This 45-year-old male patient presents with right upper quadrant pain.

â Angiomyolipoma (AML) is the most common benign solid renal neoplasm observed in clinical practice

.
â Prevalence ranges from 0.2 to 0.6%, females being affected more than males

.
â 80% are sporadic while 20% are associated with tuberous sclerosis complex

.
â As the name suggests, the tumor consists of varying amounts of three elements

â Hyperechoic due to fat component.
â Fat-poor AML can be difficult to diagnose on ultrasound and differentiation from a renal cell carcinoma (RCC) can also be challenging in these cases

.
â Up to 1/3rd cases of AML can show acoustic shadowing which is not seen with RCC

Fat in lesions can be seen as hypodense areas with average HU values less than -10 HU. The presence of fat is however nonspecific and fat has been reported in RCCs as well

.  In such cases, other associated features like larger size, necrosis, invasive/irregular margins, renal vein/ IVC involvement and metastases can help aid diagnosis

Fat-poor AMLs appear as homogeneously hyperdense on unenhanced CT.

Calcification is unusual and should raise the possibility of an alternative diagnosis like RCC

Intra-tumoral hemorrhage can be also seen, especially in tumors larger than 4cm

Larger lesions can have dilated tortuous vessels and aneurysms, which makes them susceptible to intra-tumoral hemorrhage

. A small aneurysm is seen in the lesion depicted in Figure 1.

Fat-poor AMLs show homogenous persistent and gradual enhancement

MRI
â Fat-rich AMLs appear hyperintense on T1 and T2-weighted images owing to their lipid content. Fat suppressed images and in/out phase imaging, aids confirmation of fat.
â Fat-poor AMLs appear as hypointense lesions on T2-weighted images, while fat-invisible AMLs are isointense as the signal is mainly from the muscle component of the lesion

Management:
â Classic angiomyolipoma

Lesions more than 4 cm and with intratumoral aneurysms measuring more than 5mm are at increased risk of spontaneously bleeding, and may need active management.

â Fat-poor AMLs and fat-invisible AMLs may need biopsy for confirmation as these can be difficult to differentiate from RCCs

.
â AMLs associated with tuberous sclerosis complex have increased risk of spontaneous bleeding

 and are more likely to need active management.
â Treatment options include

mTOR inhibitor sirolimus has been useful in a few subtypes.

References
1. Jinzaki M, Silverman S, Akita H, Nagashima Y, Mikami S, Oya M. Renal angiomyolipoma: a radiological classification and update on recent developments in diagnosis and management. Abdominal Imaging. 2014;39(3):588-604.
2. Vos N, Oyen R. Renal Angiomyolipoma: The Good, the Bad, and the Ugly. Journal of the Belgian Society of Radiology. 2018;102(1).
3. Park B. Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat. American Journal of Roentgenology. 2017;209(4):826-835.

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