DOI: 10.1148/rg.24si045515
RadioGraphics 2004;24:S101-S115
© RSNA, 2004
From the RSNA Refresher Courses
A Practical Approach to the Cystic Renal Mass1
David S. Hartman, MD,
Peter L. Choyke, MD and
Matthew S. Hartman, MD
1 From the Department of Radiology, Milton S. Hershey Medical Center, Penn State University School of Medicine, HO66, 500 University Dr, Hershey, PA 17033 (D.S.H.); the Department of Radiology, Clinical Center, National Institutes of Health, Bethesda, Md (P.L.C.); and the Department of Radiology, Emory University School of Medicine, Atlanta, Ga (M.S.H.). Presented as a refresher course at the 2003 RSNA scientific assembly. Received March 29, 2004; revision requested April 28 and received May 14; accepted May 25. All authors have no financial relationships to disclose. Address correspondence to D.S.H. (e-mail: dhartman@psu.edu).
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Abstract
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The pathologic and imaging features of the renal cyst have been well described. A fluid-filled lesion is considered a cystic mass (ie, not a simple cyst) when it has any of the following features: calcification, high attenuation (>20 HU) at computed tomography, signal intensity not typical of water at magnetic resonance imaging, septations, multiple locules, enhancement, wall thickening, or nodularity. There are two important causes of a cystic renal mass: a complicated simple cyst (eg, one with hemorrhage, infection, or ischemia) and cystic renal cell carcinoma. At radiologic evaluation of such masses, it is imperative that optimal imaging techniques be used. Masses with calcification, high attenuation or high signal intensity, or septations can be categorized as benign (no further evaluation required), as requiring follow-up (probably benign), or as requiring surgery. Lesions requiring surgery can be benign or malignant at microscopic examination. Lesions that are multiloculated or demonstrate enhancement, wall thickening, or nodularity usually require surgery. When multiple features are present (eg, calcification and enhancement), the mass should be managed according to its most aggressive feature. Likewise, when there are conflicting findings at evaluation with different imaging modalities, the mass should be managed according to the most aggressive finding.
© RSNA, 2004
Index Terms: Kidney, cysts, 81.311 • Kidney neoplasms, CT, 81.1211 • Kidney neoplasms, diagnosis, 81.324 • Kidney neoplasms, MR, 81.1214 Kidney neoplasms, US, 81.1298
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LEARNING OBJECTIVES FOR TEST 2
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After reading this article and taking the test, the reader will be able to:
- List the pathologic and radiologic features of a fluid-filled renal lesion that make it a cystic mass rather than a simple cyst.
- Discuss the differences in imaging features between a benign cystic renal mass and one requiring surgery.
- Describe the radiologic features of a cystic renal mass that can be managed with follow-up and the approximate risk for subsequent malignancy.
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Introduction
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An understanding of the pathologic characteristics of cystic renal masses is important in understanding their imaging features. The uncomplicated renal cyst is the most commonly encountered fluid-filled lesion of the kidney. Renal cysts are common in the general population and are detected at computed tomography (CT) in many older patients. When all imaging criteria are present, it is generally accepted that the accuracy of a well-performed sonographic, CT, or magnetic resonance (MR) imaging examination approaches 100%. Simple renal cysts have been shown to have a negligible likelihood of malignancy.
Rarely, simple cysts are complicated with hemorrhage, infection, inflammation, or ischemia. In these cases, there is a host reparative response that includes inflammatory cells, granulation tissue, and neovascularity. As a result of this response, the complicated cyst may demonstrate calcification, hemorrhage, septations, wall thickening, and/or nodularity at gross inspection.
Approximately 10% of cases of renal cell carcinoma manifest primarily as a fluid-filled cystic mass (1). The cystic mass may be unilocular or multiloculated. Multilocular cystic renal cell carcinoma may be a distinct subtype of renal cell carcinoma (2). At gross inspection, cystic renal cell carcinoma demonstrates features identical to those of a complicated cyst, as described earlier (Fig 1). The definitive differentiation between complicated cyst and cystic renal cell carcinoma requires histologic examination. Papillary renal cell carcinoma, a unique subtype of renal malignancy, can appear at CT as a homogeneous hyperattenuating mass with a homogeneous pattern of enhancement, which is less than that of the typical nonpapillary renal cell carcinoma (3).
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Figure 1a. Similarity of the gross features of complicated renal cysts and cystic renal cell carcinoma. Photographs show a hemorrhagic cyst (a) and cystic renal cell carcinoma (b). Both cystic lesions (arrows) contain fluid, which is described as resembling motor oil. Both have a thick, irregular cyst wall. Definitive diagnosis is dependent on microscopic examination.
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Figure 1b. Similarity of the gross features of complicated renal cysts and cystic renal cell carcinoma. Photographs show a hemorrhagic cyst (a) and cystic renal cell carcinoma (b). Both cystic lesions (arrows) contain fluid, which is described as resembling motor oil. Both have a thick, irregular cyst wall. Definitive diagnosis is dependent on microscopic examination.
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It must be emphasized that chemotherapy, radiation therapy, and immunotherapy are unsuccessful at curing metastatic renal cell carcinoma. The best hope for cure is complete excision or ablation of the primary malignancy before it has metastasized. Thus, a high index of suspicion must be maintained when evaluating cystic renal masses.
