Renal angiomyolipomas (AML) are a type of benign renal neoplasms encountered both sporadically and as part of a phakomatosis, most commonly tuberous sclerosis. They are considered one of a number of tumors with perivascular epithelioid cellular differentiation (PEComas) and are composed of vascular, smooth muscle and fat elements. They can spontaneously hemorrhage, which may be fatal. AMLs usually have characteristic radiographic appearances.
Angiomyolipomas are the most common benign solid renal lesion and also the most common fat-containing lesions of the kidneys. The majority of angiomyolipomas are sporadic (80%) and are typically identified in adults (mean age of presentation 43 years), with a female predilection (F:M of 2-4:1) 7,9.
The remaining 20% are seen in association with phakomatoses, the vast majority in the setting of tuberous sclerosis (80% of them get AML) although they have also been described in the setting of von Hippel-Lindau syndrome (vHL) and neurofibromatosis type 1 (NF1) 5,7. In these cases, they present earlier (usually identified by the age of 10 years), are larger, and are far more numerous. They are more likely to be fat-poor which accounts for their earlier presentation 2,6,7. They have also been associated with lymphangioleiomyomatosis (LAM) 20.
Angiomyolipomas are often found incidentally when the kidneys are imaged for other reasons, or as part of screening in patients with tuberous sclerosis.
Symptomatic presentation is most frequently with spontaneous retroperitoneal hemorrhage; the risk of bleeding is proportional to the size of the lesion (>4 cm diameter). Shock due to severe hemorrhage from rupture is described as Wunderlich syndrome 4,5,7.
Angiomyolipomas are members of the perivascular epithelioid cells tumor group (PEComas) and are composed of variable amounts of three components; blood vessels (-angio), plump spindle cells (-myo) and adipose tissue (-lipo).
Almost all classic angiomyolipomas are benign but they do have the risk of rupture with bleeding or secondary damage/destruction of surrounding structures as they grow.
Two histological types have been described
- typical (triphasic)
- atypical (monophasic or epithelioid)
A special variant called an epithelioid angiomyolipoma is composed of more plump, epithelial-looking cells, often with nuclear atypia, that have a described risk of malignant behavior. This variant, unlike conventional AMLs, may mimic renal cell carcinoma 10. Metastases have also been described 9.
The cornerstone of diagnosis on all modalities is the demonstration of macroscopic fat, however in the setting of hemorrhage, or when lesions happen to contain little fat, it may be difficult to distinguish an angiomyolipoma from a renal cell carcinoma.
In tuberous sclerosis, the lesions are larger than isolated AML and often multiple and bilateral.
- tend to appear as hyperechoic lesions on ultrasound, located in the cortex and with posterior acoustic shadowing
- in the setting of tuberous sclerosis, they may be so numerous that the entire kidney is affected, appearing echogenic with the loss of normal corticomedullary differentiation 7
- contrast-enhanced ultrasound 12
- tend to enhance peripherally
- decreased central enhancement, compared with normal cortex
Most lesions involve the cortex and demonstrate macroscopic fat (less than -20 HU). When small, volume averaging may make differentiation from a small cyst difficult. It is essential to remember that rarely renal cell carcinomas (RCC) may have macroscopic fat components and as such the presence of fat is strongly indicative of an angiomyolipoma, but not pathognomonic.
It is important to realize that ~5% of angiomyolipomas are fat-poor 15. This is especially the case in the setting of tuberous sclerosis, where up to a third do not demonstrate macroscopic fat on CT 6. Absence of ossification/calcification on imaging is in favor of AML ref.
MRI is excellent at evaluating fat-containing lesions, and two main sets of sequences are employed. Firstly, fat-saturated techniques demonstrate high signal intensity on non-fat-saturated sequences and loss of signal following fat saturation.
