Epidemiology

 Mean patient age was 2 years

Macroscopy

 fibrous lesion centered in the renal medulla containing smooth-walled cysts (mean tumor size, 5.5 cm)

Microscopically

 histologically identical to the stromal component of metanephric adenofibroma (MAF, previously termed nephrogenic adenofibroma)
 unencapsulated spindle cell lesion that entraps native kidney
 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)
 vascular alterations

Immunochemistry

 CD34+/-

Prognosis,

 surgical excision alone
 none recurrence or metastasis known

Differential diagnosis

 clear cell sarcoma of the kidney.

References:

 Argani P, Beckwith JB. Metanephric stromal tumor: report of 31 cases of a distinctive pediatric renal neoplasm. Am J Surg Pathol. 2000 Jul;24(7):917-26. PMID: #10895814#