New radiologic classification of renal angiomyolipomas
Introduction
Renal angiomyolipoma (AML) is easily diagnosed with CT or MR images because of abundant fat tissue. However, AML with a small amount of fat is frequently identified after the lesion has been managed with biopsy or surgery [1], [2]. Therefore, many investigators have reported imaging features of AML with a small amount of fat in order to differentiate this lesion from renal cell carcinoma (RCC). This subset of AML has various names, including “AML with minimal fat” [3], [4], [5], [6], [7], “lipid-poor AML” [8], [9], “fat-poor AML” [10], [11], “low-fat AML” [12], “minimal fat AML” [13], [14], [15], [16], “AML without visible fat” [17], “fat-invisible AML” [18], and so on. AML with a small amount of fat is most commonly called “AML with minimal fat” or “minimal fat AML”. AML with minimal fat originally indicated a lesion that does not contain fat attenuation at unenhanced CT (UCT) [19]. However, whether the other terminologies were used to indicate AML with minimal fat is doubtful because of many contradictions between studies in differentiating AML with a small amount of fat from RCC on CT or MR images.
A recent review article reported the classification of renal AMLs, in which clinical behavior, radiologic findings, and pathologic findings are mixed [20]. However, radiologists have difficulty accurately classifying renal AMLs with this classification because they are not usually given sufficient clinico-radio-pathologic information when CT or MR images are interpreted. Therefore, they need a new classification that is feasible to classify renal AMLs. We hypothesized that renal AMLs can be classified according to the amount of fat detected on CT or MRI and that the classification is feasible for application. The purpose of this study was to introduce a new radiological classification of renal AMLs.
Section snippets
Materials and methods
This retrospective study was approved by our institutional review board and informed consent was waived.
Results
Of the 98 AMLs, six were discordant for the diagnoses of fat-rich (n = 4) or fat-poor AML (n = 2) between an experienced radiologist and a third-year resident. The diagnoses of these discordant cases were re-evaluated with consensus reached between the two readers. The inter-reader agreement using quadratic kappa coefficients was almost perfect (kappa, 0.956; 95% confidence interval, 92.0–99.1%).
Of the 98 AMLs, 53 were classified as fat-rich AML, in which the lesion attenuation value were
Discussion
Our results showed that all renal AMLs were easily classified as fat-rich, fat-poor, or fat-invisible AML with excellent inter-reader agreement. The new radiologic classification of AMLs was based on the amount of fat that was detected on UCT or MRI. UCT or MRI diagnostic criteria were feasible for classifying renal AMLs. UCT or CSI demonstrated that a ROI should be placed within the most hypodense or the most signal-dropped area to identify a fat-rich or fat-poor AML.
The fat of renal AMLs can
Conflict of interest
None of the authors had conflicts of interest to report.
Acknowledgment
This study was not financially supported.
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