Imaging Pearls ❯ Kidney ❯ Angiomyolipoma: AML
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- “Similar to LAM, AMLs are mesenchymal tumors categorized under the PEComa family of neoplasms. These tumors originate from clonal proliferation of epithelioid cells located around blood vessels. AMLs may be found in multiple organs, most notably the kidneys, and the presence of two or more of these tumors is considered a major feature of TSC. Renal AMLs can be identified in 55%–75% of patients with TSC. They are often discovered incidentally owing to their asymptomatic nature; however, they can manifest with abdominal pain, hematuria, anemia, hypertension, and/or retroperitoneal hemorrhage. Renal AMLs can multiply and grow during puberty, suggesting an estrogenic influence.”
Tuberous Sclerosis: Current Update
Wang MX et al.
RadioGraphics 2021; 41:1992–2010 - “There are two histologic subtypes of AML: classic and epithelioid. At gross pathologic examination, classic AML appears as a well-defined expansile mass in the renal cortex or medulla. Tortuous vasculature, clear to eosinophilic cells, and spindle cells are visible at microscopy. Epithelioid AMLs occur more frequently in patients with TSC (27%) than in the general population (7%). Malignant transformation of epithelioid AMLs can occur and may be suspected in the presence of rapid growth or necrosis. In contrast, classic AMLs tend to remain benign.”
Tuberous Sclerosis: Current Update
Wang MX et al.
RadioGraphics 2021; 41:1992–2010 - “There are two histologic subtypes of AML: classic and epithelioid. At gross pathologic examination, classic AML appears as a well-defined expansile mass in the renal cortex or medulla. Tortuous vasculature, clear to eosinophilic cells, and spindle cells are visible at microscopy. Epithelioid AMLs occur more frequently in patients with TSC (27%) than in the general population (7%). Malignant transformation of epithelioid AMLs can occur and may be suspected in the presence of rapid growth or necrosis. In contrast, classic AMLs tend to remain benign.”
Tuberous Sclerosis: Current Update
Wang MX et al.
RadioGraphics 2021; 41:1992–2010 - “The hallmark finding of lipid-rich AML is the presence of intratumoral fat with an attenuation of −10 HU or lower on noncontrast CT images. These tumors demonstrate variable enhancement. It should be noted that RCC may rarely demonstrate a small amount of intratumoral fat, and this is a potential cause of diagnostic dilemmas. The presence of calcification within a fat-containing renal mass should alert the clinician to the diagnosis of RCC. In contrast, calcification is extremely rare in AML.”
Tuberous Sclerosis: Current Update
Wang MX et al.
RadioGraphics 2021; 41:1992–2010 - “Renal AMLs represent the second most frequent cause of morbidity among patients with TSC owing to the risk of spontaneous hemorrhage and rupture. Such complications occur increasingly with AMLs that are larger than 4 cm or those that are associated with aneurysms larger than 5 mm. If either of these criteria is met, embolization or nephron sparing surgery is highly recommended. Tumors that do not fulfill this criterion are followed up conservatively or treated with mTOR inhibitors, elective embolization, ablative therapies, or nephron-sparing surgery.”
Tuberous Sclerosis: Current Update
Wang MX et al.
RadioGraphics 2021; 41:1992–2010 - “Renal cysts are the second most prevalent renal feature of TSC, occurring in 17%–47% of pediatric patients with TSC. Because of their low specificity, multiple renal cysts are considered a minor feature of TSC. TSC-related renal cysts can appear with varying severity, from microscopic disease to a serious polycystic variation. The polycystic variation occurs in about 2%–3% of patients with TSC and results from contiguous mutations of TSC2 and polycystic kidney disease type 1 (PKD1) genes, which are located close to each other on chromosome 16p13.3.”
Tuberous Sclerosis: Current Update
Wang MX et al.
RadioGraphics 2021; 41:1992–2010 - “RCC occurs in only 2%–3% of persons with TSC, similar to its prevalence in the general population. However, unlike sporadic RCC, TSC-related RCC occurs in comparatively young individuals—in whom the median age at diagnosis is 28 years—and tends to be less aggressive . TSC-related RCC can be categorized into three major histologic subtypes: papillary, chromophobe, and clear cell. These subtypes tend to exhibit different imaging patterns owing to their varying histologic components..”
