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Genitourinary Imaging |
1 From the Department of Radiology, New York University Medical Center, 560 First Ave, Suite HW 202, New York, NY 10016. From the 2001 RSNA scientific assembly. Received October 17, 2001; revision requested January 10, 2002; final revision received April 29; accepted April 30. Address correspondence to G.M.I. (e-mail: gary.israel@med.nyu.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Eighty-one cystic renal masses containing calcification in a wall or septum were evaluated by means of review of computed tomographic (CT) images (n = 81), follow-up CT images (n = 28), and results of pathologic examination (n = 40) by the authors in consensus. Images were evaluated for lesion size, amount and morphology of calcification, and any association of calcification with soft-tissue structures. Lesions were categorized according to the Bosniak cyst classification system; the amount of calcification was determined with a subjective grading system. Progression of calcification was qualitatively determined with available follow-up CT scans.
RESULTS: Twenty-one lesions were Bosniak category II (benign) and showed small amounts and thin strands of calcification. Nineteen lesions containing more extensive calcification but no enhancing tissue were category IIF. Follow-up CT results available for 16 of these lesions (average follow-up length, 5 years 8 months) showed no substantial change. The three remaining lesions were proved benign at surgery. Twenty-five lesions were category III; surgical intervention was performed in 21 of these (benign, n = 12; malignant, n = 9). Sixteen lesions that contained obvious areas of enhancing soft tissue were category IV and proved malignant at surgery.
CONCLUSION: Calcification in a cystic renal mass is not as important in diagnosis as is the presence of associated enhancing soft-tissue elements. This information should enable a reasonable approach to the management of calcium-containing renal cystic lesions.
© RSNA, 2002
Index terms: Kidney, calcification, 81.814, 81.816 • Kidney, cysts, 81.3111, 81.3112 • Kidney neoplasms, CT, 81.12112
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Two patients who were believed to have Bosniak category IIF cysts (ie, complex renal cysts in need of follow-up—see below) were excluded from the study because follow-up examination results or pathologic data were not available. This yielded a total of 79 patients and 81 masses. There were 49 men and 30 women, with an average age of 64.2 years (23-86 years). For each patient who did not undergo surgery, attempts were made to obtain data from all subsequent CT examinations to ensure the longest possible follow-up; such data were available for 28 lesions. Pathologic correlation was available for 40 lesions, two of which had also been examined at follow-up CT before surgery. The remaining 15 lesions were diagnosed as benign (category II) on the basis of their appearance at CT.
The Institutional Board of Research Associates at New York University School of Medicine reviewed the manuscript of our study, determined that our study was a retrospective review of medical records that was unlikely to result in harm to subjects, and permitted the use of the data we collected for publication of this report.
Given the nature of this study, the examinations were performed with a variety of helical and conventional CT scanners with different section collimations (range, 3-10 mm). In addition, the type and amount of contrast material used varied, but all examinations were performed before and after intravenous administration of contrast material, with the exception of a single follow-up examination of one category II lesion.
Image Analysis
The images in each case were retrospectively analyzed by the authors (G.M.I., M.A.B.) in consensus. Images from the original examination were initially evaluated, and, if images from subsequent examinations were available, they were analyzed side-by-side with those of the original examination. For each cystic mass, its Bosniak classification, its greatest size in two dimensions as measured with hand-held calipers, and the amount of calcification it contained were determined. In addition, for those masses for which results of follow-up examinations were available, any change in the amount of calcification was determined. Comparison of the images with surgical findings and surgical pathologic findings was performed by the authors on the basis of surgical and pathology reports. This information was available to one of the authors (M.A.B.) prior to the consensus review.
All lesions were placed in a Bosniak cyst category (2) on the basis of the following criteria:
Category I masses are simple benign cysts with thin walls; they contain fluid with the attenuation of water but do not contain septa or calcification.
Category II masses are benign cystic lesions that may contain hairline-thin septa. Fine calcification in the walls or septa of such lesions or a short segment of slightly thickened calcification is not uncommon and is compatible with benignity. Minimal enhancement of a hairline-thin, smooth septum or wall is sometimes present.
Category IIF lesions are more complex cysts that cannot be classified neatly as being category II or category III cysts. These cysts may contain an increased number of septa and an increased amount of calcification, which may be thicker and nodular. Like category II cysts, these lesions may demonstrate minimal enhancement of a hairline-thin, smooth septum or wall but no enhancement of the tissues in which calcification is present.
Category III lesions are indeterminate masses, in that their benignity or malignancy cannot be determined with imaging studies. These lesions have thick, irregular walls or septa, and may contain either small or large amounts of calcification. Enhancement of the wall or septa can be clearly appreciated.
Category IV lesions are malignant cystic masses containing either small or large amounts of calcification within a thick, enhancing irregular wall or septum. Enhancing soft-tissue components are present adjacent to or extending from, but are independent of, the wall or septum.
