Metanephric neoplasms: the hyperdifferentiated, benign end of the Wilms tumor spectrum?

Metanephric neoplasms represent a spectrum of differentiated lesions that seem most likely to be related to Wilms tumor. These neoplasms include a pure stromal lesion, a pure epithelial lesion (MA), and a mixed epithelial-stromal lesion (MAF). The continuity of these lesions with Wilms tumor has been demonstrated best in the epithelial lesions. The relationship of Wilms tumor, MAF with mitoses or combined MA/Wilms tumor lesions, and usual MAF or usual MA may be viewed as analogous to that of neuroblastoma, differentiating neuroblastoma, and ganglioneuroma, in which progressively more mature or differentiated counterparts of malignant embryonal lesions are associated with a greater probability of benign clinical behavior. Such a spectrum already is recognized for cystic ILNR-derived nephroblastic lesions, ranging from cystic Wilms tumor, cystic partially differentiated nephroblastoma, and cystic nephroma. Although this concept implies that the more active lesions (Wilms tumor) mature with time into inactive ones (usual MAFs or MA), the converse (that an active Wilms tumor can arise within an inactive usual MAF or MA) remains possible.