| Etiology | Although previously suggested to be derived from persistent blastema, metanephric adenoma is currently thought to develop from maturation of nephroblastoma. |
| Epidemiology | Metanephric adenomas are rare (<1% of renal epithelial neoplasms) but occur across a wide range of patients, from infants to older individuals.. The peak age frequency is 40 to 69 years. There is a female predominance (2.3:1). Ten percent of patients present with polycythemia. |
| Clinics | Mostly metanephric adenomas are incidentally discovered. Some patients present with hematuria and abdominal or flank pain. |
| Pathology | Grossly, metanephric adenomas demonstrate a tan to fleshy white homogeneous cut surface (Fig. 1A) (Grignon et al., 2016). Most tumors are well-demarcated but lack a true capsule and are typically unilateral. Microscopically, the tumor is highly cellular and chiefly composed of tightly packed primitive tubules; papillary or glomeruloid structures may also be seen. The individual tumor cells are small, bland and cuboidal with scant cytoplasm, and round-to-oval nuclei with smooth chromatin, and inconspicuous nucleoli (Fig. 1B). Stromal hyalinization, dystrophic calcification and psammoma bodies are occasionally seen. Metanephric adenomas may resemble nephroblastoma or papillary renal cell carcinoma microscopically. Mitotic figures are rare or absent; necrosis is not seen. Tumor cells are immunoreactive for WT1, PAX8 and BRAF, and are typically negative for CK7 and AMACR. |
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| Figure 1B Histologically, the tumor cells are small, monomorphic and cuboidal. Architecturally, the tumors can demonstrate tubular (predominant), papillary and glomeruloid growth patterns (courtesy Chin-Chen Pan). |
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| Treatment | Surgical excision. |
| Prognosis | The vast majority of metanephric adenomas behave in a benign fashion. Cases of regional lymph node metastasis and sarcomatoid changes have been individually reported. |
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