Case 9 -

Metanephric Stromal Tumor John N Eble, M.D. Holger Moch, M.D.

Case history: While bathing her son, the mother of a 29-month-old boy found an abdominal mass. Physical examination disclosed slight flank pain and microscopic hematuria. Radiographic evaluation disclosed a large mass in his right kidney and a nephrectomy was done. Macroscopic findings: The diameter of the tumor was 5 cm. The tumor was well circumscribed but not encapsulated. The cut surface was gray. Diagnosis: Metanephric stromal tumor Metanephric stromal tumor is approximately one-tenth as common as congenital mesoblastic nephroma. These tumors often present as abdominal masses or with hematuria. The mean age at diagnosis is 24 months, but metanephric stromal tumor also has been observed in an adult. So far, all of the tumors have been cured by surgery. Gross Pathology Metanephric stromal tumors usually form a tan, lobulated mass centrally located in the kidney. With a mean diameter of 5 cm, they often are large compared to the kidney. Approximately 50% of tumors are grossly cystic and 15% are multifocal. Microscopic Pathology The border with the surrounding kidney lacks a capsule and is delicately infiltrative. It is composed of spindle and stellate cells with hyperchromatic skinny nuclei, and fine cytoplasmic processes. Metanephric stromal tumor often entraps renal tubules and blood vessels, creating "onionskin" collarettes around them in a myxoid background. More cellular areas at the periphery of the impart a striking image of variable cellularity. Epithelioid transformation of medial smooth muscle of entrapped blood vessels is common and may result in aneurysms. IN roughly 25% of metanephric stromal tumors, there is hyperplasia of juxtaglomerular cell associated with entrapped glomeruli. Approximately 20% metanephric stromal tumors contain foci of glia or cartilage. Necrosis is unusual. Metanephric stromal tumors often are immunoreactive for CD34, but this can be patchy. Reactions for desmin, cytokeratins, and S-100 protein are negative. References Argani P, Beckwith JB. Metanephric stromal tumor: report of 31 cases of a distinctive pediatric renal neoplasm. Am J Surg Pathol 24: 917-926, 2000. Arroyo MR, Green DM, et al. The spectrum of metanephric adenofibroma and related lesions: clinicopathologic study of 25 cases from the National Wilms Tumor Study Group Pathology Center. Am J Surg Pathol 25: 433-444, 2001 Bluebond-Langner R, Pinto PA, et al. Adult presentation of metanephric stromal tumor. J Urol 168: 1482-1483, 2002.