COW-2019-25

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Epithelioid angiomyolipoma (AML) is composed of plump and epithelioid cells arranged in diffuse sheets. The neoplastic cells are often monomorphic and typically show clear to granular cytoplasm. Scattered multinucleated cells can also be found as well as focal coagulative necrosis. On cytology, two types of cells may be seen: (1) clear cells with finely granular cytoplasm and small monomorphic nuclei and (2) eosinophilic cells with abundant cytoplasm, epithelioid morphology, and large nuclei with prominent nucleoli. Focal areas of more spindled cells, fatty cells and thick-walled vessels are also often present. The neoplastic cells stain diffusely for Cathepsin K, and focally for melanocytic markers, such as HMB-45, melan A and MiTF, as well as variably for smooth muscle markers (e.g. smooth muscle and muscle-specific actin).
As shown in this case, epithelioid AML often demonstrates a solid and/or diffuse growth, and distinct epithelioid cells with clear cytoplasm. This may cause confusion with renal cell carcinoma (for e.g. clear cell or unclassified type). Histologic features most helpful in differentiating epithelioid AML from clear cell renal cell carcinoma include the presence of areas of more typical AML, with adiposities and larger vessels, and the positivity for Cathepsin K, HMB45, melan A, as well as negativity for epithelial markers and PAX8.
According to the 2016 WHO Classification of Renal Neoplasms, epithelioid AML is defined as AML with at least 80% epithelioid cells. Rare epithelioid AML may demonstrate malignant behavior, particularly when the following features are present (according to some studies): pure epithelioid morphology, large size (>7 cm), mitotic count ≥ 2 per 10 high-power fields), nuclear atypia, atypical mitotic figures, necrosis, extrarenal extension, vascular and renal vein invasion.

Brimo F, Robinson B, Guo C, et al. Renal epithelioid angiomyolipoma with atypia: a series of 40 cases with emphasis on clinicopathologic prognostic indicators of malignancy.
Am J Surg Pathol 2010; 34(5):715-722.

Nese N, Martignoni G, Fletcher CD, et al. Pure epithelioid PEComas (so-called epithelioid angiomyolipoma) of the kidney: a clinicopathologic study of 41cases: detailed assessment of morphology and risk stratification.
Am J Surg Pathol 2011;35(2):161-176.

He W, Cheville JC, Sadow PM et al. Epithelioid angiomyolipoma of the kidney: pathological features and clinical outcome in a series of consecutively resected tumors.
Mod Pathol 2013;26(10):1355–1364.

Amir Akbari
amir.ubc@gmail.com
Kiril Trpkov
kiril.trpkov@albertapubliclabs.ca
University of Calgary, Calgary, Alberta, Canada

Kidney

AML, angiomyolipoma, epithelioid AML