Metanephric stromal tumor with a rare incidence of squamous epithelium: A case report and a brief review of the literature.

Metanephric stromal tumor (MST) of the kidney, a rare benign pediatric neoplasm recognized for less than 20 years, is not widely known. The authors describe a case of MST with rare squamous epithelium in a 14-month-old female. A renal mass was discovered during her fetal period. After her birth, computerized tomography revealed that the mass was localized in the inferior pole of her left kidney. She then underwent nephrectomy. The tumor was an unencapsulated but well-defined mass with a white, solid and firm cut surface and had dimensions of 4cm×3.5cm×3cm. The tumor was initially diagnosed as Wilms tumor because its frozen section exhibited spindle cells with cartilaginous and rare squamous epithelial elements. However, the paraffin-embedded section of the tumor exhibited bland stromal cells surrounding the entrapped tubules; this arrangement produced an "onion-skin" appearance. The rare squamous epithelial element appeared to originate from normal renal tubules. Immunohistochemistry results were positive for CD34 and INI1, as well as a low Ki-67 expression level, but were negative for S-100, Desmin, Actin, CD117 and Catenin-β. Fluorescence in situ hybridization analysis did not detect an ETV6 rearrangement. Morphological characteristics, immunophenotyping and molecular genetic analysis indicated MST. No recurrence or metastases occurred during the follow-up period of 36 months. Epithelial elements should be examined carefully in pediatric patients with renal masses. MST should be included in their differential diagnoses.