To determine the amount of calcification within each mass, a subjective scoring system of 1–4 was used. A score of 1 was assigned for minimal calcification, and scores of 2, 3, and 4 were assigned for mild, moderate, and severe calcification, respectively. Minimal calcification was defined as smooth, hairline-thin strands of calcification. Mild calcification was defined as calcification with some thickness and minimal nodularity. Moderate calcification was defined as calcification with further thickness and/or a grossly nodular appearance. Severe calcification was defined as grossly thickened, nodular, and extensive calcification.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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For the remaining 16 cases without pathologic correlation, the average duration of follow-up was 5 years 8 months (range, 1 year 1 month to 17 years 4 months; median, 5 years) (Figs 3–5). In four of the cases, the amount of calcification increased during the follow-up period (Fig 3). In the remaining 12 cases, the calcification did not change. In the patient who underwent repeat CT examinations during a follow-up period of 17 years 4 months, an initial calcification score of 2 was increased to a score of 4 at the last follow-up examination (Fig 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Calcification can occur in the wall or the septa of benign as well as malignant lesions (10,11). In an early description of CT findings in benign calcified cystic masses, it was stated that "small plaques of fine linear calcium can occur in the wall of benign cysts"—that is, cysts in which such a finding is observed can be considered benign if all other CT criteria for benign cysts are present (2). However, more extensive calcification would result in a lesion being upgraded to a category III lesion, which must be explored surgically. This belief was based in part on early experience with these lesions, as well as on the lower degree of detail provided by early CT scanners. However, the results of this study demonstrate that many lesions containing larger amounts of calcium can safely be followed up with serial examinations, as long as enhancing soft-tissue components are not present.
In this study, the largest amount of calcification was observed in category IIF cysts. These lesions could not be placed into category II because of the increased amount of calcium, but they did not demonstrate the type of mural enhancement associated with category III cysts. Category IIF lesions can show minimal enhancement of the wall or septa as long as the wall or septa are hairline thin and smooth. However, if there is any irregularity in an enhancing wall or septum, the lesion must be considered a category III lesion. In some category IIF lesions, it is difficult to visualize mural enhancement because of the large amounts of calcium present within the lesion. Theoretically, the use of subtraction techniques would be extremely helpful in determining whether densely calcified lesions enhance at CT, thereby enabling one to distinguish category IIF from category III lesions.
The use of magnetic resonance (MR) imaging may also be helpful in characterizing these lesions. Calcification within the wall or septa would not be depicted at MR imaging, and enhancement might be better demonstrated. However, in those cases in which there is indecision as to what category a lesion should be placed into, the lesion should be put in the next higher category. More complicated category II lesions could be placed into category IIF. Likewise, more complicated category IIF lesions should be placed into category III.
In this study, there were 19 category IIF cysts, three of which were surgically proved to be benign. No change during the average follow-up of 5 years 8 months implies that the remaining 16 cysts were benign or nonaggressive. In our experience, cystic lesions generally grow very slowly and tend to metastasize late. Therefore, it was considered safe to follow up these cases with CT, which would reveal any growth or findings of concern. A category IIF cyst may grow slightly overall, but this should not cause concern, because all cysts can grow. Furthermore, the amount of calcification can also increase over time without constituting a cause for concern. However, if the wall or septa become thicker or irregular, with any sign of an increase in soft-tissue components, the lesion would have to be considered a category III or category IV lesion. Four of the category IIF lesions in this series showed increased amounts of calcium at follow-up CT examinations. This was not of concern because no enhancing soft-tissue element was present.
Category III lesions have a wide range of appearances and can be benign or malignant. This study included 21 category III lesions with pathologic correlation. Of these 21 lesions, 11 were benign hemorrhagic or infected cysts, nine were renal cell carcinomas, and one was a multilocular cystic nephroma. The features of some category III lesions are borderline with those of category IV; these category III lesions are more likely to be malignant because they have thicker walls or septa that can simulate a soft-tissue mass.
Even though category III lesions are essentially indeterminate, in those cases that are more likely to be malignant, this concern should be communicated to the clinician; this communication may have an effect on the treatment approach. On the other hand, for category III lesions with thinner walls or septa that are more likely to be benign, it is important to communicate the possibility of benignity to the clinician. Although the criteria for category III lesions are such that a malignant lesion cannot be excluded, the probability that a category III lesion is more likely to be benign is important in patient care. In those instances, the clinician might defer surgery (depending on the clinical parameters) or more vigorously pursue a more conservative approach (eg, partial nephrectomy). Many category III lesions can be handled in this way.
However, there are some category III lesions for which it is impossible to make any firm decision as to benignity or malignancy. Most of these lesions are indeterminate even during gross inspection at surgical exploration or at gross pathologic analysis. It is only after a careful microscopic search that a definite diagnosis of neoplasm can be ruled in or out. Occasionally, even the pathologist is uncertain as to whether such a lesion was a "burnt-out" neoplasm (with few, if any, abnormal cells) or a chronic hemorrhagic cyst without abnormal cells.
