Background: Metanephric adenoma (MA) is a rare benign renal neoplasm that can occur at any age, whereas, Wilms' tumor (WT) is the most common malignant renal neoplasm in children and is occasionally seen in adults.
Cases: In case 1, a 26-year-old male had a left renal mass. Fine needle aspiration (FNA) showed 3-dimensional sheets of cells with nuclear overlapping, molding, irregular nuclear membrane and distinct nucleoli. Frequent mitotic figures could be seen. The cytologic differential diagnosis included Wilms' tumor, neuroectodermal tumor and metanephric adenoma. Nephrectomy revealed Wilms' tumor. In case 2, a 24-year-old female presented with erythrocytosis and a right renal mass. FNA showed small, uniform cells with smooth nuclear membrane, fine chromatin and inconspicuous nucleoli. A diagnosis of metanephric adenoma was made and confirmed on nephrectomy.
Conclusion: Differentiating MA from WT based on cytologic features on FNA biopsy prior to surgical resection can he difficult.