Answer: A
Histologic Description: This is a renal mass centered on the capsule which projects out to the perirenal fat. The neoplasm contains a bundled smooth muscle stroma with all the features of angiomyolipoma, and associated with epithelial cysts that represent entrapped renal tubules. The stroma consists of poorly formed fascicles of short spindle cells with eosinophilic to pale cytoplasm in a highly vascularized stroma. The epithelium has a hobnail appearance consistent with entrapped renal tubular epithelium; however, beneath it is a more cellular zone which resembles endometrial stroma. The stromal component labels for HMB45 and cathepsin K, whereas epithelium is PAX8 positive. Estrogen receptor immunoreactivity is accentuated beneath the cystic epithelium.
Differential Diagnosis: Mixed epithelial stromal tumor also is biphasic; however, the stroma is of conventional smooth muscle and doesn’t not label for HMB45 or Melan A. Endometriosis would not contain the angiomyolipomatous smooth muscle component seen in the current case, and would feature more hemorrhage and degenerative changes. Synovial sarcoma may entrapped renal tubules; however, the stromal component would be a primitive blue spindle cell lesion, not the angiomyolipomatous stroma of the current case.
AMLEC is typically subscapular and projects beyond the contour of the kidney into perirenal fat, raising clinical concerns for an aggressive capsule-invasive renal carcinoma.