Answer: Angiomyolipoma (AML)

Histology: see below

Discussion: Fat-rich angiomyolipoma are more easily diagnosed as AML on imaging studies and are therefore only rarely resected. Fat-poor examples, like the current case, are more difficult to diagnose especially at the time of frozen section, a point to remember when examining a frozen section from “tumor bed” margin during partial nephrectomy.

A member of the perivascular epithelioid cell (PEComa) group of neoplasms, renal angiomyolipomas (AML) represent 1% of renal masses. One third of AML are associated with tuberous sclerosis (TSC) syndrome. When present, the characteristic histologic triad of thick walled vessels + mature fat + smooth muscle fibers make for an easy diagnosis. However, the occasional presence of focal cytologic pleomorphism especially when coupled with hemorrhage and ensuing necrosis can lead to overdiagnosis as a sarcomatoid renal cell carcinoma. Immunohistochemical studies for HMB-45 (+), MelanA (+), Micro-ophthalmia TF(+) are helpful. Extrarenal extension, renal vein or lymph node involvement by an otherwise classic AML does not imply a malignant behavior. The risk for spontaneous hemorrhage is greater in AML lesions that are larger than 4cm in diameter and during pregnancy. Rare “Epithelioid” AML lesions with cytologic anaplasia, mitosis, necrosis, may have a malignant behavior.

An ongoing trial targeting the mTOR pathway with Rapamycin agent is underway for AML lesions in the setting of TSC syndrome given the implicated role of TSC1/TSC2 loss in the dysregulation of the above pathway.

Reference(s):
– Martignoni G, Pea M, Reghellin D, Zamboni G, Bonetti F. PEComas: the past, the present and the future. Virchows Arch. 2008 Feb;452(2):119-32.