Answer: Angiomyolipoma (fat predominant)

Histology: The lesion consists of an adipose-rich lesion with dense collagen bands. There are scattered atypical adipocytes resembling lipoblasts within the fatty component, as well as atypical hyperchromatic cells within the collagen bands. There are scattered small to medium sized vessels with focally thickened walls. Radiographic impression of this lesion was of a well circumscribed, 9 cm mass at the upper pole of the kidney. Immunostains show the lesional cells to be positive for HMB45 and calponin.

Discussion: This case is intended to review in conjuction with Case 2 from this week. Angiomyolipomas (AMLs) are lesions in the PEComa (perivascular epithelioid cell) family of tumors, which consists of clear cell sugar tumors and pulmonary lymphangioleiomyomatosis, and they may occur sporadically or in association with tuberous sclerosis. AMLs are comprised of “angio” – “myo” – and “lipo” components in varying proportions in each tumor. They contain thick walled blood vessels (“angio”); a spindled to epithelioid cellular component with smooth muscle differentiation (“myo”) that typically spin off from the vessel walls; and a fatty (“lipo”) component of mature adipose tissue. AMLs frequently arise at the periphery of the kidneys and can be confidently identified radiographically if they contain adequate amounts of fat, due to characteristic imaging features. However, the AMLs which lack a significant adipose component (i.e., fat-poor AMLs) may mimic renal cell carcinomas on imaging and thus are often excised.

By immunohistochemistry, AMLs are classically positive for HMB45 and smooth muscle markers such as actin or calponin. The differential diagnosis of an AML varies depending upon whether the AML is fat-poor or fat-predominant, as well as on the degree of cytologic atypia. Fat-poor AMLs may be confused histologically with smooth muscle neoplasms such as leiomyomas or leiomyosarcomas. Fat-predominant AMLs, such as the one in this case, may mimic liposarcomas due to the presence of atypical cells within the fat and collagen bands. This is especially treacherous in that liposarcomas are common soft tissue tumors in the retroperitoneum. Recognizing epithelioid to spindled cells centered around the vessels aids in the diagnosis of an AML. However, it’s important to note that thick walled vessels are not always present in the adipose-rich AMLs, and positive labeling for HMB45 and calponin or actin may be necessary to avoid the pitfall of misdiagnosing a liposarcoma.

Reference(s):
– Hruban RH, Bhagavan BS, Epstein JI. Massive retroperitoneal angiomyolipoma. A lesion that may be confused with well-differentiated liposarcoma. Am J Clin Pathol. 1989 Dec;92(6):805-8.
– L’Hostis H, Deminiere C, Ferriere JM, Coindre JM. Renal angiomyolipoma: a clinicopathologic, immunohistochemical, and follow-up study of 46 cases. Am J Surg Pathol. 1999 Sep;23(9):1011-20.
– Stone CH, Lee MW, Amin MB, Yaziji H, Gown AM, Ro JY, Têtu B, Paraf F, Zarbo RJ. Renal angiomyolipoma: further immunophenotypic characterization of an expanding morphologic spectrum. Arch Pathol Lab Med. 2001 Jun;125(6):751-8.