Unenhanced CT for the Diagnosis of Minimal-Fat Renal Angiomyolipoma

OBJECTIVE. Minimal-fat angiomyolipoma resembles renal cell carcinoma (RCC) on imaging. The purpose of this study was to evaluate the diagnostic accuracy of unenhanced CT to diagnose minimal-fat angiomyolipoma. MATERIALS AND METHODS. Consecutive patients who underwent unenhanced CT before resection of a solid renal mass from 2002 to 2012 were included in this study. Sixteen patients with minimal-fat angiomyolipoma and 48 patients with RCC (18 clear cell, 17 papillary, and 13 chromophobe RCCs) were studied. The mean (+/- SD) age of patients was 53.4 +/- 11.7 years for minimal-fat angiomyolipoma and 56.4 +/- 13.2 years for RCC. There were five male patients in the minimal-fat angiomyolipoma group and 26 male patients in the RCC group. Absolute density and relative density ratios were calculated and compared by the Kruskal-Wallis test and univariate regression analysis. Two radiologists, blinded to the diagnosis, independently assessed for coexisting classic angiomyolipoma, calcification within the mass, and the hypodense rim sign (defined as a low-density rim at the interface of the tumor and normal kidney). Radiologists evaluated both the axial and the coronal reformatted images. Diagnostic accuracy was compared using the Fisher exact test. RESULTS. Mean attenuation of minimal-fat angiomyolipoma was 43.1 +/- 9.8 HU, which was significantly higher when compared with RCC overall (p = 0.004) as well as with clear cell RCC (33.1 +/- 10.2 HU; p = 0.003), papillary RCC (33.0 +/- 6.6 HU; p = 0.003), and chromophobe RCC (34.3 +/- 9.9 HU; p = 0.01). The density ratio of minimal-fat angiomyolipoma to normal kidney was higher when compared with RCC overall (p = 0.002) and the respective RCC subtypes (p < 0.001, p = 0.006, and p = 0.002). The hypodense rim sign was identified in three patients with minimal-fat angiomyolipoma and five patients with RCC by both radiologists. There was no difference in the rate of detection of the hypodense rim sign in minimalfat angiomyolipoma compared with RCC (p = 0.14), and interobserver variability was fair (kappa = 0.32). Classic angiomyolipomas were identified only in patients with minimal-fat angiomyolipoma (p = 0.003). Calcification was not detected in any minimal-fat angiomyolipoma. CONCLUSION. Coexisting classic angiomyolipomas and the absence of calcification are associated with minimal-fat angiomyolipomas; by contrast, the hypodense rim sign is not useful for diagnosis. Minimal-fat angiomyolipomas have increased absolute and relative (normalized to renal parenchyma) density compared with RCC; however, overlap in density values limit diagnostic utility.