A fluid-filled renal mass is considered cystic (and therefore not an uncomplicated simple cyst) if it has one or more of the following features: calcification, high attenuation (>20 HU) at unenhanced CT, signal intensity not typical of water at MR imaging, septations, multiple locules, enhancement, wall thickening, or nodularity. Since the gross features of a complicated cyst and cystic renal cell carcinoma are similar, it would be expected that there will be overlap in their radiologic findings as well. Although definitive differentiation requires histologic evaluation, careful analysis of the pattern of calcification, internal attenuation, and septation enables the radiologist to distinguish most nonneoplastic complicated cysts from cystic renal cell carcinoma with a high degree of confidence. Cystic lesions that are multiloculated or demonstrate enhancement, wall thickening, or nodularity require surgery or careful follow-up for definitive differentiation.
The goal of the radiologist is to categorize each cystic renal mass as "nonsurgical" (ie, benign) or as "surgical" (ie, requiring surgery). Although most cystic renal masses can thus be appropriately classified (ie, nonsurgical vs surgical) and correctly managed, there is a subset of cystic masses that are probably benign but do not fulfill either the benign or the surgical criteria and thus can be managed with follow-up.
By following up cystic lesions that are probably benign, exploratory surgery can be avoided in many patients (4,5). When a lesion is to be followed up, it is imperative to communicate to the referring physician and to the patient the importance of complying with the follow-up recommendations. The time of follow-up has not been definitively determined, but an initial re-examination in 3–6 months and then annually seems reasonable. The total time that a cystic lesion should be followed up is also subjective. It has been suggested that 5-year follow-up is probably adequate in older patients, whereas a longer follow-up is prudent in younger patients (6). When follow-up images are evaluated for change, it is important to compare the current study to the earliest images, not only the most recent. Change may be slow and may be recognized only when comparing the oldest and current images.
In this article, we review how to optimize ultrasound (US), CT, and MR images and illustrate the imaging features of cystic renal masses, emphasizing those that can be considered benign, those that can be carefully followed up, and those that should be removed (or ablated) because benignity cannot be established with imaging studies.
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Imaging Techniques
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When fluid-filled renal lesions are evaluated radiologically, it is imperative that optimal techniques be used to correctly characterize each cystic mass.
Ultrasound
Complex renal lesions are often first discovered during sonography. Sonography is also used in the work-up of cystic renal lesions once they are detected with other modalities. Ensuring that the best anatomic acoustic window is used can optimize renal sonography. This is usually achieved by placing the patient in the decubitus position, although this varies with individual patient anatomy. The right kidney is usually readily visible because the liver acts as an excellent acoustic window; however, the left kidney may be obscured by colonic or small bowel gas.
The operator should select the highest possible frequency transducer compatible with obtaining an image of the kidney that is within the focal zone of the transducer. The practical range of insonating frequencies is between 2 and 5 MHz in adults, although newer variable focus transducers permit more flexibility at higher frequencies. Color and power Doppler techniques are useful adjuncts to the examination since vascularity within a nodule or septum greatly increases the likelihood of malignancy. Tissue harmonic imaging can reduce unwanted background noise, thus eliminating low-level echoes within an otherwise simple cystic lesion (7). In one study, accuracy for classifying cystic lesions increased from 64% to 84% by using pulse inversion harmonic imaging (7). Recent reports suggest that intravenous microbubble contrast agents combined with pulse inversion harmonic imaging are useful in characterizing cystic renal lesions (8), although this technique is not yet in widespread clinical use. When this technique is combined with high mechanical index imaging, which transmits sufficient energy to burst the microbubbles, enhancement refresh rates can help quantitate the perfusion within a lesion.
The classic simple renal cyst demonstrates anechoic contents, a sharp, smooth back wall, refraction along the sidewalls, and positive through transmission, which is the result of unattenuated ultrasound waves reflecting strongly from surfaces beyond the cyst. Hemorrhagic or proteinaceous cysts may contain low-level echoes or even layers of echoes. Tissue harmonic imaging may be useful in confirming this finding and eliminating "dirty echoes" within an otherwise pristine cyst (7). Cyst septations yield strong back echoes and may create reverberation artifacts within the cyst. Calcifications within a cyst may cause shadowing of the insonating beam, thus obscuring other parts of the lesion. Solid components of the cystic renal neoplasms typically demonstrate internal echoes and fail to show enhancement beyond the lesion. Color and power Doppler techniques may be useful in this setting.
Sonography is particularly useful in guiding nephron-sparing interventions once it has been decided to treat a complex renal lesion. Intraoperative US is performed with "finger" or laparoscopic probes (9,10). The finger probe is draped in a sterile, protective plastic sheath with sterile gel, and water is used as an acoustic coupler within the surgical wound. This allows the lesion to be localized and treatment directed to it, thus minimizing damage to the remainder of the kidney. Similarly, sonography can be employed to direct percutaneous therapies such as radiofrequency ablation and cryotherapy of complex renal lesions.
Computed Tomography
CT remains the major method of imaging and characterizing cystic renal lesions. The decision about whether a renal lesion is neoplastic is based on its enhancement properties and its morphologic features. The latter will be discussed in more detail later.
Enhancement at CT is caused by the microvasculature, which can vary from dense and permeable in the case of renal cell carcinoma to minimal in the case of papillary renal cancer. Enhancement is dependent on the dose and rate of administration of iodinated contrast material as well as the timing of the imaging. Ideally, iodinated contrast media should be delivered at a rate of 3–4 mL/sec; however, perfectly acceptable nephrographic phase images can be obtained at rates of only 1–2 mL/sec in cases where venous access is suboptimal. Peak enhancement can be early (during the arterial phase [20–30 seconds]) or delayed (during the nephrographic phase [80–100 seconds]), depending on the cell type of the solid component of the cystic lesion. A precontrast image is usually needed to establish enhancement, although "deenhancement" is an equally valid indicator of vascularity (11). This leads to the recommendation that cystic renal lesions are best evaluated with triple phasic multi–detector row CT (ie, unenhanced, arterial phase, and nephrographic phase).
