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Pitfalls in Renal Mass Evaluation and How to Avoid Them1 CME FEATURE See accompanying test at http:// www.rsna.org /education /rg_cme.html
LEARNING OBJECTIVES FOR TEST 2 After reading this article and taking the test, the reader will be able to: ■■Describe
pitfalls in renal mass evaluation due to the limitations of CT and MR imaging equipment.
pitfalls in renal mass evaluation due to errors in image interpretation.
pitfalls in renal mass evaluation due to an inadequate CT or MR imaging technique.
Gary M. Israel, MD • Morton A. Bosniak, MD Characterization of renal masses with computed tomography (CT) and magnetic resonance (MR) imaging is usually clear-cut and accurate. However, potential pitfalls exist in diagnosis of renal masses, and it is necessary to understand these pitfalls to avoid misdiagnosis and possibly unnecessary surgery. Although some of the pitfalls are related to technical factors of the CT and MR imaging equipment, others are related to errors in image interpretation. To maximize detection and characterization of renal masses, the study should include images obtained before and after administration of intravenous contrast material, including images obtained during the nephrographic phase of enhancement. One should be aware of the potential unreliability of absolute Hounsfield unit measurements and of the existence of possible CT pseudoenhancement. When CT results are indeterminate, MR imaging may be helpful in demonstrating enhancement in renal masses. Before diagnosing a renal mass as a malignant neoplasm or suggesting surgery for a renal mass, one should consider alternative benign diagnoses; when appropriate, previous images or a supporting history should be obtained. ©
RSNA, 2008 • radiographics.rsnajnls.org
TEACHING POINTS See last page
Abbreviations: MLCN = multilocular cystic nephroma, MLCRCC = multilocular cystic renal cell carcinoma RadioGraphics 2008; 28:1325–1338 • Published online 10.1148/rg.285075744 • Content Codes: 1 From the Department of Diagnostic Radiology, Yale University School of Medicine, 333 Cedar St, New Haven, CT 06520-8042 (G.M.I.); and the Department of Radiology, New York University School of Medicine, New York, NY (M.A.B.). Received October 17, 2007; revision requested January 11, 2008, and received January 18; accepted January 22. All authors have no financial relationships to disclose. Address correspondence to G.M.I. (e-mail: [email protected]
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With the use of computed tomography (CT) and magnetic resonance (MR) imaging, the diagnosis of most renal masses is relatively straightforward and accurate. However, even with these techniques, there still remain some masses in which a correct diagnosis is not made. This could be due to the pathologic and vascular nature of the lesion, but failure to appreciate the limitations of these imaging modalities, errors in imaging technique, errors in image interpretation, and failure to obtain an adequate history or previous images should also be considered. This article reviews the potential pitfalls in characterizing renal masses with CT and MR imaging and offers suggestions to help avoid errors in the diagnosis of renal masses. Renal masses include those due to infection, localized cystic disease of the kidney, complex cystic renal masses (Bosniak category III) that are likely benign, and solid masses. Pitfall: Failure to properly characterize a renal mass secondary to limitations of the equipment. Solution: Be aware of the potential unreliability of absolute Hounsfield unit readings and of the existence of possible CT pseudoenhancement, so that benign cystic renal masses are not mischaracterized as solid and potentially malignant. At CT, enhancement in renal masses is determined by comparing Hounsfield unit readings from the unenhanced exmination with those from the contrast-enhanced examination. Although there is no universally agreed on number that can be used as unequivocal evidence of enhancement, a change of 20 HU or greater would be strong evidence of enhancement (1). However, it must be recognized that the use of Hounsfield unit numbers for tissue characterization may be unreliable. Early in the history of CT, it was recognized that there was a wide range of Hounsfield unit attenuation for a given tissue as a result of CT scanner performance alone (2). Variability of Hounsfield unit attenuation was found to be dependent on the type of scanner, the position of an object within a scanner, change in kilovolt peak, and change in scanning time. With the introduction of helical scanners in the 1990s, this unreliability was again recognized (3).
RG ■ Volume 28 • Number 5
The Hounsfield unit numbers acquired with the multidetector CT scanners of today have also been shown to vary significantly (4). Therefore, it is important to be aware of the potential unreliability of Hounsfield unit numbers when using them to determine the presence or absence of enhancement in a mass. It should be stressed that any enhancement that is identified must be unequivocal. In some cases, it can be helpful to use the gallbladder, a simple renal cyst (if present), or even an extrarenal urinefilled pelvis (on the early scan) as an internal reference standard to compare the Hounsfield unit measurements of the standard with the Hounsfield unit measurements of the renal mass, to determine if what appears to be enhancement is real or artificial. Pseudoenhancement is defined as an artificial elevation of the Hounsfield unit measurements of a renal cyst measured on the contrast-enhanced CT images and is thought to be secondary to the image reconstruction algorithm used to adjust for beam-hardening effects (5–11). When compared with the unenhanced Hounsfield unit value, pseudoenhancement may create Hounsfield unit values that are artificially higher, suggesting enhancement within the lesion, so that a simple renal cyst might be mischaracterized as a solid mass. Therefore, being aware of pseudoenhancement and knowing when to suspect it are important. Pseudoenhancement most frequently occurs in small (