Tuberous Sclerosis: Current Update
Wang MX et al.
RadioGraphics 2021; 41:1992–2010
- OBJECTIVE. The purpose of this article is to describe useful imaging features for differentiating angiomyolipoma (AML) subtypes from renal cell carcinoma subtypes.
CONCLUSION. A newer radiologic classification of renal AML consists of fat-rich AML (≤ –10 HU), fat-poor AML (> –10 HU; tumor-to-spleen ratio < 0.71; signal intensity index, > 16.5%), and fat-invisible AML (> –10 HU; tumor-to-spleen ratio, > 0.71; signal intensity index, < 16.5%). Each subtype must be differentiated from the renal cell carcinoma subtype because of overlapping imaging features.
Renal Angiomyolipoma Based on New Classification: How to Differentiate It From Renal Cell Carcinoma
Byung Kwan Park
AJR 2019; 212:582–588 - “Song et al. [1] reported on a new radiologic classification of renal angiomyolipoma (AML) in which they classified the tumor into three subtypes: fat-rich AML, fat-poor AML, and fat-invisible AML. Each AML subtype is defined according to the amount of fat, which is quantified with CT or MRI. Fat-rich AML measures –10 HU or less at CT. Both fat-poor AML and fat-invisible AML measure more than –10 HU.”
Renal Angiomyolipoma Based on New Classification: How to Differentiate It From Renal Cell Carcinoma
Byung Kwan Park
AJR 2019; 212:582–588 - Fat-rich AML is defined as a lesion mea- suring –10 HU or less on unenhanced CT images .This subtype constitutes approximately 95% of renal AMLs. Almost all fat-rich AMLs are easily diagnosed with unenhanced CT alone because fat, a hallmark of AML, is clearly visualized.”
Renal Angiomyolipoma Based on New Classification: How to Differentiate It From Renal Cell Carcinoma
Byung Kwan Park
AJR 2019; 212:582–588 - “Approximately 5% of renal AMLs histo- logically have a small amount of fat and correspond to fat-poor AML or fat-invisible AML in the Song classification. Among small (< 4 cm) renal tumors, 4–13% are histologically confirmed as AML because imaging shows too little fat. Many authors have shown that AML with a small amount of fat is likely to have female predominance, small size, and a more homogeneous texture than RCC”
Renal Angiomyolipoma Based on New Classification: How to Differentiate It From Renal Cell Carcinoma
Byung Kwan Park
AJR 2019; 212:582–588 - Fat-rich AML should be differentiated from fat-containing RCC if this situation is rarely encountered. A small amount of fat relative to a large tumor, calcification, or necrosis is more suggestive of RCC than of AML. In exceptional cases, biopsy is recommended if these findings are detected.”
Renal Angiomyolipoma Based on New Classification: How to Differentiate It From Renal Cell Carcinoma
Byung Kwan Park
AJR 2019; 212:582–588 - “Fat-invisible AML should be differentiated from non–clear cell RCC. Both tumors tend to be hyperattenuating on unenhanced CT images, negative on chemical shift MR images, hypointense on T2- or fat- suppressed T2-weighted MR images, hyper- intense on DW images, and hypointense on ADC maps. Therefore, biopsy is necessary to differentiate these tumors.”
Renal Angiomyolipoma Based on New Classification: How to Differentiate It From Renal Cell Carcinoma
Byung Kwan Park
AJR 2019; 212:582–588 - CT or MRI features can provide clues for differentiating fat-rich AML from fat-con- taining RCC and fat-poor AML from clear cell RCC. Fat-invisible AML, however, is difficult to differentiate from non–clear cell RCC on the basis of CT or MRI features. Radiologists should not be reluctant to perform biopsy when lesion differentiation is not clear at CT or MRI.
Renal Angiomyolipoma Based on New Classification: How to Differentiate It From Renal Cell Carcinoma
Byung Kwan Park
AJR 2019; 212:582–588
- “Patients with lymphangioleiomyomatosis (LAM) have a frequency of angiomyolipoma (AML) that varies from 20% to 54% depend-
ing on the method of patient or case collec- tion, imaging modality used, diagnostic criteria, and statistics of sampling variation. This frequency of AML is much higher than the reported frequency of 1–3% in the general population.”
