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 presentatio n was that of an abdominal mass. Gross examination typically revealed a fib rous lesion centered in the renal medulla containing smooth-walled cysts (m ean tumor size, 5.5 cm). MST is histologically identical to the stromal com ponent of metanephric adenofibroma (MAF, previously termed nephrogenic aden ofibroma) and is an unencapsulated spindle cell lesion that entraps native kidney. Characteristic histologic features of MST include alternating cellu larity that imparts a nodular low-power appearance, onion-skin cuffing arou nd entrapped renal tubules, heterologous differentiation (glia or cartilage ), and vascular alterations (angiodysplasia of entrapped arterioles, juxtag lomerular cell hyperplasia in entrapped glomeruli). Three tumors in which t he vascular alterations were particularly florid were associated with extra renal vasculopathy and attendant morbidity. A majority of cases stained fur CD34, although the degree of staining was variable. Most patients were tre ated with surgical excision alone, and none experienced recurrence or metas tasis. Recognition of this entity can spare a child potentially toxic adjuv ant chemotherapy that might be used for lesions in its differential diagnos is, specifically clear cell sarcoma of the kidney.