[Metanephric stromal tumor: report of two cases and bibliographic review].

Objective: Metanephric Stromal Tumors (MST) are pediatric renal neoplasms not very common in adults. This study revises its classification, incidence and evolution and also some specific characteristics of the cases diagnosed in adults.

Methods: We present two cases of MST diagnosed in adults of 72 and 77 years old respectively. Abdominal pain due to a more than 4 Kg. mass was the initial presentation in both cases. The tumors were completely resected. Four and ten years after excision patients are alive without disease.

Results: Characteristic histologic features include a proliferation of fusocellular cells with alternating cellularity that imparts a nodular appearance and onion-skin cuffing around entrapped renal tubules or vascular structures. No mitoses or atypia was found but extensive necrosis and fibrosis were present. A majority of stromal cells were vimentine and CD-34 positive. Stains for CK and EMA highlighted entrapped native tubules. Both cases were previously classified as mesoblastic nephromas. According to the 2002 ONS classification of tumours of the urinary system, they have been revised and re-classified as MST CONCLUSION: MST are pediatric benign tumors exceptionally diagnosed in adults. Metanephric stromal tumors are divided into 3 categories based on the presence of epithelial cells, stroma and epithelial cells plus stromal. Complete excision is the treatment of choice and the prognosis is excellent.