A change of less than 10 HU from pre- to postcontrast images is usually considered typical of a benign cyst (12). This criterion was suggested by Bosniak (13). Very rarely, the solid component of a renal cystic tumor may enhance less than 10 HU, but this is exceptional. In contradistinction, a change within the solid portion of a cystic renal lesion of more than 15 HU is almost always indicative of a pathologic process although not always a malignancy, as cystic angiomyolipomas, oncocytomas, and infections may enhance.
A change in attenuation between 10 and 15 HU after intravenous administration of contrast material is considered suspicious but not diagnostic for a neoplasm. For instance, could the change be due to volume averaging, imperfect placement of the region of interest, motion, or streak artifacts (pseudoenhancement)? Furthermore, are there overriding anatomic features such as a nodule or wall thickening that make the lesion suspicious even without definitive enhancement? Could US or MR imaging be performed to confirm the finding? After these options are considered, if it is still thought that the lesion is enhancing between 10 and 20 HU, this finding is considered highly suspicious for a neoplasm. In general, papillary cancers enhance less intensely than other cell types and account for these minimally enhancing lesions (3). These lesions accumulate contrast material more slowly, so delayed images may be helpful in confirming enhancement.
The introduction of multi–detector row CT has made the technique of renal CT somewhat more complex than before. Moreover, specific recommendations are highly dependent on the manufacturer. However, some general principles apply. Every attempt should be made to keep the exposure, field of view, and section thickness setting constant during the three phases of the examination to aid in the assessment of enhancement (14). Although section thickness of 3–5 mm is considered sufficient for identifying enhancing foci within complex renal lesions, thinner and overlapping (1 mm thick with 0.5-mm overlap) sections have been suggested to decrease volume average artifacts (15). Spectacular results can be obtained, for instance, with 1-mm collimation; however, this produces large data sets and higher than desirable radiation exposures (15). Indeed, attempts to reduce the radiation exposure may lead to increased noise and decreased accuracy.
Although multi–detector row CT is ideal for multiplanar reformation and volume rendering of cystic renal masses, it is still unclear whether image processing adds substantially to the diagnosis of cystic renal lesions.
MR Imaging
MR imaging can play a major role in the evaluation of cystic renal masses. In patients with allergies to iodinated contrast media or in whom renal insufficiency prevents the administration of iodinated contrast media, gadolinium-based contrast agents can be employed to detect enhancement. Therefore, the MR examination of a complex renal cystic lesion consists of T1-weighted imaging, T2-weighted imaging, and enhanced T1-weighted imaging, preferably performed as a set of dynamically acquired sets. The T1-weighted sequence, which is usually performed with a breath-held spoiled gradient-echo acquisition, detects the presence of high-signal-intensity hemorrhage, protein, or lipid. The T2-weighted sequence, which is usually a fast or turbo spin-echo sequence, is used to detect the signal from the cystic fluid and identify defects within the cyst fluid or cyst wall that might represent septa or nodules. Gadolinium chelate–enhanced T1-weighted MR imaging is used analogously to CT with iodinated contrast material and requires pre- and postcontrast imaging. Since there is no "radiation penalty" with MR imaging, this is best performed with a series of two- or three-dimensional breath-held spoiled gradient-echo images, typically in the coronal plane.
Unlike CT, where attenuation measurements are reproducible, the arbitrary signal intensity units of MR imaging must be standardized for each examination. This can be accomplished only by performing contrast-enhanced imaging within a single series with no "tuning" between acquisitions. Breath-held images obtained before and then every minute for 5 minutes after the intravenous administration of a gadolinium chelate agent allow serial assessment of a cystic lesion. Using a relative percentage signal intensity change of 15% as a threshold, Ho et al (16) found that MR imaging was 100% sensitive and 95% specific for distinguishing benign and malignant renal lesions (Fig 2). An alternative to this approach is to use image subtraction (postcontrast image – precontrast image). This approach is valid so long as coregistration is possible, but motion is often quite variable and can lead to the false impression of lesion enhancement.
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Figure 2a. Use of MR imaging to evaluate the relative enhancement of a hyperattenuating and high-signal-intensity cyst in a 53-year-old asymptomatic man. CT demonstrated a 3-cm-diameter hyperattenuating (40 HU) lesion in the left kidney. (a-c) Serial precontrast (a), corticomedullary phase (b), and nephrographic phase (c) T1-weighted breath-hold gradient-echo MR images show no significant enhancement of the lesion, with relative signal intensities of 138, 138, and 139, respectively. (d) Longitudinal sonogram also shows that the mass is cystic.
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Figure 2b. Use of MR imaging to evaluate the relative enhancement of a hyperattenuating and high-signal-intensity cyst in a 53-year-old asymptomatic man. CT demonstrated a 3-cm-diameter hyperattenuating (40 HU) lesion in the left kidney. (a-c) Serial precontrast (a), corticomedullary phase (b), and nephrographic phase (c) T1-weighted breath-hold gradient-echo MR images show no significant enhancement of the lesion, with relative signal intensities of 138, 138, and 139, respectively. (d) Longitudinal sonogram also shows that the mass is cystic.
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Figure 2c. Use of MR imaging to evaluate the relative enhancement of a hyperattenuating and high-signal-intensity cyst in a 53-year-old asymptomatic man. CT demonstrated a 3-cm-diameter hyperattenuating (40 HU) lesion in the left kidney. (a-c) Serial precontrast (a), corticomedullary phase (b), and nephrographic phase (c) T1-weighted breath-hold gradient-echo MR images show no significant enhancement of the lesion, with relative signal intensities of 138, 138, and 139, respectively. (d) Longitudinal sonogram also shows that the mass is cystic.
