Imaging characteristics of minimal fat renal angiomyolipoma with histologic correlations

Objectives. To review our experience with minimal fat angiomyolipomas (AMLs) and correlate the confirmed pathologic diagnosis with preoperative radiologic features to evaluate the feasibility of an accurate diagnosis preoperatively. On rare occasions, renal AMLs contain minimal amounts of fat that are not identified on cross-sectional imaging. Methods. From November 1998 to August 2003, 6 patients (seven lesions) underwent renal surgery with the preoperative diagnosis of renal cell carcinoma as determined by preoperative imaging studies, and the finding of AML was unexpected. A single pathologist and radiologist reviewed all pathologic specimens and radiologic examinations. The characteristic findings were recorded and compared with those in published reports. Results. Pathologic review of the specimens demonstrated microscopic amounts of mature adipocytes, abnormally thickened blood vessels, and smooth muscle cells in all seven lesions. The mean estimated fat content was 4.1% (range 3% to 10%). Hounsfield unit measurement of the lesions on unenhanced computed tomography images revealed all lesions to be hyperdense relative to the normal kidney parenchyma and enhanced by at least 90 Hounsfield units or more with the administration of intravenous contrast. Enhancement was homogenous throughout each of the seven masses. Conclusions. Minimal fat renal AML tumors are typically hyperdense relative to the normal kidney parenchyma and demonstrate homogenous enhancement with the administration of intravenous contrast on computed tomography. These results may have important implications when planning partial versus radical nephrectomy by open or minimally invasive techniques.