Renal angiomyolipoma

CT and MRI presentation of typical angiomyolipoma

Classical angiomyolipoma is visualized as a large mass in the renal parenchyma (may involve the renal pelvis), consisting predominantly of adipose tissue. Key radiological signs are based on the detection of macroscopic fat:

• Unenhanced CT (non-contrast): a well-defined mass is visualized within the parenchyma (e.g., medially in the renal pole).
• MRI in-phase (in-phase): the tumor demonstrates high signal intensity compared to normal renal tissue.
• MRI out-of-phase (out-of-phase): a characteristic loss of signal intensity is observed precisely at the boundary between the internal fat of the neoplasm and the surrounding parenchyma. This is a key sign indicating the presence of a fat component.

Multiple lesions and tuberous sclerosis

In addition to solitary tumors, multiple angiomyolipomas occur, which are closely associated with tuberous sclerosis (tuberous sclerosis).

On contrast-enhanced CT (in coronal reconstructions), they are defined as multiple bilateral low-density lesions corresponding to the density of pure fat (pure fat). When evaluating very small foci (e.g., about 3 mm in size) on out-of-phase MRI, an important feature is observed: the "edge" artifact (second-order chemical shift) merges. As a result, a complete signal drop-out (complete drop-out) occurs on the tomogram, reliably confirming the fatty nature of even the smallest nodules.

Atypical form and risk of complications

Atypical angiomyolipomas: Sometimes, against the background of multiple classical fat-containing components, a larger solid mass with minimal fat content can be identified—the so-called lipid-poor angiomyolipoma (lipid-poor). The clinical significance of this radiological picture is immense: it requires strict differential diagnosis, as the solid mass may turn out to be early-stage renal cell carcinoma (renal cell carcinoma).

Complications and management: The main threat of angiomyolipoma is its pronounced tendency to bleed. If hemorrhage occurs, CT reveals an extensive high-density perirenal collection corresponding to blood. When bleeding develops, emergency vascular embolization is indicated to stop it immediately upon patient admission.

Differential diagnosis with other masses

When solid or cystic alterations are detected in the kidney, angiomyolipoma must be differentiated from several other pathologies:

• Oncocytoma (oncocytoma): a tubular adenoma that can reach large sizes (from 1 to 20 cm). Although previously considered benign, oncocytomas are now proven to be capable of metastasis. On ultrasound, this is a solid mass with variable echogenicity and often a stellate hypoechoic center. On contrast-enhanced CT, it is a well-circumscribed solid mass, sometimes with a low-density central scar. They can extend into and engulf the paranephric fat, making them difficult to differentiate from angiomyolipoma. An accurate diagnosis requires a biopsy, and treatment involves partial nephrectomy (partial nephrectomy) to preserve function.
• Hemangioma (haemangioma): a rare cavernous-type vascular tumor. Manifests with hematuria in middle age. On contrast CT, it rapidly enhances and merges with vessels. Pyelography may show deformation of the pelvicalyceal system or a filling defect (due to the tumor itself or a blood clot).
• Multilocular cystic nephroma (multilocular cystic nephroma): a rare benign tumor occurring in young children and adult women. On CT, it appears as a unilateral encapsulated mass with enhancing septa. The cystic part has water density and does not enhance. A key sign is herniation (herniation) into the renal sinus.

Malignant parenchymal tumors

The differential diagnosis always includes renal cell carcinoma (renal cell carcinoma). Most cases occur sporadically in the 5th–7th decades of life. However, there is now an increasing detection rate of cancer in young patients, which is often associated with hereditary cancer syndromes (about 10 are known).

The most significant of these is von Hippel-Lindau disease (VHL disease). This genetic syndrome is associated with the development of renal cysts, clear cell renal cell carcinoma, and a wide range of tumors in other locations.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• MRI artifacts: second-order chemical shift ("edge" artifact) and its clinical significance
• Von Hippel-Lindau (VHL) disease and associated renal pathologies
• Management tactics and indications for partial nephrectomy in oncocytoma
• Multilocular cystic nephroma: mechanisms of herniation formation
• Radiological signs of early renal cell carcinoma

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

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