Although the Bosniak cyst classification system is based on CT findings, clinical history may be important in the process of making decisions about the treatment of a lesion. For instance, if there is a history of urinary tract infection in a patient with a complicated cystic lesion that would ordinarily be considered a category III lesion, aspiration of the lesion to recover pus or inflammatory debris would be indicated, rather than surgical exploration. Also, trauma—including that caused by previous needle aspiration of a benign cyst—can lead to the formation of a calcified, thick-walled lesion. If such a lesion was initially believed to be a benign cyst, it could be followed up as a category IIF lesion.
Category IV cysts are clearly cystic neoplasms. They are malignant because they contain enhancing soft-tissue components that are adjacent to but separate from the wall of the lesion. The amount and morphology of calcification within these lesions have no bearing on the diagnosis of malignancy.
It should be kept in mind that there are a number of other conditions that may manifest as peripherally calcified or rim-calcified cystic renal masses. Peripherally calcified lesions near the hilum of the kidney could represent renal artery aneurysms or arteriovenous fistulas (10,11). These diagnoses would be evident at CT if a sufficient bolus of contrast material were delivered. Also, an echinococcal cyst sometimes manifests as a peripherally calcified lesion, and, while uncommon in North America, this cyst is endemic in some parts of the world and is sometimes seen in patients who travel or immigrate to the United States (12,13). Furthermore, in patients with localized tuberculosis, the calcified wall of a hydrocalyx may manifest as a calcified cystic mass. If a history of tuberculosis is not obtained, a careful search for clues of previous tuberculosis infection, such as an amputated calyx, may prove helpful in diagnosis.
This study had several limitations. First, the results of the CT examinations were retrospectively collected; this introduced a case-selection bias, so the group of cysts we evaluated may not be all inclusive. This is particularly true of category II cysts with minimal calcification. They may not have been recorded in the database for inclusion in this series because they were clearly benign and therefore did not require follow-up CT examinations or surgical exploration.
Second, the lesions were analyzed in consensus, and therefore, interobserver variability could not be accessed. In addition, the CT protocol was not standardized in all patients. Optimizing the parameters of the CT examination would enable more accurate lesion characterization. Although CT was performed before and after intravenous administration of contrast material in all patients in our series (with the exception of a single examination in one patient), not all CT examinations were performed with collimation of 5 mm or less. In some early CT examinations performed at our institution, and in some of the CT examinations performed at other institutions, 10-mm-thick sections were used.
Also, papillary cystadenocarcinoma–type lesions are typically hypovascular and can be missed if the CT examination is performed too early after contrast material administration. For some of the submitted cases, only CT images obtained during the corticomedullary phase of enhancement were available. In general, because the lesions discussed in this article tended to be hypovascular, it is important for images to be acquired at least 90 seconds after contrast material injection to be certain that any vascularity is appreciated (14,15). It has been demonstrated that success in evaluating and accurately characterizing these lesions is dependent on the use of top-quality technique (7). The CT examination must be performed before and after the administration of an adequate amount of intravenous contrast material, and the contrast material should be delivered as a bolus injection to facilitate the evaluation of any possible tissue enhancement. Slow infusion of contrast material may result in low levels of enhancement not being demonstrated at CT.
Another limitation of this study was that pathologic proof was available for only three of the 19 category IIF lesions. Since these lesions are not usually explored surgically, follow-up examinations must be performed to document stability and thus benignity. How long follow-up must be conducted before benignity is proven has not been established, but stability of a lesion during a 5-year period is certainly indicative of benignity. The average duration of CT follow-up of the category IIF lesions in our series was more than 5 years.
Also, we did not systematically analyze the Hounsfield unit values of the contents of each lesion. Radiologists may be more comfortable in categorizing a cystic mass as benign if its contents have the attenuation of fluid. However, many benign lesions contain hemorrhagic fluid, while cystic neoplasms can contain low-attenuating fluid. This issue should not be troublesome, however, because cystic masses should be evaluated before and after the administration of contrast material so that any enhancing component would become evident. Finally, the sample size in our study was relatively small, and additional studies with increased numbers of patients and longer follow-up periods are necessary.
In summary, calcium per se can be seen in benign and malignant cystic lesions. On the basis of the results of this study, calcification in a cystic renal mass is not as important in diagnosis as is the presence of associated enhancing soft-tissue elements. Cystic lesions that show considerable calcification—even thick and nodular calcification—but no evidence of tissue enhancement at CT (category IIF lesions) can be managed with follow-up CT examinations to monitor their stability. Lesions that show associated soft-tissue enhancement of the wall or septa (category III lesions) in most instances must be evaluated surgically, although many of them will prove to be benign.
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FOOTNOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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