The second method is to use in-phase and out-of-phase imaging, which generates India ink artifact at the interface between fat and non-fat components. This can occur either at the interface between the angiomyolipoma and surrounding kidney or between fat and non-fat components of the mass 8. Chemical shift signal intensity loss, along with other features, may suggest a fat-poor AML 15.
Digital subtraction angiography (DSA)
Angiomyolipomas are hypervascular lesions demonstrating often characteristic features:
- arterial phase: a sharply marginated hypervascular mass with a dense early arterial network, and tortuous vessels giving the "sunburst" appearance
- venous phase: whorled "onion peel" appearance of peripheral vessels
- micro- or macro-aneurysms 2
- absent arteriovenous shunting
Treatment and prognosis
Angiomyolipomas found incidentally usually require no therapy (when small), although follow-up is recommended to assess for growth. Small solitary AMLs (<20 mm) probably do not require follow-up due to their slow growth 13.
Larger AMLs, or those that have been symptomatic, can be electively embolized and/or resected with a partial nephrectomy.
The traditional convention is that intervention is considered when an angiomyolipoma reaches 40 mm in size 18.
Lesions that present with retroperitoneal hemorrhage often require emergency embolization as a life-saving measure.
mTOR inhibitors (e.g. everolimus) have been shown to significantly decrease AML size and may help to preserve renal function in tuberous sclerosis patients.
When an AML has typical appearances there is essentially no differential. If atypical, especially when fat-poor, other lesions to consider include:
renal cell carcinoma (RCC)
- macroscopic fat in RCC almost always occurs in the presence of ossification/calcification
- focal or diffuse signal loss in opposed phase suggests microscopic fat (in contrast to the curvilinear India ink artifact seen with macroscopic fat), which is very common in clear cell renal cell carcinoma and less common in fat-poor AML 19
retroperitoneal liposarcoma invading the kidney:
- presence of a large vessel extending into the renal cortex suggestive of AML; liposarcoma is hypovascular
- renal parenchymal defect at the site of tumor contact favors exophytic angiomyolipoma - claw sign
- calcifications suggest liposarcoma
- oncocytoma: may contain fat
- Wilms tumor: may contain fat
- adrenal myelolipoma
- perirenal fat entrapment / renal junctional parenchymal defect 11
- 1. Eble JN. Angiomyolipoma of kidney. Semin Diagn Pathol. 1998;15 (1): 21-40. Pubmed citation
- 2. Logue LG, Acker RE, Sienko AE. Best cases from the AFIP: angiomyolipomas in tuberous sclerosis. Radiographics. 23 (1): 241-6. doi:10.1148/rg.231025109 - Pubmed citation
- 3. Umeoka S, Koyama T, Miki Y et-al. Pictorial review of tuberous sclerosis in various organs. Radiographics. 28 (7): e32. doi:10.1148/rg.e32 - Pubmed citation
- 4. Moratalla MB. Wunderlich's syndrome due to spontaneous rupture of large bilateral angiomyolipomas. Emerg Med J. 2009;26 (1): 72. doi:10.1136/emj.2008.062091 - Pubmed citation
- 5. Lowe LH, Isuani BH, Heller RM et-al. Pediatric renal masses: Wilms tumor and beyond. Radiographics. 20 (6): 1585-603. Radiographics (full text) - Pubmed citation
- 6. Choyke PL. Imaging of hereditary renal cancer. Radiol. Clin. North Am. 2003;41 (5): 1037-51. Pubmed citation
