Case report: two rare cases of mixed epithelial and stromal tumor of the kidney and a review of the literature.

Mixed epithelial and stromal tumor of the kidney (MESTK) is rare renal neoplasm, which usually behaves benignly, while very rarely malignancies have also been reported. Histologically, MESTK is composed of both mesenchymal and epithelial components, where the epithelial components are arranged in a tubular or tubule-cystic pattern against a background of ovarian-like stromal proliferation. MESTK is more commonly seen in perimenopausal women or in patients on long-term estrogen replacement therapy. Given the popularity of routine health screening, patients main present asymptomatically. We report one rare case of MESTK, which was diagnosed in a 30-year-old woman. A computed tomography (CT) scan revealed one well-defined, uneven mass in the left kidney. The tissue obtained by fine-needle aspiration showed relatively homogeneous cells. Renal cell carcinoma could not be excluded, and left complete nephrectomy was performed, according to the patient's wishes. Another case of MESTK we present here was diagnosed in an 18-year-old male adolescent, who did not have a history of estrogen treatment, with estrogen treatment seen rarely in the clinical setting. Renal cell carcinoma was suspected, and a left partial nephrectomy was performed. Based on histopathological examination, the diagnosis was MESTK for both cases. Both patients were periodically monitored for one year following surgery and showed no imaging findings of recurrence or metastases. MESTK is benign tumor, and hence preoperative diagnosis is crucial to avoid overtreatment. To improve the current understanding of this disease, comprehensive studies on their pathogenesis and preoperative diagnosis are needed.