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 termed nephrogenic adenofibroma) and 16 related tumors from the files of the National Wilms Tumor Study Group Pathology Center (NWTSGPC). All tumors contained a variable amount of a bland spindle cell stroma, which is essentially identical to the recently described metanephric stromal tumor (MST). Features that distinguish this stroma from congenital mesoblastic nephroma (CMN) include intratumoral angiodysplasia, concentric cuffing of entrapped tubules ("onion skinning"), and heterologous differentiation. The epithelial components of these lesions spanned a wide range of appearances. All tumors contained at least focally an inactive embryonal epithelium identical morphologically to metanephric adenoma (MA), and hence each case could be classified as containing MAF. The epithelium of nine tumors had this appearance throughout, and hence these were considered usual MAFs. The epithelium of four tumors demonstrated increased mitotic activity but was otherwise similar to MA. The epithelial component of seven tumors spanned a morphologic spectrum from inactive MA to malignant epithelial predominant Wilms tumor (WT), with gradual transitions noted in several cases. Five other tumors contained a carcinomatous component distinct from these lesions but identical morphologically 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 morphologic spectrum between benign pure stromal (MST) and pure epithelial (MA) lesions, and can merge with the morphology of WT, supporting the concept that these are all related lesions. A relationship to PRCC is also evident.