Fat-poor angiomyolipoma and renal cell carcinoma: differentiation with MR imaging and accuracy of histopathologic evaluation

We read the article by Dr Sasiwimonphan and colleagues in the April 2012 issue of Radiology (1). We believe that the study was very interesting but there are some concerns to be discussed. In Materials and Methods, the description of histologic features of tumors is inaccurate. Two subtypes of renal angiomyolipoma (AML) have been defined on the basis of the adipose tissue amount: fat-poor renal AML and monotypic renal AML. The fat content of fat-poor renal AMLs per high-power field is less than 25% (2). The smooth muscle or epithelioid monotypic (monophasic) renal AML contains only 5% or less of adipose tissue. The monotypic adipose renal AML contains at least 95% of adipose tissue (3). The authors’ conclusions are not exhaustive because they do not classify the renal AMLs into fat-poor and monotypic subtypes (1). Dr Sasiwimonphan and colleagues excluded eight patients with 10 renal AMLs “without fat” on computed tomographic (CT) scans. The term “macroscopic” should be used only in the histologic reports. In the Material and Methods, the authors have not specified histologically the presence of necrosis, hemorrhage, and calcifications in the renal cell carcinomas (RCCs). These features are commonly seen in the RCC and may be considered for diagnosis with magnetic resonance (MR) imaging. The inaccuracy of histologic description decreases the value of the conclusions reported in the study of Dr Sasiwimonphan and colleagues. In our experience, a comparison between small (<4-cm) RCCs (n = 20) and fat-poor renal AMLs (n = 14) with use of chemical shift showed that a 20% threshold for the signal intensity index (SII) is too low (our study demonstrates that the average SII in the RCC group was 6.1%, whereas four RCCs showed SII >25%, with a range between 32.1% and 35.6%) (4). Our results are different from those reported by Dr Sasiwimonphan and colleagues. In the differentiation of small RCCs from fat-poor renal AMLs, we consider the time profiles of percentage of enhancement and contrast-to-noise ratio enhancement at dynamic MR imaging (5).