Gross Features: These rare tumors mostly present as single, solid, well demarcated
tumors with light brown to sandy cut surfaces of glassy quality. They are sharply
demarcated and composed of several nodules arising in the cortex and streaked with
small fibrous septa. Necrosis and softening are not found, although cystic regression
may occur in the center.
Microscopic Features: Metanephric adenomas are composed of densely packed small tumor
cells with overlapping nuclei and invisible cell borders in solid appearing areas
turning to a polar orientation in areas of more differentiated tubular,
tubulopapillary or glomeruloid structures. Electron microscopically, the cytoplasm is
scanty with only a few organelles, rudimentary apical microvilli and some basal
membrane formations. The nuclei are innocent looking and mitotic figures are
uncommon. The stroma is scanty. Psammoma-like calcified bodies seem to be typical,
especially in sclerotic zones at the periphery .
Differential Diagnosis: Chromophil RCC, Wilms' tumor