Imaging features of renal lipid-poor angiomyolipomas: a pictorial review

Renal lipid-poor angiomyolipomas (lipid-poor AMLs) are usually difficult to differentiate from clear-cell renal cell carcinomas (RCCs), papillary RCCs, epithelioid AMLs, and oncocytomas, while the differentiation is important due to different treatments and prognoses. This pictorial review presents clinical, histopathological, and imaging (ultrasound [US], CT, and MRI) features of lipid-poor AMLs and the keys to differentiate them from the main subtypes of RCCs, epithelioid AMLs, and oncocytomas. Lipid-poor AMLs are small lesions, mostly occur in young female patients. They are often hyperechoic or isoechoic on US and show homogeneous hyperdensity without calcification on CT. MRI typical features include hypointensity on T2WI, discrete and focal signal intensity (SI) decrease on opposed-phase image, and with wash-in and wash-out enhancement. Clear-cell RCCs show heterogeneous hyperintensity on T2WI and diffuse decrease in SI on opposed-phase images. Papillary RCCs show SI drop on in-phase images (which reflects intratumoral hemorrhage) and a progressive enhancement. Epithelioid AMLs are big (7 cm) and heterogeneous with potential malignancy. Immunohistochemistry markers (such as HMB-45 antigen and melan-A) matter for their diagnosis. Oncocytomas may show typically central stellate scars with segmental enhancement inversion. In addition, combining clinical information, CT, and MRI features could further facilitate diagnostic accuracy.