Answer: B

Histological description: Grossly, the tumor was well-circumscribed with minimally infiltrative borders. The tumor has a uniform appearance of bland epithelioid to slightly spindled cells with nuclei having delicate chromatin. The cytoplasm was lightly eosinophilic and in areas had a filamentous appearance. Focally, the stroma had thicker eosinophilic collagen bundles. The lesion lacked necrosis, increased mitoses, and atypical mitoses. In areas, epithelioid and spindle cells radiated off of vessel walls. The tumor lacked an adipose component.

Discussion: Angiomylipoma (AML) in the kidney are more often sporadic but can be seen in the setting of Tuberous Sclerosis where presentation is in younger individuals and the lesions are multifocal and bilateral. In contrast to leiomyomas that arise from vessels in the renal hilum, AMLs typically arise from the cortex and can extend out of the kidney or even be predominantly or exclusively in the perirenal fat. AMLs are most often spindled with smooth muscle cells having identical nuclei to low grade leiomyosarcomas, yet AMLs have cytoplasm that is looser and filamentous. The smooth muscle component in AML can be fascicular and indistinguishable from a smooth muscle tumor, and in other areas more haphazard. AMLs, including epithelioid cases that lack atypical features, are benign although they can involve lymph nodes. Some AMLs are associated with significant morbidity and even mortality due to massive retroperitoneal hemorrhage. There do exist atypical epithelioid AMLs where epithelioid cells have overall high N/C ratios with variable amount of pale clear to eosinophilic cytoplasm. There are populations of pleomorphic cells with multinucleation and abundant cytoplasm termed “amoeboid cells”. In these cases, malignant behavior correlates with the presence of 3 of the 4 following features: 1) >70% atypical epithelioid areas; 2) >2 mitoses per 10 HPF; 3) atypical mitotic figures; or 4) necrosis. AMLs are typically positive for HMB45, Melan A, and Cathepsin K. Cytokeratins and PAX8 are negative. The key features to recognize in the current case were the characteristic bland epithelioid to slightly spindled cells with lightly eosinophilic filamentous cytoplasm and the radiating cells coming off of some of the vessels. Also, for the diagnosis of AML, it is not necessary for the lesion to have an adipose tissue component, and typically lesions that come to resection are fat-poor AMLs as AMLs with a prominent fatty component can be recognized on imaging and unless symptomatic and not resected.