Mixed epithelial and stromal tumor of the kidney with prominent peritumoral adipose tissue: a case report radiologically mimicking angiomyolipoma with epithelial cysts

Mixed epithelial and stromal tumor of the kidney (MEST) is a rare renal neoplasm with biphasic differentiation comprising epithelial and stromal components. It typically appears as a mixed cystic-solid mass on imaging, and the prognosis is favorable in the vast majority of cases. We report a case of MEST with a prominent fat component on imaging: a 54-year-old woman was admitted with a 1-month history of left flank distension and pain. Contrast-enhanced computed tomography (CT) showed a mixed cystic-solid mass at the lower pole of the left kidney, with a prominent low-attenuation fat-density area surrounding the mass. Preoperatively, the lesion was considered an unusual angiomyolipoma with epithelial cysts (AMLEC). Because the lesion was small, located at the lower pole, and relatively well circumscribed, the patient underwent laparoscopic left partial nephrectomy. Postoperative histopathologic examination confirmed that the left renal mass was benign; however, immunohistochemistry (IHC) demonstrated positivity for EMA and PCK in the epithelial component and for PR and ER in the stromal component, supporting a diagnosis of MEST. The adipose tissue submitted separately with the tumor was located around the tumor and could not be definitively confirmed as an intratumoral component. This case demonstrates that MEST may also exhibit prominent adipose tissue on imaging and illustrates the diagnostic pitfall in which fat-related imaging findings cause MEST to mimic AMLEC, making the early distinction between MEST and cystic change in fat-containing renal tumors challenging.