The spectrum of metanephric adenofibroma and related lesions - Clinicopathologic study of 25 cases from the National Wilms Tumor Study Group Pathology Center

The authors report nine new metanephric adenofibroma (MAFs; previously term ed nephrogenic adenofibroma) and 16 related tumors from the files of the Na tional Wilms Tumor Study Group pathology Center (NWTSGPC). All tumors conta ined a variable amount of a bland spindle cell stroma, which is essentially identical to the recently described metanephric stromal tumor (MST). Featu res that distinguish this stroma from congenital mesoblastic nephroma (CMN) include intratumoral angiodysplasia, concentric cuffing of entrapped tubul es ("onion skinning"), and heterologous differentiation. The epithelial com ponents of these lesions spanned a wide range of appearances. All tumors co ntained at least focally an inactive embryonal epithelium identical morphol ogically to metanephric adenoma (MA), and hence each case could be classifi ed as containing MAF. The epithelium of nine tumors had this appearance thr oughout, and hence these were considered usual MAFs, The epithelium of four tumors demonstrated increased mitotic activity but was otherwise similar t o MA. The epithelial component of seven tumors spanned a morphologic spectr um from inactive MA to malignant epithelial predominant Wilms tumor (WT), w ith gradual transitions noted in several cases. Five other tumors contained a carcinomatous component distinct from these lesions but identical morpho logically to papillary renal cell carcinoma (PRCC). In one of these cases, this component had metastasized to the regional lymph nodes at the time of diagnosis. No tumor recurred during follow-up, although almost all patients received adjuvant therapy for WT regardless of their tumor's histology and NWTSGPC diagnosis. In conclusion, MAF is a biphasic tumor that spans the m orphologic spectrum between benign pure stromal JMST) and pure epithelial ( MA) lesions, and can merge with the morphology of WT, supporting the concep t that these are all related lesions. A relationship to PRCC is also eviden t.