Mixed Epithelial and Stromal Tumor of the Kidney with Sarcomatous Transformation Metastatic to the Lung. A Case Report.

Background: Malignant transformation of mixed epithelial and stromal tumor (MEST) of the kidney is a very rare entity. We report an unusual case with a review of the literature.

Case: A 25-year-old woman underwent surgery for an 8-cm biphasic renal tumor, which was diagnosed as MEST of the kidney. High cellularity, mitoses, and nuclear atypia seen in the stromal component were compatible with a sarcomatous transformation in MEST. The patient also had pulmonary metastatic nodules that were resected 2 months after the nephrectomy. Metastatic pulmonary tumors were morphologically identical to the sarcomatous component of the renal tumor and showed pulmonary epithelial hyperplasia with a phyllodes-like pattern.

Conclusion: Although MEST of the kidney is known as a benign tumor, malignant transformation, mostly sarcomatous type, should be considered in the diagnosis.