Computed Tomography and Magnetic Resonance Findings of Fat-Poor Angiomyolipomas.

Introduction: Approximately 5% of angiomyolipomas (AMLs) are classified as "fat poor" due to lack of visually detectable fat on imaging, making them difficult to distinguish from renal cell carcinoma. Recent investigations have proposed CT and MR imaging features suggestive of fat-poor AML (fp-AML). Herein, we determined the frequency of these features in a cohort of fp-AMLs by retrospective review of preoperative imaging.

Methods: A pathology database query from January 2005 to August 2013 identified 49 renal specimens of AML with available imaging. A retrospective review of all CT and MR images of these 49 cases was conducted. Cases with visually detectable fat on imaging were excluded.

Results: A total of 26 fp-AMLs were identified. Thirteen lesions had available unenhanced CT images, of which eight (62%) were hyperdense compared to the adjacent renal parenchyma, while five (38%) were isodense. Twenty lesions had enhanced CT images: 14 (70%) and 6 (30%) with homogeneous and heterogeneous enhancement, respectively. Of the nine lesions with enhanced MR sequences, five (56%) were homogeneously enhancing, and four (44%) were heterogeneously enhancing. Eight of nine (89%) lesions had hypointense signal intensity (SI) on T2-weighted MR sequences, while one (11%) had hyperintense SI. None of the eight lesions displayed a decrease in signal on fat-suppressed sequences.

Conclusions: In this study, we confirmed common imaging features of fp-AML: high attenuation on unenhanced CT sequences, homogeneous enhancement on CT, and hypointensity on T2-weighted MR. When these features are present, a renal mass biopsy may be prudent.