Gross Features: Chromophil renal cell tumors are characterized by a spherical
outline. Tumors with a diameter up to some millimeters, usually adenomas, tend to be
beige or white colored, while larger tumors, usually carcinomas, exhibit extensive
greasy brown colored central necrosis resulting from a poor vascular supply and
frequent hemorrhages. Sometimes there are yellow glittering spots (aggregates of
foamy macrophages) mostly in the periphery just beneath the fibrous pseudocapsule.
Microscopic Features: Light microscopically, the basic chromophil cell type has pale
cytoplasm and crowded centrally located small nuclei. Electron microscopically, the
cytoplasm contains only a few organelles, mainly endoplasmic reticulum. Rudimentary
microvilli develop from the lumenal surface and there are extensive basal infoldings
covered with basement membrane material, resembling those of the proximal tubule.
Increasing dedifferentiation results in enlarged nuclei with prominent nucleoli and
an eosinophilic or granular cytoplasm due to an accumulation of mitochondria. As a
rule, chromophilic tumors have a papillary or tubulopapillary growth pattern which
may appear solid when tightly packed or in undifferentiated areas. The papillary
stalks are often expanded by collections of foamy macrophages and edema fluid.
Psammoma bodies are common.
Differential diagnosis: In dedifferentiated (solid) tumors, clear cell RCC is a
consideration.