- 7. Harriet J. Paltiel Sonography of Pediatric Renal Tumors January 2007 (volume 2 issue 1 Pages 89-104 DOI: 10.1016/j.cult.2007.01.004)
- 8. Israel GM, Hindman N, Hecht E et-al. The use of opposed-phase chemical shift MRI in the diagnosis of renal angiomyolipomas. AJR Am J Roentgenol. 2005;184 (6): 1868-72. AJR Am J Roentgenol (full text) - Pubmed citation
- 9. Lai HY, Chen CK, Lee YH et-al. Multicentric aggressive angiomyolipomas: a rare form of PEComas. AJR Am J Roentgenol. 2006;186 (3): 837-40. doi:10.2214/AJR.04.1639 - Pubmed citation
- 10. Cui L, Zhang JG, Hu XY et-al. CT imaging and histopathological features of renal epithelioid angiomyolipomas. Clin Radiol. 2012;67 (12): e77-82. doi:10.1016/j.crad.2012.08.006 - Pubmed citation
- 11. Hélénon O, Merran S, Paraf F et-al. Unusual fat-containing tumors of the kidney: a diagnostic dilemma. Radiographics. 1997;17 (1): 129-44. doi:10.1148/radiographics.17.1.9017804 - Pubmed citation
- 12. Malhi H, Grant EG, Duddalwar V. Contrast-Enhanced Ultrasound of the Liver and Kidney. Radiol. Clin. North Am. 2014;52 (6): 1177-1190. doi:10.1016/j.rcl.2014.07.005 - Pubmed citation
- 13. Maclean DF, Sultana R, Radwan R et-al. Is the follow-up of small renal angiomyolipomas a necessary precaution?. Clin Radiol. 2014;69 (8): 822-6. doi:10.1016/j.crad.2014.03.016 - Pubmed citation
- 14. Ellingson JJ, Coakley FV, Joe BN et-al. Computed tomographic distinction of perirenal liposarcoma from exophytic angiomyolipoma: a feature analysis study. J Comput Assist Tomogr. 2008;32 (4): 548-52. doi:10.1097/RCT.0b013e3181507534 - Free text at pubmed - Pubmed citation
- 15. Shetty AS, Sipe AL, Zulfiqar M, Tsai R, Raptis DA, Raptis CA, Bhalla S. In-Phase and Opposed-Phase Imaging: Applications of Chemical Shift and Magnetic Susceptibility in the Chest and Abdomen. (2019) Radiographics : a review publication of the Radiological Society of North America, Inc. 39 (1): 115-135. doi:10.1148/rg.2019180043 - Pubmed
- 16. Nicolas Vos, Raymond Oyen. Renal Angiomyolipoma: The Good, the Bad, and the Ugly. (2018) Journal of the Belgian Society of Radiology. 102 (1): 41. doi:10.5334/jbsr.1536 - Pubmed
- 17. Buj Pradilla MJ, Martí Ballesté T, Torra R, Villacampa Aubá F. Recommendations for imaging-based diagnosis and management of renal angiomyolipoma associated with tuberous sclerosis complex. (2017) Clinical kidney journal. 10 (6): 728-737. doi:10.1093/ckj/sfx094 - Pubmed
- 18. Dawson C. Guidelines for the active surveillance of angiomyolipoma. Urology Clinical Governance Lead (2017). Pchurology.co.uk
- 19. Schieda N, Davenport MS, Pedrosa I, Shinagare A, Chandarana H, Curci N, Doshi A, Israel G, Remer E, Wang J, Silverman SG. Renal and adrenal masses containing fat at MRI: Proposed nomenclature by the society of abdominal radiology disease-focused panel on renal cell carcinoma. (2019) Journal of magnetic resonance imaging : JMRI. 49 (4): 917-926. doi:10.1002/jmri.26542 - Pubmed
- 20. Meraj R, Wikenheiser-Brokamp KA, Young LR, McCormack FX. Lymphangioleiomyomatosis: new concepts in pathogenesis, diagnosis, and treatment. (2012) Seminars in respiratory and critical care medicine. 33 (5): 486-97. doi:10.1055/s-0032-1325159 - Pubmed
- 21. Idir Ouzaid, Riccardo Autorino, Richard Fatica, Brian R. Herts, Gordon McLennan, Erick M. Remer, Georges‐Pascal Haber. Active surveillance for renal angiomyolipoma: outcomes and factors predictive of delayed intervention. (2014) BJU International. 114 (3): 412. doi:10.1111/bju.12604