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Figure 2d. Use of MR imaging to evaluate the relative enhancement of a hyperattenuating and high-signal-intensity cyst in a 53-year-old asymptomatic man. CT demonstrated a 3-cm-diameter hyperattenuating (40 HU) lesion in the left kidney. (a-c) Serial precontrast (a), corticomedullary phase (b), and nephrographic phase (c) T1-weighted breath-hold gradient-echo MR images show no significant enhancement of the lesion, with relative signal intensities of 138, 138, and 139, respectively. (d) Longitudinal sonogram also shows that the mass is cystic.
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Radiologic Evaluation
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The suggestions presented in this section are meant to be guidelines. In every case, it is important to consider pretest probability and the patient’s ability to tolerate uncertainty. Patients with von Hippel–Lindau disease or other syndromes associated with renal malignancy have a much higher risk of renal cancer than age-matched controls. In these patients, any cystic renal mass should be presumed to be a cystic renal cell carcinoma and should be removed or carefully followed up. Likewise, young patients with cystic renal masses should be followed up more frequently than older patients. The older patient has a much higher probability of having a cyst or a complicated cyst than does the younger patient. Likewise, some patients, when informed of all risks and options, prefer radiologic follow-up if there is only a small likelihood of malignancy. Others cannot tolerate any uncertainty and desire immediate definitive diagnosis and treatment despite the risk of surgery or ablation.
Calcification
Calcification has been reported in 1%–3% of cases of renal cysts (17). Nonenhanced CT is superior to US in evaluating the calcified cystic mass, as the latter is plagued by calcification-induced shadowing and reverberation. MR imaging is not sensitive for detecting calcification, and areas of calcification will have low signal intensity. In all calcified cystic masses, it is important to obtain nonenhanced and contrast-enhanced CT and/or MR images. Enhanced MR imaging is probably superior to enhanced CT in detecting enhancement in a calcified mass as calcification is usually less apparent at MR imaging than at CT, thus making the enhancement more apparent.
Benign Calcification.—
A calcified cystic mass can be considered nonsurgical when there is a small amount of calcium smoothly deposited in the wall or septum of an otherwise simple cyst (18). Milk of calcium in the dependent portion of a cystic mass can also be considered benign (Fig 3).
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Figure 3a. Benign calcification in a 50-year-old man with abdominal pain. (a) Axial unenhanced CT scan shows a cystic lesion in the left kidney. The dependent calcification was freely moveable with change of position and represents milk of calcium. There is a second small fleck of calcium in the cyst wall at the 10-o’clock position. (b) Contrast-enhanced CT scan shows no irregularity, nodularity, or enhancement of the cyst wall.
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Figure 3b. Benign calcification in a 50-year-old man with abdominal pain. (a) Axial unenhanced CT scan shows a cystic lesion in the left kidney. The dependent calcification was freely moveable with change of position and represents milk of calcium. There is a second small fleck of calcium in the cyst wall at the 10-o’clock position. (b) Contrast-enhanced CT scan shows no irregularity, nodularity, or enhancement of the cyst wall.
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Surgical Calcification.—
A calcified cystic mass is considered surgical if there is associated enhancement, nodularity, or wall thickening within the mass (Fig 4).
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Figure 4a. Surgical calcification in a 67-year-old asymptomatic man. (a) Axial unenhanced CT scan shows an irregular nodular calcification (large arrow) in a cystic mass of the left kidney. In addition, there is septal calcification (small arrow) and nephrolithiasis. (b) Contrast-enhanced CT scan shows focal nodular enhancement (arrow) adjacent to the irregular calcification.
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Figure 4b. Surgical calcification in a 67-year-old asymptomatic man. (a) Axial unenhanced CT scan shows an irregular nodular calcification (large arrow) in a cystic mass of the left kidney. In addition, there is septal calcification (small arrow) and nephrolithiasis. (b) Contrast-enhanced CT scan shows focal nodular enhancement (arrow) adjacent to the irregular calcification.
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Follow-up Calcification.—
Thick and nodular calcification that is not associated with enhancement, wall thickening, or nodularity can be seen in complicated, nonmalignant cysts and can be followed up (Fig 5) (6,19).
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Figure 5a. Follow-up calcification. Unenhanced (a) and contrast-enhanced (b) CT scans show an incidentally detected calcified mass. Despite the density of the calcification, the lesion does not enhance after intravenous administration of contrast material. This lesion could have been managed with follow-up; however, surgery was performed and revealed a heavily calcified and septated cystic lesion with no evidence of tumor.
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Figure 5b. Follow-up calcification. Unenhanced (a) and contrast-enhanced (b) CT scans show an incidentally detected calcified mass. Despite the density of the calcification, the lesion does not enhance after intravenous administration of contrast material. This lesion could have been managed with follow-up; however, surgery was performed and revealed a heavily calcified and septated cystic lesion with no evidence of tumor.
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High Attenuation or High Signal Intensity
A mass is considered hyperattenuating if it measures greater than 20 HU at unenhanced CT. A mass is considered to have high signal intensity if it has higher signal intensity than water at T1-weighted MR imaging. Most of these high-attenuation or high-signal-intensity lesions are cysts containing hemorrhage, blood breakdown products, high protein content, or colloid (20). Rarely, solid neoplasms including lymphoma, myomatous hamartoma, renal cell carcinoma, and metastasis measure greater than 20 HU on unenhanced scans or have higher signal intensity than water on unenhanced MR images and are therefore considered hyperattenuating or hyperintense. The mechanisms by which these tumors become hyperattenuating or have high signal intensity are unknown (21).
