Nephrogenic
adenofibroma is a novel kidney
tumor of young people (mean age of presentation, 13 years), who present with
polycythemia,
hypertension, or
hematuria, which resolve following
nephrectomy. The typical
nephrectomy specimen contains a solitary, nonencapsulated, vaguely circumscribed, irregularly shaped or spherical, firm mass with either tan, gray-white, or pale yellow coloration.
Cysts are sometimes present within the
tumor. The histologic appearance is distinctive and characterized by a marked proliferation of spindled mesenchymal cells resembling the classical type of
congenital mesoblastic nephroma, encasing discrete nodules of embryonal epithelium similar to the hyperplastic nephrogenic rests (nephroblastomatosis) usually associated with
Wilms' tumor. The mesenchymal component consists of a fascicular proliferation of tightly interlaced, uniform, benign-appearing spindled cells that immunostatin for
vimentin and
fibronectin, but not
desmin or actin. The epithelial component consists of discrete islands of blastemal cells that are partially or fully differentiated toward tubular, tubulopapillary, or papillary structures. Psammoma bodies are plentiful. Embryonal epithelium immunostains for
cytokeratin but not
epithelial membrane antigen. The overall histologic appearance of the mesenchymal and epithelial components is benign, and preliminary clinical data suggest that the
tumor has a benevolent course. Two cases, however, contained small, well-circumscribed papillary lesions near the renal pelvis that resembled low-grade
collecting duct carcinoma. The clinical implications of the latter finding are unclear.