Answer: Angiomyolipoma

Histology: The lesion consists of alternating hyaline bundles of collagen. In between the collagen bundles are rounded and slightly spindled cells with vacuolated lightly eosinophilic cytoplasm. No adipose tissue is noted.

Discussion: The key to recognizing that this is an angiomyolipoma (AML) is first the keloidal collagen, which in the kidney is most commonly seen in AML. Second the cells in between the collagen are typical of AML consisting lightly eosinophilic cytoplasm which is either vacuolated or contains thin filaments that I often refer to as “spider web-like”. Currently, most AMLs that are resected do not contain a significant amount of adipose tissue, as the typical AML can be diagnosed radiologically and not resected. In the current case, the unique few vessels of AML with the modified smooth muscle cells spinning off the outer portion of the vessel wall is also not readily apparent. In this case given the lack of the adipose and vascular components it is reasonable to do immunohistochemistry for HMB45 and MelanA which were positive. An alternative diagnosis for this case would have been PEComa, given the absence of fat and abnormal vessels. However, in the kidney the convention is to call these lesions fat-poor AMLs, a term that urologists would recognize whereas they would be unfamiliar with the term PEComa. A solitary fibrous tumor (SFT) could have this pattern, but CD34 was negative along with the positive HMB45 and MelanA, ruling out an SFT. One should be aware that HMB45 and MelanA may be very focal in AMLs. AMLs are typically benign where extra-renal extension and involvement of perirenal lymph node are not evidence of malignant behavior. There are rare malignant AMLs which are epithelioid with marked pleomorphism, necrosis, increased mitoses, and atypical mitoses, features lacking in the current case.