At unenhanced CT or MR imaging examination, it is impossible to determine if a hyperattenuating or high-signal-intensity lesion is a cyst or a solid tumor. Both may be homogeneous and sharply marginated. The distinction is dependent on the presence of vascularity (ie, enhancement) or a solid appearance at US examination. In all cases, it is therefore imperative to perform nonenhanced and contrast-enhanced imaging or sonography. If iodinated contrast material cannot be administered, MR imaging with and without contrast material may be used. Even with enhanced scans, the wall and internal composition may be difficult to evaluate because the high-attenuation fluid inside the lesion masks these structures.
Benign Hyperattenuating or High-Signal-Intensity Lesions.—
A hyperattenuating or high-signal-intensity lesion can be considered a benign hyperattenuating cyst if it is sharply marginated or homogeneous or demonstrates a hematocrit effect on nonenhanced and contrast-enhanced scans and demonstrates no significant enhancement (see discussion earlier) (Figs 2, 6). If sonography is performed, the mass should be cystic but does not need to have all of the sonographic criteria for a simple cyst (Fig 6c). Israel and Bosniak (6) believe that to be considered nonsurgical, a hyperattenuating mass should measure less than 3 cm in diameter with at least one-quarter of the lesion extending outside of the renal parenchyma so a portion of its wall can be evaluated. We are aware of only one reported case that met these criteria yet had well-differentiated renal cell carcinoma in the cyst wall (22).
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Figure 6a. Hyperattenuating, septated benign cyst in an 84-year-old man with mild renal insufficiency. (a) Unenhanced CT scan shows a 4-cm-diameter lesion with high attenuation (68 HU). (b) Contrast-enhanced CT scan shows no significant enhancement (70 HU). (c) Longitudinal sonogram shows that the lesion is cystic and has one delicate septation (arrow). Although this lesion meets the criteria for a benign hyperattenuating cyst, some recommend follow-up of all hyperattenuating lesions larger than 3 cm in diameter (6).
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Figure 6b. Hyperattenuating, septated benign cyst in an 84-year-old man with mild renal insufficiency. (a) Unenhanced CT scan shows a 4-cm-diameter lesion with high attenuation (68 HU). (b) Contrast-enhanced CT scan shows no significant enhancement (70 HU). (c) Longitudinal sonogram shows that the lesion is cystic and has one delicate septation (arrow). Although this lesion meets the criteria for a benign hyperattenuating cyst, some recommend follow-up of all hyperattenuating lesions larger than 3 cm in diameter (6).
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Figure 6c. Hyperattenuating, septated benign cyst in an 84-year-old man with mild renal insufficiency. (a) Unenhanced CT scan shows a 4-cm-diameter lesion with high attenuation (68 HU). (b) Contrast-enhanced CT scan shows no significant enhancement (70 HU). (c) Longitudinal sonogram shows that the lesion is cystic and has one delicate septation (arrow). Although this lesion meets the criteria for a benign hyperattenuating cyst, some recommend follow-up of all hyperattenuating lesions larger than 3 cm in diameter (6).
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Surgical Hyperattenuating or High-Signal-Intensity Lesions.—
A lesion is considered surgical when any of the following CT, MR imaging, or US features is present: lack of a smooth contour or interface, heterogeneous texture (nonenhanced or contrast-enhanced images), significant enhancement, or solid appearance at sonographic examination (Fig 7) (21).
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Figure 7a. Surgical hyperattenuating mass in a 63-year-old asymptomatic man. (a-c) Unenhanced (a), nephrographic phase (b), and excretory phase (c) CT scans show a well-defined, homogeneous, 3-cm-diameter lesion. The region of interest for measurement was in the center of the mass. The mass enhances 13 HU on the nephrographic phase image (b), then de-enhances 7 HU on the excretory phase image (c). If nephrographic phase images had not been obtained, the apparent enhancement would have been 6 HU. (d) Longitudinal sonogram shows that the lesion (arrows) is solid. (e) Gross photograph of the excised mass shows a well-defined 3-cm-diameter mass, which was a papillary renal cell carcinoma.
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Figure 7b. Surgical hyperattenuating mass in a 63-year-old asymptomatic man. (a-c) Unenhanced (a), nephrographic phase (b), and excretory phase (c) CT scans show a well-defined, homogeneous, 3-cm-diameter lesion. The region of interest for measurement was in the center of the mass. The mass enhances 13 HU on the nephrographic phase image (b), then de-enhances 7 HU on the excretory phase image (c). If nephrographic phase images had not been obtained, the apparent enhancement would have been 6 HU. (d) Longitudinal sonogram shows that the lesion (arrows) is solid. (e) Gross photograph of the excised mass shows a well-defined 3-cm-diameter mass, which was a papillary renal cell carcinoma.
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Figure 7c. Surgical hyperattenuating mass in a 63-year-old asymptomatic man. (a-c) Unenhanced (a), nephrographic phase (b), and excretory phase (c) CT scans show a well-defined, homogeneous, 3-cm-diameter lesion. The region of interest for measurement was in the center of the mass. The mass enhances 13 HU on the nephrographic phase image (b), then de-enhances 7 HU on the excretory phase image (c). If nephrographic phase images had not been obtained, the apparent enhancement would have been 6 HU. (d) Longitudinal sonogram shows that the lesion (arrows) is solid. (e) Gross photograph of the excised mass shows a well-defined 3-cm-diameter mass, which was a papillary renal cell carcinoma.
