A benign renal epithelial tumor resembling differentiation toward early embryonic metanephric tubules.

Metanephric adenoma belongs to the category of Metanephric tumor in the WHO histological classification of tumors of the kidney.

Although previously suggested to be derived from persistent blastema, metanephric adenoma is currently thought to develop from maturation of nephroblastoma.

Metanephric adenomas are rare (

Mostly metanephric adenomas are incidentally discovered. Some patients present with hematuria and abdominal or flank pain.

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.

Surgical excision.

The vast majority of metanephric adenomas behave in a benign fashion. Cases of regional lymph node metastasis and sarcomatoid changes have been individually reported.

No consistent cytogenetic abnormalities have been identified in metanephric adenoma. The gains of chromosomes 7 and 17 noted in papillary renal cell carcinoma, as well as the chromosome gains of 1q, 7q, and 12, and losses of 11p and 16q commonly seen in Wilms tumor have not been detected in metanephric adenoma.

BRAF (v-raf murine sarcoma viral oncogene homolog B1)

7q34

BRAF V600E mutations are found in approximately 90% of metanephric adenomas.