New radiologic classification of renal angiomyolipomas

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Abstract

Purpose

To introduce a new radiologic classification of renal angiomyolipoma (AML).

Materials and methods

Between 1995 and 2014, CT or MR images in 98 patients with histologically proven 98 AMLs were reviewed independently by a radiologist and a resident. The lesions were classified as (a) 53 fat-rich AML (≤−10HU), (b) 22 fat-poor AML (>−10HU) with tumor-to-spleen ratio (TSR) <0.71 or signal intensity index (SII) >16.5%, and (c) 23 fat-invisible AML (>−10HU) with TSR ≥0.71 and SII ≤16.5%. Inter-reader agreement was assessed with a weighted kappa value. Fat-poor and fat-invisible AMLs were compared in terms of attenuation value, TSR, and SII using unpaired t-test.

Results

The weighted kappa value was 0.956 (95% confidence interval, 92.0–99.1%). When a region of interest (ROI) was placed within the most hypodense area on unenhanced CT or within the most signal-dropped area on chemical shift image, the mean attenuation values, TSRs, and SIIs of fat-poor versus fat-invisible AMLs were 19.5 ± 8.1 HU versus 38.1 ± 9.9 HU, 0.59 ± 0.19 versus 0.96 ± 0.01, and 43.7 ± 16.9% versus −5.4 ± 21.1%, respectively (p < 0.0001). When a ROI was placed within the other area on CT or chemical shift images, 90.1% (48/53) of fat-rich AMLs were mis-classified as fat-poor or fat-invisible AML and 50% (11/22) of fat-poor AMLs as fat-invisible AML.

Conclusion

The new radiologic classification of renal AML is feasible for clinical practice. ROI location is important in differentiating the types of AMLs.

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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