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Figure 7d. Surgical hyperattenuating mass in a 63-year-old asymptomatic man. (a-c) Unenhanced (a), nephrographic phase (b), and excretory phase (c) CT scans show a well-defined, homogeneous, 3-cm-diameter lesion. The region of interest for measurement was in the center of the mass. The mass enhances 13 HU on the nephrographic phase image (b), then de-enhances 7 HU on the excretory phase image (c). If nephrographic phase images had not been obtained, the apparent enhancement would have been 6 HU. (d) Longitudinal sonogram shows that the lesion (arrows) is solid. (e) Gross photograph of the excised mass shows a well-defined 3-cm-diameter mass, which was a papillary renal cell carcinoma.
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Figure 7e. Surgical hyperattenuating mass in a 63-year-old asymptomatic man. (a-c) Unenhanced (a), nephrographic phase (b), and excretory phase (c) CT scans show a well-defined, homogeneous, 3-cm-diameter lesion. The region of interest for measurement was in the center of the mass. The mass enhances 13 HU on the nephrographic phase image (b), then de-enhances 7 HU on the excretory phase image (c). If nephrographic phase images had not been obtained, the apparent enhancement would have been 6 HU. (d) Longitudinal sonogram shows that the lesion (arrows) is solid. (e) Gross photograph of the excised mass shows a well-defined 3-cm-diameter mass, which was a papillary renal cell carcinoma.
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Follow-up Hyperattenuating or High-Signal-Intensity Lesions.—
Israel and Bosniak (4,6) believe that lesions that fulfill all of the above criteria for a nonsurgical hyperattenuating mass but are totally intrarenal and/or larger than 3 cm or are in kidneys with multiple complicated cystic lesions should be followed up.
Septations
Septations may result from healing or organization of a cyst that has hemorrhaged or been infected. Septations also may result from two adjacent cysts that share a common wall. Septations are often partial (21). Calcification may be deposited in the septations in a fine curvilinear pattern that is best evaluated with CT (23). Septations are more prominent and obvious at MR imaging and US than at CT (6).
Benign Septations.—
A septated cyst can be considered nonsurgical if the septa are thin (1 mm or less) and smooth and attach to the walls of the cyst without associated nodularity (Figs 6c, 8) (18,20,24). Nonsurgical septations may be calcified. For a cyst to be considered nonsurgical, there must be only a small number of septations.
Surgical Septations.—
A septated cyst is considered surgical if the septa are thick, irregular, or nodular or demonstrate significant enhancement. The best location to demonstrate nodularity is at the intersection of the septation and the cyst wall (Fig 9).
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Figure 9. Surgical septation in a 58-year-old man with an incidentally discovered cystic lesion of the right kidney. CT scan shows a thick, irregular, enhancing septation (nonenhanced scan not shown). A low-grade renal carcinoma was diagnosed at microscopic evaluation.
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Follow-up Septations.—
Israel and Bosniak (6) opine that if the septal thickness is "greater than hairline" but smooth, the cyst is probably still benign and can be followed up. If there is an increase in the thickness or in the irregularity of the septa at follow-up examination, surgical exploration is necessary (6).
Multiple Locules
Although the exact number is arbitrary, when a mass has more than three or four septations, it should be considered multiloculated (Fig 10). The multiple septa that separate the multiple locules are more prominent and obvious at MR imaging and US than at CT. Although many diverse disease processes may result in a multiloculated renal mass, the two most common in the adult are renal cell carcinoma and multilocular cystic nephroma (25). The multilocular cystic nephroma is an uncommon, nonhereditary cystic neoplasm. It is characterized by a well-developed capsule, fibrous stroma, and septa that separate multiple epithelium-lined, noncommunicating cysts (26). Although the vast majority of multilocular cystic nephromas are benign, metastases have been reported (26). Multiloculated renal cell carcinoma more commonly affects men than women, whereas in the adult, multilocular cystic nephroma is much more common in women.
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Figure 10a. Septated or multiloculated cystic lesion in a 60-year-old woman being evaluated for an abnormal uterus. Axial CT scan (a) and coronal reformatted image (b) show a well-defined, 3-cm-diameter cystic mass in the upper pole of the left kidney. The lesion appears to contain three or four septations, which are smooth without large nodular elements. At surgery, a multiloculated renal cell carcinoma was removed.
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Figure 10b. Septated or multiloculated cystic lesion in a 60-year-old woman being evaluated for an abnormal uterus. Axial CT scan (a) and coronal reformatted image (b) show a well-defined, 3-cm-diameter cystic mass in the upper pole of the left kidney. The lesion appears to contain three or four septations, which are smooth without large nodular elements. At surgery, a multiloculated renal cell carcinoma was removed.
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It is not possible to definitely differentiate most cases of multilocular cystic nephroma from multiloculated renal cell carcinoma radiologically (Figs 10, 11). Radiologic features that favor renal cell carcinoma include the presence of blood in the tumor or in the perinephric space at CT or MR imaging, relatively large solid areas within the tumor mass, intravascular extension, or distant metastases. Findings suggestive of multilocular cystic nephroma include absence of intratumoral or perinephric hemorrhage and herniation of a portion of the mass into the renal pelvis.
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Figure 11a. Multiloculated renal mass evaluated with different imaging modalities. (a) Longitudinal sonogram of a 42-year-old man shows a multiloculated mass, which was diagnosed as renal cell carcinoma. GB = gallbladder. (b) Axial T2-weighted MR image of the same patient shows the 11-cm-diameter, sharply marginated, multiloculated renal cell carcinoma. (c) Multilocular cystic nephroma in a 49-year-old woman in whom a renal cyst was aspirated 20 years earlier. CT scan shows an 11-cm-diameter multiloculated mass. Note that a portion of the mass projects into the renal pelvis (arrow), a feature suggestive of multilocular cystic nephroma.
