Renal angiomyolipoma (AML) is one of the most common benign kidney tumors, composed of varying proportions of fat, smooth muscle, and blood vessels. While typically noninvasive, some cases exhibit local invasion, extending into the perirenal fat or, in rare instances, the collecting system, renal vein, inferior vena cava, and even the right atrium. Here, we present a case of sporadic renal AML with renal vein invasion.

A 55-year-old woman with no prior history of hypertension or diabetes presented with a painful left inguinal mass with whitish discharge for three weeks. Suspecting an inguinal abscess or malignancy, she was admitted for further evaluation. Surgical debridement of the abscess was performed smoothly. A preoperative abdominal CT scan incidentally identified a large, lipid-rich right renal tumor (8.7 × 6.2 cm), consistent with AML, showing renal vein invasion (Novick Classification Level 1) (Figure 1,2). Given her obesity (BMI: 33.9 kg/m²), endocrine assessment led to a new diagnosis of type 2 diabetes, gout, and hypertriglyceridemia. Despite the risk of rupture and embolism, the patient initially opted for sirolimus 1 mg every other day, postponing surgery. She experienced occasional watery diarrhea and heartburn, but the side effects were tolerable. After three months, a follow-up CT scan showed tumor shrinkage and a mild reduction in enhancement (Figure 3,4). Renal function remained normal, but considering the IVC thrombosis and its potential progression to pulmonary embolism, nephrectomy and thrombus extraction were recommended. However, she remained hesitant and opted for close monitoring with regular CT scans.

The majority of AMLs (>80%) are sporadic, with the remainder mostly associated with tuberous sclerosis complex (TSC). Rapidly growing epithelioid AML has been reported to undergo malignant transformation, while benign renal AML may also exhibit invasive behavior without malignant features. mTOR inhibitors, including sirolimus and everolimus, are guideline-endorsed treatments for TSC-related AML. In 85% of TSC patients, mutations in TSC1/TSC2 lead to hyperactive mTOR signaling, dysregulating cellular processes. Consequently, sirolimus and everolimus directly inhibit TSC-AML growth via the mTOR pathway. However, the molecular characteristics of sporadic AML remain unclear. Western blot analysis has confirmed the loss of TSC2, but not TSC1, in sporadic AML, raising uncertainty about mTOR inhibitor efficacy. Nonetheless, a retrospective study found that 44.5% of patients achieved ≥25% tumor reduction, and 18.5% had ≥50% shrinkage after everolimus treatment. Furthermore, a Phase II trial on everolimus in sporadic AML reported tumor shrinkage >25% in 58% of patients. However, 40% of participants discontinued treatment due to side effects, raising concerns about tolerability. Given its similar mechanism, sirolimus was expected to show comparable efficacy.

This case highlights a large AML with renal vein invasion, demonstrating the potential efficacy of sirolimus in sporadic AML. While guidelines remain undefined, sirolimus may offer a non-invasive alternative to surgery or transcatheter arterial embolization. However, tolerability concerns must be considered, along with the risk of pulmonary vein embolism due to detachment of necrotic tissue from cytoreduction. Further research and discussion are needed to refine treatment strategies for sporadic AML.