Gross Features: Depending on size, chromophobe renal cell tumors consist of one or
more solid nodules with slightly lobulated surfaces. The cut surface of the fresh
specimen appears homogeneously orange and turns beige or sandy after formalin
fixation. The uniform pale cut surface interspersed with a few hemorrhages is a very
characteristic gross feature of this mostly moderately differentiated tumor type,
while a slightly brown cut surface is usually associated with well differentiated
tumors.
Microscopic Features: The basic chromophobe cell type is a large polygonal cell with
transparent slightly reticulated cytoplasm with prominent cell membranes, reminiscent
of plant tissue. Another diagnostic hallmark is the lack of cytoplasmic coloring
with routine dyes but a diffuse cytoplasmic staining reaction with Hale's iron
colloid stain, which is a characteristic feature of this special tumor type only.
Electron microscopically, the cytoplasm is crowded by loose glycogen deposits and
numerous sometimes invaginated and sometimes studded vesicles, 150-300 nm in diameter
resembling those of the type B intercalated cells of the cortical collecting duct.
Well differentiated tumors have condensed and hyperchromatic (sometimes binucleated)
nuclei which become more atypical with increased grade. In general, the growth
pattern is compact or tubulocystic. Microscopic calcifications sometimes are
present.
Differential diagnosis: Clear cell RCC is the principal consideration for the
classic type with pale cytoplasm, while renal oncocytoma is the major consideration
when eosinophilic cytoplasm predominates.