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Figure 11b. Multiloculated renal mass evaluated with different imaging modalities. (a) Longitudinal sonogram of a 42-year-old man shows a multiloculated mass, which was diagnosed as renal cell carcinoma. GB = gallbladder. (b) Axial T2-weighted MR image of the same patient shows the 11-cm-diameter, sharply marginated, multiloculated renal cell carcinoma. (c) Multilocular cystic nephroma in a 49-year-old woman in whom a renal cyst was aspirated 20 years earlier. CT scan shows an 11-cm-diameter multiloculated mass. Note that a portion of the mass projects into the renal pelvis (arrow), a feature suggestive of multilocular cystic nephroma.
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Figure 11c. Multiloculated renal mass evaluated with different imaging modalities. (a) Longitudinal sonogram of a 42-year-old man shows a multiloculated mass, which was diagnosed as renal cell carcinoma. GB = gallbladder. (b) Axial T2-weighted MR image of the same patient shows the 11-cm-diameter, sharply marginated, multiloculated renal cell carcinoma. (c) Multilocular cystic nephroma in a 49-year-old woman in whom a renal cyst was aspirated 20 years earlier. CT scan shows an 11-cm-diameter multiloculated mass. Note that a portion of the mass projects into the renal pelvis (arrow), a feature suggestive of multilocular cystic nephroma.
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Less commonly, certain renal cystic diseases and inflammatory, traumatic, and vascular lesions may manifest as a multiloculated renal mass. With the exception of arteriovenous fistula, which can be reliably diagnosed radiologically, and multiloculated abscess, which can be suggested clinically, most multiloculated renal masses are treated with excision or nephrectomy.
Rarely, an aggregate of multiple simple cysts will cluster in one portion of the kidney and mimic a multiloculated mass. This entity is known as localized cystic disease (27). This entity is nonfamilial and not related to other renal cystic diseases. It is usually clinically silent and detected as an incidental finding. Radiologic features that help distinguish localized cystic disease from a multiloculated mass include the presence of normal parenchyma between individual cysts, the presence of one or more cysts at some distance from the main aggregate of cysts, and the lack of a well-defined pseudocapsule around the multiple cysts (Fig 12).
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Figure 12a. Localized cystic disease of the kidney in a 43-year-old man being evaluated for abdominal trauma. The localized cystic disease was an incidental finding. Axial contrast-enhanced CT scans show a cluster of cysts separated by normal enhancing renal parenchyma in the upper pole of the right kidney. There is no pseudocapsule surrounding the cysts. Note that the small cyst at the 6-o’clock position (arrow in a) is clearly separate from the aggregate of cysts.
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Figure 12b. Localized cystic disease of the kidney in a 43-year-old man being evaluated for abdominal trauma. The localized cystic disease was an incidental finding. Axial contrast-enhanced CT scans show a cluster of cysts separated by normal enhancing renal parenchyma in the upper pole of the right kidney. There is no pseudocapsule surrounding the cysts. Note that the small cyst at the 6-o’clock position (arrow in a) is clearly separate from the aggregate of cysts.
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Wall Thickening, Nodularity, and Enhancement
The simple, uncomplicated cyst is characterized by an extremely thin cyst wall without nodularity. As it does not enhance, the wall of a simple cyst is barely perceptible. When a simple cyst is completely intrarenal, its wall thickness cannot be accurately evaluated.
Cystic renal cell carcinoma often has thickening or nodularity in a portion of its wall. The finding of wall thickening can be quite subtle as the wall may measure only 2–3 mm in thickness (Fig 13). The recognition of wall thickening in a small cystic lesion is especially challenging. Wall thickening may also be seen in nonneoplastic lesions, including infected cyst, abscess, cyst with hemorrhage, pancreatic pseudocyst, and organizing hematoma with a pseudocapsule (Fig 14).
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Figure 13a. Cystic renal cell carcinoma in a 71-year-old man undergoing follow-up for esophageal carcinoma. (a) Unenhanced CT scan shows an appearance suggestive of a thick wall. (b) Contrast-enhanced CT scan clearly shows the thick wall (arrow).
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Figure 13b. Cystic renal cell carcinoma in a 71-year-old man undergoing follow-up for esophageal carcinoma. (a) Unenhanced CT scan shows an appearance suggestive of a thick wall. (b) Contrast-enhanced CT scan clearly shows the thick wall (arrow).
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Figure 14. Cystic inflammatory mass in a 26-year-old woman with abdominal pain. Contrast-enhanced CT scan shows a thick-walled (arrows) cystic mass of the left kidney. Note that the cyst wall enhances less than the normal renal parenchyma but more than the cyst fluid.
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Discrete nodularity within a cystic mass is another feature that may be seen in cystic renal cell carcinoma. Asymmetric wall thickening or septations seen en face can resemble nodules. Even small areas of nodularity can be quite conspicuous at sonographic examination because of the acoustic window of the cyst fluid. Debris within a cyst may also mimic nodularity but should not be confused with a true nodule, as the former is usually moveable as the patient is placed in different positions.
The value of identifying enhancement in cystic masses cannot be overemphasized. The vast majority will demonstrate an asymmetric pattern of enhancement that is readily apparent. Enhancement within a cystic lesion often makes areas of wall thickening and nodularity more conspicuous (Fig 13). When there are only small foci of enhancement, accurate measurement of the degree of enhancement can be difficult. In this setting, the lesion may need to be electronically zoomed before attempting region-of-interest measurements. Nodular enhancement is usually readily detected by placing a region of interest on pre- and postcontrast images.