Active Surveillance of Nonfatty Renal Masses in Patients With Lymphangioleiomyomatosis: Use of CT Features and Patterns of Growth to Differentiate Angiomyolipoma From Renal Cancer Nilo A. Avila et al. AJR 2017; 209:611–619 - “AMLs with no visible fat that are hyperattenuating relative to the normal renal parenchyma on unenhanced CT have been called “AML with minimal fat,” and AMLs that are hypoattenuating relative to the normal renal parenchyma but with attenuation values not in the range of fatty tissue on unenhanced CT have been termed “AML with diffusely scattered fat”. In this article, we use the term “nonfatty renal mass” to denote any renal mass with no visible fat on CT and the term “nonfatty AML” to denote an AML with no visible fat on CT.”
Active Surveillance of Nonfatty Renal Masses in Patients With Lymphangioleiomyomatosis: Use of CT Features and Patterns of Growth to Differentiate Angiomyolipoma From Renal Cancer Nilo A. Avila et al. AJR 2017; 209:611–619 - “Renal AML can be classified according to amount of fat as fat rich, fat poor, or fat invisible. To detect fat, one needs to thoroughly evaluate the entire AML by controlling the size and shape of the ROI. Fat-invisible AML should be biopsied, and fat-poor AML requires further investigation to determine whether biopsy is necessary to differentiate it from renal cell carcinoma. If differentiation between AML and renal cell carcinoma is not clear with CT and MRI, percutaneous biopsy may be performed.”
Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat Byung Kwan Park AJR 2017; 209:826–835 - “Angiomyolipoma (AML) is the most common benign solid renal tumor. Most AMLs contain fat that is clearly visible on CT and MR images, so these tumors can be easily diagnosed without biopsy or surgery. Approximately 5% of renal AMLs, however, have too little fat to be identified in a CT or MRI examination. Preoperatively, these AMLs are difficult to differentiate from renal cell carcinoma (RCC) with radiologic examinations, and they frequently are diagnosed after surgery.”
Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat Byung Kwan Park AJR 2017; 209:826–835 
Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat Byung Kwan Park AJR 2017; 209:826–835-
“Unenhanced CT clearly depicts a hypoattenuating area (≤ –10 HU) suggesting fat in fat-rich AML. There- fore, detecting fat is not a problem in most fat-rich AMLs. However, some fat-rich AMLs have very small foci of fat measuring less than –10 HU, so these hypoattenuating areas may not be recognized at preoperative CT. Therefore, meticulous care should be taken not to miss a small focus of fat. Thin (< 5 mm) slice thickness (1.5–3 mm) should be used because thick (≥ 5 mm) slice thickness may not depict fat attenuation.”
Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat Byung Kwan Park AJR 2017; 209:826–835 -
“Unenhanced CT cannot show fat attenuation in fat-invisible AMLs . These lesions appear homogeneously hyperattenuating because they have too little fat . The attenuation of fat-invisible AML is higher than that of fat-poor AML because the amount of fat in the former is lower than that in the latter. For this reason, the attenuation values of fat-invisible AML are fairly constant compared with those of fat- poor AML wherever an ROI is placed.”
Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat Byung Kwan Park AJR 2017; 209:826–835
- “Contrast-enhanced CT is not necessary for diagnosing fat-rich AML but should be performed when there is potential for tumor bleeding. Frequently, large fat-rich AMLs contain a lot of tortuous or dilated vessels, which are susceptible to bleeding. Contrast-enhanced CT is essential to identify these abnormal vessels before embolization.”
Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat Byung Kwan Park AJR 2017; 209:826–835 -
“Contrast-enhanced CT frequently shows heterogeneous enhancement within fat-poor AMLs because a small amount of fat is localized or scattered. Therefore, this type of AML may be misdiagnosed as clear cell RCC, which is heterogeneously enhancing on contrast-enhanced CT images. Still, it is unclear that fat-poor AML requires MRI or percutaneous biopsy. By definition, MRI is necessary to identify fat-poor AML. However, clear cell RCCs may have similar MRI features to those of these AML.”
Renal Angiomyolipoma: Radiologic Classification and Imaging Features According to the Amount of Fat Byung Kwan Park AJR 2017; 209:826–835
