Background: Metanephric adenoma of the kidney is a rare, usually solitary, and benign tumor of the kidney, predominately affecting female patients in the fifth or sixth decade of life. It is considered by some specialists to be the hyperdifferentiated benign end of the Wilms tumor spectrum.
Case Presentation: The case was a 49-year-old woman with abdominal pain, a palpable mass in the right abdomen, and mild hematuria, without any other symptoms or laboratory findings. An ultrasonography and computed tomography revealed a tumor in the upper pole of the right kidney with a diameter of 5.3 cm. There was no infiltration of the neighboring structures and tissues. The differential diagnosis included papillary renal cell carcinoma, adult Wilms tumor, and metanephric adenoma. A partial nephrectomy was followed. The histological examination of the tumor revealed relatively small unvarying basophilic epithelial cells with scant cytoplasm, uniform nuclei, and some areas with nuclear grooves, delicate chromatin, and indistinct nucleoli in a loose non-cellular stroma and developed in a tightly packed alveolar, tubular, and rarely papillary pattern. The mitotic rate was extremely low. Tumor cells were positive for paired box gene 8, cluster differentiation 57, and Wilms tumor 1 and negative for racemase, cytokeratin 7, and EMA. The cell proliferation rate Ki-67 was extremely low, and there was diffuse, strong cytoplasmic positivity for BRAF V600E staining. Based on morphology and immunohistochemistry, the diagnosis of metanephric adenoma was made.
Conclusion: Although metanephric adenomas may be difficult to diagnose clinically, the histological examination and the immunohistochemistry assay, including BRAF V600 staining, can make a safe diagnosis, avoiding the administration of incorrect treatment.
Key words: Kidney, metanephric adenoma, benign, tumor, BRAF
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