Wall thickening and enhancement must not be confused with the normal renal parenchyma that at least partially surrounds a cystic lesion ("parenchymal beak"). Making this distinction may be especially problematic when evaluating the axial images of a lesion that arises in the upper or lower pole (Fig 15) (28). Coronal or sagittal images are very useful for demonstrating this parenchymal beak in polar lesions. A corollary of this principle is that the wall of a cystic mass that projects beyond the renal margin can be better evaluated for subtle thickening than the portion of the wall that abuts renal parenchyma. It is of interest that women are three to four times more likely to have a benign cause for a complex cyst than men (29).
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Figure 15a. Cyst with apparent wall thickening due to normal parenchyma in an asymptomatic 68-year-old woman. (a) Contrast-enhanced CT scan shows a cystic mass with apparent wall thickening. (b) CT scan obtained immediately superior to a shows a fluid-filled mass without a perceptible wall. (c) Coronal MR image shows that the apparent wall thickening is due to the normal parenchymal beak. Lines A and B indicate the respective levels where a and b were obtained.
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Figure 15b. Cyst with apparent wall thickening due to normal parenchyma in an asymptomatic 68-year-old woman. (a) Contrast-enhanced CT scan shows a cystic mass with apparent wall thickening. (b) CT scan obtained immediately superior to a shows a fluid-filled mass without a perceptible wall. (c) Coronal MR image shows that the apparent wall thickening is due to the normal parenchymal beak. Lines A and B indicate the respective levels where a and b were obtained.
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Figure 15c. Cyst with apparent wall thickening due to normal parenchyma in an asymptomatic 68-year-old woman. (a) Contrast-enhanced CT scan shows a cystic mass with apparent wall thickening. (b) CT scan obtained immediately superior to a shows a fluid-filled mass without a perceptible wall. (c) Coronal MR image shows that the apparent wall thickening is due to the normal parenchymal beak. Lines A and B indicate the respective levels where a and b were obtained.
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Uniform enhancement of a renal mass is much less common but may be more subtle and therefore more difficult to detect than focal enhancement (Fig 7). Careful measurement with a region of interest is imperative, as the eye is insensitive at detecting small degrees of uniform enhancement. This uniform pattern of enhancement is especially important to recognize in a homogeneous hyperattenuating or high-signal-intensity mass, as its presence indicates that the mass has vascularity and is therefore not cystic. Another way of stating this principle is that a nonenhanced or contrast-enhanced CT scan can demonstrate if a lesion is homogeneous or heterogeneous but not whether it is fluid filled or solid. The determination of cystic versus solid requires the demonstration of enhancement with CT or MR imaging, MR with T1-weighted and T2-weighted images, or sonographic evaluation (Figs 2, 6, 7).
The presence of enhancement within a cystic lesion dramatically changes the likelihood that it represents a malignancy. Several series have reported that the chance of neoplasia increases to 40%–80% when enhancement is observed within a lesion. Benign neoplasms and nonneoplastic inflammatory and traumatic lesions may also enhance due to the granulation tissue, which includes inflammatory neovascularity. A second proposed mechanism for enhancement in nonneoplastic lesions is the interposition of normal vascularized tissue within the cystic mass (Fig 16). Because it is usually not possible to confidently differentiate nonneoplastic from neoplastic lesions by means of wall thickening, nodularity, or enhancement, they are almost always considered surgical.
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Figure 16a. Enhancement of a traumatic hemorrhagic cyst in a 43-year-old tae kwon do enthusiast who was kicked in the left side 6 months earlier. (a) Unenhanced CT scan shows a multiloculated renal lesion with a perinephric component. (b) Contrast-enhanced CT scan shows that the lesion is thick walled with a markedly enhancing nodule (arrow). At surgery, a posttraumatic, intrarenal and perirenal, partially liquefied hematoma simulating a cyst was found. The enhancing nodule proved to be residual normally functioning renal tissue.
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Figure 16b. Enhancement of a traumatic hemorrhagic cyst in a 43-year-old tae kwon do enthusiast who was kicked in the left side 6 months earlier. (a) Unenhanced CT scan shows a multiloculated renal lesion with a perinephric component. (b) Contrast-enhanced CT scan shows that the lesion is thick walled with a markedly enhancing nodule (arrow). At surgery, a posttraumatic, intrarenal and perirenal, partially liquefied hematoma simulating a cyst was found. The enhancing nodule proved to be residual normally functioning renal tissue.
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Discussion
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Bosniak Classification
In 1986, in a seminal article, Bosniak (18) proposed a classification to characterize cystic renal masses. In his original classification, there were four categories: category I, simple benign cysts; category II, benign cystic lesions that are minimally complicated; category III, more complicated cystic lesions; and category IV, lesions that are clearly malignant cystic carcinomas. In 1993, in another landmark article, Bosniak (4) revised the classification system to include a subset of minimally complicated lesions that could be managed with follow-up, category IIF. Recently, the significance of larger amounts of calcification was modified from requiring surgery to evaluation with follow-up (19). In 2003, the effectiveness of follow-up of moderately complex cystic renal masses was reported (6).
All of these articles emphasized the importance of well-performed CT as the best imaging modality for cystic mass categorization. Numerous studies have documented the value of this classification system, and it has been widely embraced by radiologists and urologists (14,30–32).
The imaging strategies and recommendations stated in this article are based on the principles of the Bosniak classification with only slight modifications and include the use of US and MR imaging. In all classifications of cystic renal masses, several of the parameters used for categorization are qualitative rather than quantitative (eg, a small amount of calcification). Bosniak <
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Abstract
LEARNING OBJECTIVES FOR TEST...
