Answer: B

Histological Description: Adjacent to the renal pelvis is a spindle cell tumor composed of well-formed fascicles. Nuclei are cigar-shaped with occasional nuclear hyperchromasia yet still relatively uniform. Scattered mitotic activity is present. The lesion lacks necrosis. Adjacent renal parenchyma is uninvolved.

Discussion: First, in terms of the distinction of AML and a smooth muscle tumor involving the kidney, there is a difference in location. AMLs involve the kidney or peri-renal tissue, but typically not the hilum of the kidney. In contrast, smooth muscle tumors arise from the large vessels of the renal hilum. Histologically, smooth muscle cells have identical nuclei to those in low grade leiomyosarcomas yet the cytoplasm is looser and filamentous, whereas in smooth muscle the overall appearance is more dense pink due to more compact eosinophilic cytoplasm. Smooth muscle tumors also tend to have a more regular fascicular growth pattern compared to AML. Immunohistochemically, both AML and smooth muscle tumors express desmin and smooth muscle actin. However, AMLs are also positive for HMB45 and Melan A, yet can be very focal. S100 protein can be positive. Typically, Cathepsin K is diffusely positive in AMLs but not smooth muscle tumors. A more recently described marker GPNMB is also positive in AML and negative in smooth muscle tumors. In the current case, immunohistochemistry confirmed the diagnosis of a smooth muscle tumor. Once it is recognized that this is a smooth muscle tumor, the differential is between a leiomyoma and leiomyosarcoma. In this site, a leiomyoma is diagnosed only if the lesion is virtually identical to normal smooth muscle in terms of cellularity and no atypia and no mitotic activity, yet recognizable as a tumor because it is forming a spherical mass. In this case, the lesion is hypercellular with occasional hyperchromatic nuclei and scatter mitotic activity diagnostic of a low grade (Grade 1) leiomyosarcoma.