Renal angiomyolipoma (renal AML) is a rare benign mesenchymal neoplasm of the kidney that is a member of the perivascular epithelioid cell tumors (PEComa) family. It often affects middle-aged females and usually arises sporadically in renal parenchyma and grows slowly. Tuberous Sclerosis Complex (TSC) is an inherited disorder that is related to the formation of renal AML as multifocal and bilateral tumors (2-5).
Fine Needle Aspiration (FNA) serves as a simple and cost-effective technique to be used as an initial diagnostic tool for the evaluation of mass lesions. Samples from the FNA of classic renal AML show moderate to high cellularity. The most common identified cells are spindle-shaped cells with or without epithelioid components. These cells are round to polygonal in shape with indistinct cell borders. The cytoplasm is fibrillary or finely vacuolated. The other two components are adipocytes and elements of abnormal vessels. The proportion of these three components may differ (2,6).
Based on the WHO classification, renal epithelioid angiomyolipoma (renal eAML) is composed of at least 80% predominantly round to polygonal epithelioid cells (1). Epithelioid cells may exhibit variable degrees of nuclear atypia including pleomorphism and enlarged sizes with prominent nucleoli. Occasional mitosis and necrosis are observed. Bare nuclei, intranuclear inclusions, and multinucleated giant cells are present in some cases (2). This subset of renal eAML is associated with aggressive behavior (recurrence and/or metastasis) that has shown to be more likely in the presence of severe nuclear atypia and higher rates of necrosis (2,6). The most common extrarenal location is the liver (2).
Renal AMLs (classic and epithelioid) are positive for most of the melanocytic (HMB45, microphthalmia-associated transcription factor; MITF, and Melan-A) and smooth muscle markers (SMA and less commonly Desmin). On the other hand, they do not express S100 and Cytokeratin (2,4-6). Moreover, Cathepsin K has been newly introduced as a diagnostic marker for both of the renal AML types. The staining pattern is helpful in distinguishing renal eAML from other neoplasms in the differential (2).