World Health Organization Classification of Tumours Pathology and ...

Metanephric adenoma and metanephricadenofibromaJ.N. EbleD.J. GrignonH. MochDefinitionMetanephric adenoma is a highly cellularepithelial tumour composed of small, uniform,embryonic-appearing cells.ICD-O codesMetanephric adenoma 8325/0Metanephric adenofibroma 9013/0Metanephric adenosarcoma 8933/3EpidemiologyMetanephric adenoma occurs in childrenand adults, most commonly in the fifthand sixth decades. There is a 2:1 femalepreponderance {561}. Patients withmetanephric adenofibroma have rangedfrom 5 months to 36 years (median = 30months) {120}. There is a 2:1 ratio of malesto females. A single case of high grade sarcomaarising in association withmetanephric adenoma (metanephricadenosarcoma) has been reported {2072}.Clinical featuresApproximately 50% of metanephric adenomaare incidental findings with otherspresenting with polycythemia, abdominalor flank pain, mass, or hematuria.Presenting symptoms of metanephricadenofibroma have included polycythemiaor hematuria; some have beenincidental findings. Arroyo et al. {120}described several cases in which eitherWilms tumour or carcinoma occurred inassociation with metanephric adenofibroma.Other than one patient withregional metastases from the carcinoma,these patients have had no progression.MacroscopyMetanephric adenomas range widely insize; most have been 30 to 60 mm indiameter {561}. Multifocality is uncommon.The tumours are typically well circumscribedbut not encapsulated. Thecut surfaces vary from grey to tan to yellowand may be soft or firm.Foci of haemorrhage and necrosis arecommon; calcification is present inapproximately 20%,and small cysts in10% {561,1237}.Metanephric adenofibromas are typicallysolitary tan partially cystic masses withindistinct borders {120}.HistopathologyMetanephric adenoma is a highly cellulartumour composed of tightly packedsmall, uniform, round acini with anembryonal appearance. Since the aciniand their lumens are small, at low magnificationthis pattern may be mistaken fora solid sheet of cells. Long branchingand angulated tubular structures also arecommon. The stroma ranges from inconspicuousto a loose oedematous stroma.Fig. 1.57 Metanephric adenoma.Hyalinized scar and focal osseous metaplasiaof the stroma are present in 10-20% of tumours {561}. Approximately50% of tumours contain papillary structures,usually consisting of tiny cysts intowhich protrude blunt papillae reminiscentof immature glomeruli. Psammomabodies are common and sometimesnumerous. The junction with the kidney isusually sharp and without a pseudocapsule.The cells of metanephric adenomaare monotonous, with small, uniformnuclei and absent or inconspicuousnucleoli. The nuclei are only a little largerthan those of lymphocytes and are roundor oval with delicate chromatin. The cytoplasmis scant and pale or light pink.Mitotic figures are absent or rare.Metanephric adenofibroma is a compos-ABFig. 1.58 Metanephric adenoma. A Well circumscribed tumour without encapsulation. B Complicated ductal architecture with psammoma bodies.44 Tumours of the kidney