Small (<1 cm) incidental echogenic renal cortical nodules: chemical shift MRI outperforms CT for confirmatory diagnosis of angiomyolipoma (AML)

Non-calcified echogenic renal cortical nodules are commonly detected with abdominal ultrasound (US). The majority of these nodules represent benign angiomyolipomas (AMLs), which are present in 0.3–2.1 % of the population at autopsy [1]. The increased echogenicity (in the absence of calcification) of renal AML is due to the presence of gross or mature fat within the nodule [2]. Although renal AMLs are typically markedly echogenic, echogenicity may vary depending upon the relative proportion of fat, smoothmuscle and blood vessels within the nodule [3, 4]. Renal cell carcinoma (RCC) is traditionally considered to be less echogenic than renal cortical parenchyma, although the echogenicity of RCC varies with its size. Forman et al. [5] demonstrated that one-third of RCCs less than 3 cm in size are as echogenic as “classic” AML. In a recent meta-analysis, Farrelly et al. [6] demonstrated that nearly half of small RCCs are more echogenic than renal cortical parenchyma and 11.5 % are as echogenic as renal sinus fat. The increased echogenicity of small RCC is attributed to cell arrangement with increased internal interfaces and the presence of internal degeneration or haemorrhage [7]. Nodule heterogeneity, intratumoural cysts and the presence of a hypoechoic rim are specific sonographic findings that favour echogenic RCC; while posterior acoustic shadowing is a specific sonographic finding that favours AML [6]. Although these differentiating sonographic findings are specific, they lack the sensitivity required to discriminate betweenAML and RCCwhen a small echogenic renal cortical nodule is detected in everyday practice [6]. Given that small RCCs are commonly echogenic and may mimic renal AMLs at US and that sonographic differentiating features are insensitive, an imaging quandary occurs. Confirmatory imaging with computed tomography (CT) is generally accepted for larger lesions to detect the presence of gross fat and confirm the presumed diagnosis of AML (Fig. 1). For smaller lesions (<1 cm), recommendations vary considerably and management is controversial (Fig. 1). Some radiologists recommend no further follow-up imaging, considering them all small AMLs [2]. Sonographic follow-up is prescribed by others, since small incidentally discovered renal nodules grow slowly [8] and are associated with a low risk of metastatic disease [9]. However, metastatic disease from small RCC does occur [9] and both small RCC and AML can grow slowly [2], making differentiation based on growth difficult. A meta-analysis on the topic by Farelly et al. [6] concluded that, based on the limited available literature, all incidentally detected echogenic renal cortical nodules undergo confirmatory imaging with CT. At our institution, we observe a variety of practice patterns amongst radiologists, with many recommending confirmatory CT for echogenic renal cortical nodules measuring <1 cm in size. The diagnosis of AML with CT is predicated on the ability to demonstrate gross fat within the nodule, which is essentially pathognomonic of AML; with only rare case reports of gross fat within both papillary and clear cell RCC [10]. A representative region of interest (ROI) measurement within the nodule measuring of fat density (less than −10 to −20 Hounsfield units [HU]) is considered diagnostic of AML [11, 12]. For larger nodules this is readily accomplished and an accurate diagnosis of AML is achieved (Fig. 2). For smaller nodules (<1 cm), particularly when the nodule is embedded within N. Schieda (*) : L. Avruch Department of Medical Imaging, The Ottawa Hospital, The University of Ottawa, 1053 Carling Avenue, Ottawa, Ontario, Canada K1Y 4E9 e-mail: [email protected]