Background: Epithelioid angiomyolipoma is defined as potentially malignant mesenchymal neoplasm, characterized by proliferating epithelioid cells, whereas classic angiomyolipoma, composed of fat, smooth muscle cells and dysmorphic vessels, is defined as a potentially benign. The usual or classic angiomyolipoma is often found incidentally on imaging studies, relatively easily identified due to the presence of fat, in contrast to the epithelioid angiomyolipoma that can pose diagnostic challenges. Case presentation: We report a 51-year-old female patient in which an ultrasonography examination showed a solid mass close to the right renal pelvis with hypoechoic and hyperechoic areas. A differential diagnosis of atypical sinus lipomatosis, lipoma and a transitional cell carcinoma was postulated whereas in a subsequent computed tomography a classic angiomyolipoma was postulated. A re-examination by contrast enhanced ultrasound revealed a striking perfusion difference of the hypoechoic and hyperechoic areas. The hypoechoic area showed homogenous and prolonged enhancement whereas the hypoechoic area displayed a marked slower contrast material flooding and a relatively rapid wash out. The histological analysis from the biopsy of the hyperechoic area showed a classic angiomyolipoma, whereas the sample of the hypoechoic central portion revealed an epithelioid angiomyolipoma. A nephrectomy was performed because of the malignant potential of the epithelioid variant of the angiomyolipoma. Conclusions: A solid kidney mass with two sharply defined parts, one-part compatible with a classical angiomyolipoma and the other being suspected of carcinoma, is rare, but also illustrative and instructive. The combination of different imaging modalities in the work up of a solid renal mass facilitated to discriminate benign from malignant areas. Keywords: Epithelioid angiomyolipoma, Contrast enhanced ultrasound, Computed tomography, Biopsy Background angiomyolipoma and epithelioid angiomyolipoma have Epithelioid angiomyolipoma is defined as a potentially been associated with tuberous sclerosis [2, 3].The usual malignant mesenchymal neoplasm, characterized by pro- or classic angiomyolipoma is often found incidentally on liferating epithelioid cells, whereas classic angiomyoli- imaging studies, relatively easily identified due to the poma, composed of fat, smooth muscle cells and presence of fat, in contrast to the epithelioid angiomyoli- dysmorphic vessels, is considered benign . Classic kid- poma that can pose diagnostic challenges as it mimics a ney angiomyolipoma appears in 0.3% of the general large variety of neoplasms. population and accounts for 3% of solid renal masses Ultrasound contrast agent consist of gas microbubbles whereas epithelioid angiomyolipoma is very rare. Classic enclosed in lipid shells. These microspheres are about the half size of a red blood cell and remain only intravas- cular and thus do not cross into the interstitial space * Correspondence: email@example.com Ultrasound Learning Center EFSUMB, Klinik Hirslanden, Zürich, Switzerland whereas most computed tomography and magnetic res- Klinik für Innere Medizin und Nephrologie, Klinik Hirslanden, Witellikerstrasse onance imaging agents cross the vessels . Ultrasound 40, 8032 Zürich, Switzerland contrast agent therefore is not excreted in the collecting Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Tuma et al. BMC Nephrology (2018) 19:123 Page 2 of 5 system of the kidney facilitating the quantification of hypoechoic solid renal mass of 2.5 cm diameter without tumor perfusion by analyzing tumor vascular enhancement posterior acoustic shadowing closely located to the renal patterns . Contrast-enhanced ultrasound allows to con- sinus. The kidney was not congested. Atypical sinus lipo- tinuously acquire images creating a time-intensity curve as matosis, lipomas or a transitional cell carcinoma was sus- compared with magnetic resonance imaging or computed pected and subsequently a computer tomography scan tomography with limited number of time points . Fur- was performed showing a homogenous mass that met the thermore, ultrasound contrast agents can be administrated criteria for a classic angiomyolipoma (Fig. 1). A computed multiple times for repeated acquisitions and exhibit no risk tomography scan repeated 2 years later demonstrated a for nephrotoxicity, cerebral deposition or nephrogenic sys- possible tumor growth to 3 cm in diameters. Another 2 temic fibrosis and lacks radiation burden. years later, an ultrasound examination revealed two differ- On conventional B-mode ultrasound, classical renal ent areas of the solid mass: a hyperechoic outer rim (echo- angiomyolipoma most commonly present as a uniform genicity tumor to normal kidney cortex ratio (TQ) of hyperechoiclesiondue to thepresenceoffat.The assess- 2.27) and inner hypoechoic portion (TQ of 0.47.) as dis- ment of hyperechoic renal mass with computer tomog- played in Fig. 2a. Echogenicity was measured according to raphy or magnetic resonance imaging can aid in diagnosing prior reported method [10–12]. A contrast enhanced angiomyolipoma: negative attenuation on computer tomog- ultrasound was performed displaying a distinct different raphy and signal dropping on fat-suppressed magnetic res- perfusion pattern of these two areas. In the hyperechoic onance sequences are findings which refer to an peripheral area, we noticed a strong perfusion that started angiomyolipoma. Applying contrast-enhanced ultrasound, nearly simultaneously with the renal cortex and a marked renal angiomyolipomas are characterized by homogenous slower contrast material flooding with a relatively rapid enhancement and a prolonged enhancement time during wash out in the hypoechoic portion of the tumor (Fig. 2b). the corticomedullary and late phase. Xu et al. demonstrated The finding of the contrast enhanced ultrasound of the that an early washout, a heterogeneous enhancement, and hyperechoic part of the tumor was in line with a classic thepresenceofaperitumoralrim arelesions suspectfor angiomyolipoma whereas the dignity of the hypoechoic renal cell carcinoma.  