Diagnosis: Hematoma secondary to a bleeding renal angiomyolipoma

Coronal (left image), axial (middle image), and sagittal (right image) contrast-enhanced CT images demonstrate retroperitoneal hemorrhage surrounding an exophytic, fat-containing, hypervascular right renal mass (yellow arrows). Prominent intratumoral vessels are noted within the lesion (red arrows). Despite predominantly exophytic location, the lesion definitively arises from the renal cortex in the posterior interpolar region, as indicated by the cortical defect (blue arrow).

Discussion

Angiomyolipoma and risk of hemorrhage

As the name implies, angiomyolipoma (AML) is a benign renal tumor comprised of vascular, smooth muscle, and fatty elements.
Isolated or sporadic AMLs account for the vast majority of cases, in which tumors are typically unilateral and occur in middle-aged women. Up to 80% of patients with tuberous sclerosis have multiple, bilateral AMLs.
Although they are usually an incidental finding, AMLs may present in the acute setting with hemorrhage, especially if they grow larger than 4 cm. It is thought that abnormal intratumoral vessels are prone to microaneurysm formation and, hence, hemorrhage.
Clinical presentation of a bleeding AML includes hypotension, decreasing hematocrit, flank pain, and/or hematuria.

Imaging of AML

The diagnosis of AML relies on identification of intratumoral macroscopic (gross) fat, which is characterized by negative density on CT (typically <-20 HU), or intratumoral fat signal (saturation on fat-suppressed images) on MRI. Overwhelmingly, AML is a diagnosis made by radiologists.
Imaging diagnosis may be difficult in lipid-poor AMLs (4% of all AMLs). The finding of a small, hyperdense renal mass on CT should raise the suspicion of lipid-poor AML, particularly in a young female patient. Biopsy can be used to make the diagnosis and avoid unnecessary surgery.
On MR imaging, an AML is seen as a mass that contains focal macroscopic fat signal. Intra-lesional fat signal will follow retroperitoneal or mesenteric fat on all pulse sequences, with the fat-suppressed sequences most helpful. Note that in- and out-of-phase imaging, which is used to evaluate for the presence of water and fat in the same voxel, is not helpful to detect macroscopic fat.
In extremely rare instances, renal cell carcinomas (RCCs) can contain macroscopic intratumoral fat, which may be secondary to lipid-producing necrosis within a large RCC, intratumoral bone metaplasia with fatty marrow elements (usually associated with calcification), or entrapment of perirenal or sinus fat. Unlike a rare fat-containing RCC, AML should not have any calcification.

Clinical synopsis

This patient was admitted and treated with aggressive fluid resuscitation. After CT diagnosis, she was referred to interventional radiology, where she had an uneventful endovascular embolization. Following the procedure she was asymptomatic and hemodynamically stable, and she was discharged the following day.

Self-assessment

Name two factors that predispose to hemorrhage in renal AMLs.	Large size (typically larger than 4 cm in diameter) Prominent microaneurysms Presence of symptoms (which may reflect interval growth)
What are the diagnostic criteria for renal AML?	An AML is diagnosed by the presence of macroscopic fat. A renal mass containing fat (<-20 HU) on CT is considered virtually pathognomonic for AML. Ultrasound is not reliable for the diagnosis of macroscopic fat, as the finding of a hyperechoic renal mass is nonspecific. A small RCC may also present as a hyperechoic mass; however, a small hyperechoic renal mass is much more likely to represent an AML. On MRI, the mass should contain internal signal that is similar to the surrounding retroperitoneal fat. Fat-suppression techniques are particularly useful. Note that in- and out-of-phase imaging is not helpful, as this imaging technique detects intracytoplasmic lipid (e.g., to diagnose an adrenal lipoma or hepatic steatosis) rather than macroscopic fat.
What is the differential diagnosis for macroscopic fat-containing renal tumors?	Angiomyolipoma Lipoma (unusual and does not differ in appearance from AML with predominant adipose component) Retroperitoneal liposarcoma (typically lacks the renal cortical defect or indentation in the renal parenchyma seen in exophytic AMLs, and does not feature enlarged vessels arising from the kidney) Wilms tumor (rare) Renal cell carcinoma (very rare)