Renal Angiomyolipomas with minimal fat content: A Radiologic Chameleon
Number:28ESURABS0057
Type:Educational Electronic Presentation
Authors: Sofia Frade Dos Santos, Paolo Niccolò Franco, Alejandra García Baizán, María Milagros Otero García
Keywords:renal angiomyolipoma(AML), fat poor AML, fat invisible AML, renal cell carcinoma, radiologic classification
SECTIONS
Educational objective

-  To review relevant demographic, clinic, and histologic features of renal angiomyolipomas.

-  To describe and illustrate the two classifications of renal angiomyolipomas, with a focus on the radiologic one.

-  To highlight clues to the differential diagnosis, especially between angiomyolipomas with minimal fat content (fat poor/fat invisible) and renal cell carcinoma.

-  To propose an algorithm that may assist the radiologist and the clinician in identifying renal angiomyolipomas.

Topic Review

1. Introduction

The radiologic diagnosis of a classic renal angiomyolipoma (AML) is straightforward due to abundant fat tissue. However, approximately 5% of the AMLs have a minimal fat content and can be confused with renal cell carcinoma (RCC) (1), thus behaving as a “radiologic chameleon”.
Indeed, AMLs with minimal fat content account for a large percentage of solid renal tumors preoperatively misdiagnosed as RCC (20–60%). (2)
Therefore, accurate preoperative diagnosis of renal AMLs can be quite challenging but, at the same time, critical to prevent unnecessary nephrectomies and preserve renal function.

This subset of AML with minimal fat content has been given various names, including “lipid-poor AML”, “fat-poor AML”, “low-fat AML”, “minimal fat AML”,“AML without visible fat”,...

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Figure 1 – Correspondence between two existing classifications of renal angiomyolipomas: Jinzaki et al. and Song et al., the latter based only on radiological criteria.
Figure 2 – Jinzaki’s classification of renal angiomyolipomas: features across different imaging modalities. Note: for the ROI (region of interest) measurement (in unenhanced CT), it is important to have thin CT slices (≤ 3 mm). Using unenhanced CT or chemical shift imaging (CSI) MRI, a ROI should be placed within the most hypodense or the most signal-dropped area (small differences in attenuation numbers and signal intensities can lead to a different classification).
Conclusions

AMLs with minimal fat content still represent a challenge for radiologists.

In many cases, biopsy (or surgery) is needed to solve the diagnostic dilemma and exclude RCC. However, the combination of clinical and imaging features may help to detect these AMLs and, in selected cases, invasive procedures may be avoided or delayed.