver, pancreas, and spleen. Most patients develop renal cysts by their third decade. Antenatal diagnosis is sometimes also possible on demonstration of enlarged cystic kidneys, ascites, and hepatomegaly. The presence of even one cyst in a genetically predisposed child is suspicious of ADPKD. On CT and MR, variable sized cysts which are usually ≥3 cm in diameter are seen diffusely involving bilateral enlarged kidneys. Imaging also helps to identify infection, hemorrhage, and rupture. B, Advancing disease: renal enlargement and more cysts.
On MRI, the renal cysts are hypointense on T1-weighted and hyperintense on T2- weighted images, with infected and proteinaceous cysts showing intermediate signal intensities MRI based renal and cyst volumetry enables assessment of disease progression. With increasing renal volume, the glomerular filtration rate may decline and the patient may present with abdominal pain, hypertension, or hematuria. Renal blood flow may also decrease suggesting progression of the disease. Imaging is critical in evaluation of patients who have a first-degree relative with ADPKD. It is especially so in evaluation of prospective donors who plan to donate a kidney to their kin with ADPKD. Severe cases can be diagnosed easily on imaging but a milder form of the disease, especially with PKD2 mutation, can be difficult to diagnose. Genetic testing helps in the diagnosis of ADPKD in only 70% cases.
Coronal T2-weighted MR image shows the kidneys expanded by innumerable cysts, a finding consistent with ADPKD. Several hepatic cysts are also seen.
ADPKD: Associations  Cerebral berry aneurysms  Found in 6% of patients with ADPKD without a family history of aneurysms  Found in up to 22% of patients with ADPKD with a family history 16  Intracranial dolichoectasia: 2-3%  Hypertension: up to 80% of adults  Ascites: due to rupture liver cyst or portal hypertension  Colonic diverticulosis  Small bowel diverticula (perhaps)  Bicuspid aortic valve  Mitral valve prolapse: up to 25%  Aortic dissection  Multiple biliary hamartomas (Von Meyenberg complexes)  Cysts in other organs  Liver: most common, 75% by age 60 years  Ovaries  Spleen: ~5 %  Seminal vesicles: 60% by age 40 years  Prostate: 11%  Pancreas: ~10%:  Pancreatic cysts are more common in Von Hippel-lindau disease (VHL)
ARPKD Autosomal recessive polycystic kidney disease is characterized by ectasia of the renal collecting ducts, interstitial fibrosis, and variable degrees of portal hepatic fibrosis (congenital hepatic fibrosis), which often causes portal hypertension 10% to 90% of collecting ducts are involved. Severity of the disease depends on, and is inversely related to, the age at presentation. Patients with milder renal involvement may present at any age from infancy to early adulthood. The severity of renal involvement and extent of liver involvement in these patients are inversely related. On noncontrast CT examination, the kidneys are smooth, enlarged, and low in attenuation, likely a reflection of the large fluid volume in the dilated ducts. Administration of contrast medium reveals a striated nephrogram, representing accumulation of contrast in the dilated tubules. Tubular ectasia is confined mainly to the renal medulla, with only occasional macrocysts. The renal cortex is less severely affected. CT may also show evidence of congenital hepatic fibrosis and portal hypertension. The liver contour is often irregular, and dilated intrahepatic bile ducts, ascites, splenomegaly, and enlarged portosystemic collateral veins may be seen
Associated oligohydramnios may be identified antenatally (pulmonary hypoplasia with neonatal mortality) Enlarged kidneys usually retain a reniform shape Cysts Because the cysts are sub-millimetric, they act as thousands of tiny interfaces that scatter and bounce the ultrasound waves back to the transducer. This creates a blended, grainy mixture of bright white reflections ("salt") and darker fluid areas ("pepper") Cysts may become discernible with time as they enlarge, but rarely exceed 1-2 cm in diameter (unlike ADPKD) 13 High-resolution ultrasound (linear-array transducer, 7.5 MHz or greater) may allow visualization of cylindrical cysts in the medulla and cortex, representing ectatic collecting ducts 8 Medullary pyramids Initially may appear hypoechoic compared to the cortex: which can give a peripheral halo during this stage Become increasingly hyperechoic with time, reducing corticomedullary differentiation Differences from ADPKD:  Smaller cysts  Cyst free parenchyma do not show normal echogenicity in ARPKD  Loss of distinction between cortex and medulla in ARPKD
Autosomal Recessive Kidney Disease. Two-year-old patient with renal insufficiency. Sagittal sonogram shows marked renal enlargement and innumerable microcysts. A characteristic perirenal halo may be seen, representing the peripherally compressed hypoechoic normal cortex around the hyperechoic medulla. Tiny, punctate, echogenic foci develop with time due to calcium deposition and correlate with renal failure. These foci do not show posterior acoustic shadowing but may cause ring-down artifacts.
ARPKD
ARPKD: Associations Congenital hepatic fibrosis is present in virtually 100% of the cases Other associated clinical manifestations are primarily portal hypertension, splenomegaly, and cholangitis.
VHL Disease Is an autosomal dominant disease that is linked to a tumor- suppression gene on chromosome 3. The most common visceral manifestations of VHL are renal cysts (59%–63% of patients), renal cell carcinoma (24%–45%), pheochromocytoma (0%– 60%), pancreatic cysts (50%–91%), and pancreatic neoplasms such as serous cystadenomas (12%) and neuroendocrine tumors of the pancreas (5%–17%). Of these, renal cell carcinoma is a significant source of mortality. Other findings are retinal angiomas, CNS hemangioblastomas In patients with VHL, cysts that appear simple at imaging often contain small foci of renal cell carcinoma. As time progresses, these small foci may grow, forming a solid mass as the original cyst involutes. Therefore, in a patient with VHL, no cyst can be confidently dismissed at imaging as free of tumor cells.