part remained unclear. A target biopsy of both parts was The evaluation of the dignity of solid renal masses on performed. The histological examination showed a classic imaging is often challenging. Computer tomography, angiomyolipoma in the biopsy specimen of the outer rim magnetic resonance imaging and ultrasound can often (Fig. 3a) and an epithelioid angiomyolipoma in the biopsy not accurately distinguish between benign and malignant specimen of the center part (Fig. 3b). Immunohistochem- renal masses [8, 9] whereas the combination of various istry showed strong positivity for melanocytic markers examination techniques can complete each other in the and smooth muscle markers, confirming the diagnosis of work up of a patient with a renal mass. epithelioid angiomyolipoma. A subsequent nephrectomy was performed confirming the diagnosis. Case presentation A 52-year-old female patient with recurrent urinary tract Discussion and conclusions infections underwent abdominal ultrasound examination. The incidence of occasional detection of renal masses The right kidney ultrasound showed a sharply restricted, has increased with the wider application of various Fig. 1 a Computer tomography scan without contrast material in 2008 displaying a solid mass close to the right kidney sinus of 2.81 × 2.34 cm in diameters. b Computer tomography scan with contrast material in 2008 displaying an early enhancement of tumor ventral margin in the arterial phase Tuma et al. BMC Nephrology (2018) 19:123 Page 3 of 5 Fig. 2 a B-mode ultrasound reveals two areas with different echogenicity within the tumor: Outer rim is hyperechoic with an echogenicity tumor to normal kidney cortex ratio (TQ) of 2.27 (a value larger than 2.0 is characteristic for angiomyolipoma) and an inner hypoechoic portion (TQ 0.47) . b Time intensity curves (TIC) of the contrast enhanced ultrasound (CEUS) showing an early and strong enhancement of the hyperechoic outer rim (yellow line), similar to renal cortex (blue line), and a later and weaker enhancement of hypoechoic inner portion (red line). The strong enhancement of the outer tumor rim is clearly seen on the ventral side imaging modalities [13, 14]. The individual imaging carcinoma was postulated whereas based on a subse- methods alone can often not determine the dignity of quent computed tomography a classic angiomyolipoma kidney tumors. The majority of benign kidney tumors was postulated. consist of oncocytomas and angiomyolipomas . Be- However, a re-examination by contrast enhanced nign tumors do not require intervention unless they ultrasound revealed a striking perfusion difference of the cause the patient discomfort. The usual or classic angio- hypoechoic and hyperechoic areas. Prior studies suggest myolipoma is relatively easily identified by applying that contrast-enhanced ultrasound is a valuable method current imaging modalities. The high fat content in clas- in distinguishing angiomyolipoma from renal cell carcin- sic angiomyolipomas causes the tumors to appear very oma [7, 16–18]. The outer rim of the renal mass de- echogenic in the B-mode of ultrasound and in computed scribed in our patient displayed the classical features of tomography an intensity of − 20 Hounsfield units is re- an angiomyolipoma: negative attenuation on the com- corded. In our case, a differential diagnosis of atypical puter tomography scan, hyperechoic appearance on the sinus lipomatosis, lipoma and a transitional cell B-Mode and homogenous and prolonged enhancement Fig. 3 a Histology of the biopsy specimen taken from the hyperechoic tumor rim showing many vessels and fatty tissue confirming the diagnosis of angiomyolipoma. b Histology of the biopsy specimen taken from the inner hypoechoic tumor showing pleomorphic multinucleated giant cells and cells with predominant eosinophilic cytoplasm revealing an epithelioid variant of an angiomyolipoma Tuma et al. BMC Nephrology (2018) 19:123 Page 4 of 5 of the ultrasound contrast agent. Classic angiomyolipo- Authors’ contributions JT, ALS and WG analyzed and interpreted the patient data regarding mas are composed of fat, smooth muscle cells and dys- imaging and biopsy results. HM performed the histological examination of morphic vessels and thus the ultrasound contrast agent, the kidney. GS performed the kidney biopsy. All authors read and approved which does not trespass the vessels, enhanced this part the final manuscript. of the tumor rapidly and homogenously. The inner por- Ethics approval and consent to participate tion of the renal mass was hypoechoic and showed a low According to the Swiss Law (Humanes Forschungsgesetz) and the Kantonale enhanced by the ultrasound contrast material due to the Ethikkommission Zürich (KEK) a case report with 5 and less subjects is not a research project and therefore not subject to approval. absence of fat and a considerable low number of vessels as shown by the histological analysis. Epithelioid variant Consent for publication of angiomyolipoma (epithelioid angiomyolipoma) is a The patient gave written informed consent for the publication. rare tumor defined as a potentially malignant mesenchy- Competing interests mal neoplasm that has been described first by The authors declare that they have no competing interests. Martignoni as a distinct variant of angiomyolipoma . Epithelioid angiomyolipoma is characterized by the pres- Publisher’sNote ence of plump and spindled epithelioid cells with varying Springer Nature remains neutral with regard to jurisdictional claims in degrees of nuclear atypia and pleomorphic multinucle- published maps and institutional affiliations. ated cells are often also present [1, 20]. According to the Author details current WHO classification, tumors with 80% and more Ultrasound Learning Center EFSUMB, Klinik Hirslanden, Zürich, Switzerland. epithelioid cells are considered epithelioid angiomyoli- 2 3 Institut für Pathologie, Universitätsspital, Zürich, Switzerland. Institut für poma . Kidney epithelioid angiomyolipoma have Radiologie, Kantonsspital, Winterthur, Switzerland. Stiftung für Wissenstransfer, Hefenhofen, Switzerland. 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Published: May 30, 2018