Angiomyolipoma is a usually benign mesenchymal neoplasm composed of smooth muscle, blood vessels, and adipose tissue in varying amounts (Morita et al. 2012). It occurs predominantly in the kidney but can rarely occur in extrarenal locations including the liver, lungs, and the mediastinum (Candaș et al. 2013). Though they may be sporadic in origin, angiomyolipomas are associated with tuberous sclerosis and TSC2/PKD1 contiguous gene syndrome (Martignoni et al. 2002). Patients with a genetic syndrome tend to present at a younger age than patients without a genetic mutation. Angiomyolipomas belong to the perivascular epithelioid cell tumor (PEComa) family, which means they have perivascular and epithelioid features and can coexpress melanocytic and muscle markers. The PEComa family includes angiomyolipoma (AML), clear-cell sugar tumor (CCST) of the lung, lymphangioleiomyomatosis (LAM), clear-cell myomelanocytic tumor of the falciform ligament/ligamentum teres, and other rare clear-cell tumors (Martignoni et al. 2008).
Clinically, angiomyolipomas usually behave benignly. However, the epithelioid variant has risk of malignant behavior and thus carries a worse prognosis (Park et al. 2016). They also carry a risk of hemorrhage, risk of invasion of adjacent organs, and risk of metastasis. Patients with mediastinal angiomyolipomas may present with chest pain and/or shortness of breath (Liang et al. 2015). A characteristic feature on imaging is the presence of macroscopic adipose tissue. CT-guided percutaneous transthoracic needle biopsy may be performed prior to definitive treatment. The definitive treatment of angiomyolipomas is surgical excision.
On gross examination, angiomyolipomas are nonencapsulated, well- circumscribed, and may have a pushing border. Capsular invasion is present in approximately 25% of cases. On sectioning, angiomyolipomas may have a predominantly red, yellow, or white cut surface depending on proportion of vascular tissue, adipose tissue, and smooth muscle in the tumor. Sporadic cases are usually unifocal and unilateral. A genetic syndrome should be considered if a patient presents with multifocal or bilateral disease. Though they may invade local lymph nodes, this is not considered a sign of malignancy (Llarena Ibarguren et al. 1991).
Differential diagnosis includes thymoma especially if the thymoma has a spindled cell morphology and solitary fibrous tumor among other entities. These however will not be positive for melanocytic markers.