VHL Disease Visceral manifestations of VHL. (a, b) Coronal T2WI (a) and coronal T1+C fat sat gradient-recalled-echo MR image (b) show two complex enhancing cystic lesions (arrowheads) in the left kidney, findings compatible with cystic renal cell carcinomas. Several additional less complex cystic renal lesions are also seen. An enhancing solid and cystic lesion seen in the left adrenal gland (arrow) is a finding compatible with pheochromocytoma. (c) Axial T2WI shows two cystic pancreatic lesions (arrow), findings compatible with a serous cystadenoma of the pancreas.
Tuberous Sclerosis (Bourneville disease) Is an AD neurocutaneous syndrome that causes hamartomas in the kidneys, lungs, liver, brain, heart, eyes, and skin. It is caused by mutations in the genes TSC1 and TSC2, which encode for hamartin and tuberin, respectively. The m/c renal manifestation of tuberous sclerosis is angiomyolipoma, which occurs in 55%–75% of patients followed by simple renal cysts. It is unclear whether patients with tuberous sclerosis are at increased risk for RCC. However, the average age of patients with tuberous sclerosis and renal cell carcinoma is 28 years, which is 25 years younger than the average age of patients with renal cell carcinoma in the general population. At imaging, differentiation of lipid-poor angiomyolipoma from renal cell carcinoma is difficult. Rapid growth and the presence of calcification are two imaging findings that suggest renal cell carcinoma. However, a biopsy may be necessary for differentiation. As patients with TS age, their risk for end-stage renal disease increases because of the mass effect of cysts and angiomyolipomas on the renal parenchyma. Angiomyolipomas also can rupture and cause substantial hemorrhage.
Tuberous Sclerosis Tuberous sclerosis. Axial CECT images show the kidneys to be enlarged and replaced by numerous cysts and soft-tissue masses. In the right kidney, a focus of fat (arrow in a) and a small intralesional aneurysm (arrow in b) are findings consistent with angiomyolipomas. The right kidney was subsequently removed because of hemorrhage and was found to be replaced by cysts and angiomyolipomas. The angiomyolipomas were composed mostly of smooth muscle elements and very little fat.
Medullary Cysts Medullary Sponge Kidney Medullary sponge kidney (MSK) is defined as dilated, ectatic collecting tubules. It may be focal or diffuse. The etiology is unknown. Mostly sporadic, a few cases with autosomal dominant inheritance The incidence of MSK in the general population is not known, but it is found in up to 12% of patients with renal calculi. Associations with hemi hypertrophy, Ehlers-Danlos syndrome, congenital HPS, HPT, Caroli’s disease, and ARPKD have been reported. The onset of symptomatic MSK is usually in the third to fourth decades of life, patients may present with hematuria, renal colic, or urinary tract infection. Tubular ectasia may be difficult to impossible to recognize at ultrasound. When nephrocalcinosis is present, multiple echogenic shadowing foci are seen localized to the medullary pyramids. This entity typically involves both the kidneys. Unilateral or segmental involvement may be seen at times.
At excretory urography, papillary blush is classically noted as contrast pools in the dilated medullary collecting ducts. This gives a classic “paint brush” appearance. Striated appearance of the medullary collecting ducts with calyceal distortion may also be encountered as the disease increases in severity. US examination reveals echogenic medullary pyramids. Tiny calculi can be seen involving the medullary pyramids. On CT examination, the typical papillary blush appearance can be noted in the urographic phase with associated calculi in the dilated collecting ducts suggestive of medullary nephrocalcinosis. CT urography scores over EU in identification of dilated medullary collecting ducts, calculi, obstruction, and infection.
Nephronophthisis and Medullary Cystic Kidney Disease Medullary cystic disease complex applies to two tubulointerstitial diseases— nephronophthisis and medullary cystic disease—which are progressive and the most common heritable cause of renal failure in children and young adults. It shows autosomal dominant inheritance. Genetic mutations result in protein derangement which further results in ciliary dysfunction and formation of medullary cysts. Nephronophthisis has three forms—infantile, juvenile, and adolescent—based on the age of onset. The juvenile form is the most common with presentation at 4–6 years of age. Renal failure can ensue at an average age of 13 years. MCKD presents in young adults. Patients with both the forms of the disease present with polyuria, polydipsia, and salt wasting. They may also develop hyperuricemia and gout.
Imaging features are similar for both the disease entities. At US imaging, kidneys are normal to small sized with increased echogenicity and poor corticomedullary differentiation. Multiple small-sized cysts <3 cm can be seen at the corticomedullary junction and in renal medulla, but these are typically seen after the onset of end-stage renal disease. Renal cysts increase with increasing age. When present, the typical location of the cysts and characteristic clinical findings help to differentiate MCKD from other renal cystic diseases. Patients progress to renal failure and renal transplantation is the treatment of choice. The disease does not involve the transplanted kidney. The kidneys are enlarged for age (right = 8.3 x 4.3 x 4.0 cm, left = 9.1 x 3.5 x 3.5 cm). Multiple small cysts located in the medulla and corticomedullary junctions, with sparing of the cortices. Scattered punctate calcifications in the medulla. Cortical echogenicity is increased.
References: •Textbook of Radiology And Imaging, David Sutton, 8th edition •Grainger and Allison Diagnostic radiology, 7th edition •John R. Haaga et al., CT and MRI of the whole body, 6th edition •Carol M Rumack, Diagnostic Ultrasound, 6th edition