Metanephric stromal tumor: report of 31 cases of a distinctive pediatric renal neoplasm.

We report 31 cases of a novel pediatric renal neoplasm, metanephric stromal tumor (MST). Mean patient age was 2 years, and the most common presentation was that of an abdominal mass. Gross examination typically revealed a fibrous lesion centered in the renal medulla containing smooth-walled cysts (mean tumor size, 5.5 cm). MST is histologically identical to the stromal component of metanephric adenofibroma (MAF, previously termed nephrogenic adenofibroma) and is an unencapsulated spindle cell lesion that entraps native kidney. Characteristic histologic features of MST include alternating cellularity that imparts a nodular low-power appearance, onion-skin cuffing around entrapped renal tubules, heterologous differentiation (glia or cartilage), and vascular alterations (angiodysplasia of entrapped arterioles, juxtaglomerular cell hyperplasia in entrapped glomeruli). Three tumors in which the vascular alterations were particularly florid were associated with extrarenal vasculopathy and attendant morbidity. A majority of cases stained for CD34, although the degree of staining was variable. Most patients were treated with surgical excision alone, and none experienced recurrence or metastasis. Recognition of this entity can spare a child potentially toxic adjuvant chemotherapy that might be used for lesions in its differential diagnosis, specifically clear cell sarcoma of the kidney.