A renal angiomyolipoma (AML) is a non-cancerous growth primarily found in the kidney. This common benign kidney tumor is composed of fat cells, smooth muscle, and blood vessels. These growths can vary significantly in size, from a few millimeters to over 20 centimeters.
What is Renal Angiomyolipoma
Renal angiomyolipomas are benign tumors, meaning they do not spread to other parts of the body. While well-defined, they can grow significantly and may lead to complications, such as a mass effect or spontaneous bleeding due to abnormal blood vessels within the tumor.
There are two main categories of renal AMLs: sporadic and those associated with Tuberous Sclerosis Complex (TSC). The majority of cases (80% to 90%) occur sporadically, appearing in individuals without an underlying genetic condition. The remaining 10% to 20% of cases are linked to genetic conditions, primarily TSC. TSC is an inherited disorder that causes non-cancerous tumors to grow in various organs, including the brain, skin, heart, lungs, and kidneys.
Individuals with TSC often develop multiple and bilateral angiomyolipomas, affecting both kidneys. This occurs due to mutations in the TSC1 or TSC2 genes, which produce proteins regulating cell growth and size. When these genes mutate, cell growth can become uncontrolled, leading to tumor formation. The prevalence of angiomyolipoma is estimated to be between 0.3% and 3% worldwide.
How Renal AML is Discovered
Renal angiomyolipomas are frequently discovered incidentally during imaging tests performed for other health concerns. This is because many individuals with AMLs do not experience symptoms. When symptoms do arise, they are often associated with larger tumors or complications like bleeding.
Potential symptoms include flank pain (discomfort in the side or back) and hematuria (blood in the urine). In less common instances, particularly if bleeding occurs, symptoms might include high blood pressure or signs of anemia. The most concerning symptom is spontaneous retroperitoneal hemorrhage, a serious and potentially life-threatening bleeding into the abdominal space, which is more common with tumors larger than 4 cm.
Diagnostic imaging plays a central role in identifying renal angiomyolipomas. Ultrasound, CT scans, and MRIs are commonly used to confirm the diagnosis. These techniques detect the characteristic fat content within the tumor, helping distinguish it from other kidney masses. While imaging is usually sufficient, a kidney biopsy might be considered in complex cases where differentiating AML from other kidney tumors, such as renal cell carcinoma, is difficult.
Managing Renal Angiomyolipoma
The management of renal angiomyolipoma depends on several factors, including the tumor’s size, the presence of symptoms, and its growth rate. Treatment plans are individualized to each patient’s situation.
For small, asymptomatic AMLs, watchful waiting (active surveillance) is a common approach. This involves regular imaging follow-ups, typically with ultrasound, CT, or MRI, to monitor the tumor for changes in size or symptom development. This strategy aims to avoid unnecessary interventions while ensuring any progression is detected early.
When AMLs cause symptoms or are at high risk of bleeding, interventional procedures are considered. Embolization is a minimally invasive procedure often recommended for bleeding AMLs or those over 4 cm. This technique blocks the tumor’s blood supply, which can reduce its size and mitigate hemorrhage risk.
Surgical removal is another option, particularly for very large tumors, complex cases, or when other treatments are not effective. Partial nephrectomy, which removes only the tumor while preserving healthy kidney tissue, is often preferred to maintain kidney function. In rare instances, a total nephrectomy (removal of the entire kidney) may be necessary.
For larger AMLs, especially those associated with Tuberous Sclerosis Complex, drug therapy with mTOR inhibitors like sirolimus or everolimus can be effective. These medications target a specific cellular pathway to shrink tumors and reduce bleeding risk. mTOR inhibitors are increasingly considered a primary treatment for TSC-associated AMLs, aiming to reduce tumor burden and preserve kidney function.