Accepted on 28 Aug 2022            Submitted on 12 Jun 2022

A 61-year-old female was admitted to the emergency department for epigastric pain following epigastric hernia surgery. An abdominal unenhanced computed tomography (CT) demonstrated a reoccurrence of epigastric eventration. As an incidental finding, two round well-defined lesions of fat density (–78 HU) containing tortuous foci of enhancing vessels features strongly suggestive of renal AMLs (asterisk in Figure 1). Both AMLs showed a fourfold increase in size to prior imaging 10 years ago; the right posterior cortical AML measured 6.6 cm and left anterior hilar AML measured 3.2 cm (arrowheads in Figure 2).

Figure 1 

Figure 2 

The lipomatous component unexpectedly extended into the left renal vein, forming one 4.5 cm fat density intravascular flap (arrows on Figures 1 and 3A) and a second, smaller, 2 cm intravascular fat density lesion located more proximally, in a tributary vein (arrowheads in Figures 1 and 3). A contrast-enhanced CT outlined the intravenous extension more clearly, showed no extension into the inferior vena cava, and excluded pulmonary embolism (PE).

Figure 3 

Total left nephrectomy and right partial nephrectomy were planned. Histologic examination confirmed the diagnosis of AML without any evidence of sarcomatoid dedifferentiation.

AML is a common asymptomatic benign renal neoplasm mostly detected incidentally during abdominal cross-sectional imaging. It is composed of thick-walled blood vessels, smooth muscle, and mature adipose tissue. In 20% of cases there will be an association with tuberous sclerosis manifesting as multifocal and bilateral AMLs. A common symptomatic presentation occurring with large AMLs (>4 cm) and leading to surgical treatment or embolization is spontaneous retroperitoneal hemorrhage sometimes complicated by hypotensive shock.

A rare case of AML showing liposarcomatous transformation has also been reported. Aggressive patterns with intravascular growth is a rare condition only described in a couple of case reports [1]. Mostly the extension occurs in the renal veins and inferior vena cava (IVC). Extension of a lipomatous thrombus in a renal vein tributary, as in our case, has not yet been described. The increased risk for PE emphasizes the need for surgery and sometimes for IVC filter placement [2].

Even a benign, and relatively frequently encountered, AML can rarely exhibit aggressive behavior, with intravascular potentially life-threatening fat extension. The radiologist should pay attention to this and attentively review the renal veins and IVC for a fat density component that suggests intravascular extension or embolization [3]. Therapeutic options, that is, potentially lifesaving surgery must be discussed with the patient.

The authors have no competing interests to declare.

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