Renal oncocytoma and chromophobe renal cell carcinoma - A comparison of colloidal iron staining and electron microscopy

Chromophobe renal cell carcinoma (RCC) is characterized by diffuse cytoplas mic staining with Hale colloidal iron (HCI) and the presence of numerous mi crovesicles. The eosinophilic variant morphologically may resemble renal on cocytoma. The latter commonly shows focal cytoplasmic HCI reactivity, but m icrovesicles have not been previously reported. We examined 19 chromophobe RCCs and 28 oncocytomas for their HCI staining patterns. Electron microscop y was performed on 13 chromophobe RCCs and 10 oncocytomas. In all cases of chromophobe RCC, more than 75% of cells showed a diffuse cytoplasmic HCI po sitivity, whereas a variable proportion of cells in 20 oncocytomas showed f ocal cytoplasmic staining in a perimembranous, apical, or perinuclear patte rn. Ultrastructurally, chromophobe RCCs contained abundant microvesicles wi th varying numbers of mitochondria, whereas all oncocytomas contained abund ant mitochondria with focal collections of microvesicles. The microvesicles , in perimembranous, apical, or perinuclear clusters or singly scattered th roughout the cytoplasm, were morphologically indistinguishable from those i n chromophobe RCCs. In most cases, the microvesicle location and HCI staini ng pattern correlated. Chromophobe RCC and oncocytoma have distinctive morp hologic features that usually allow their recognition. In difficult cases, HCI staining and electron microscopy may help, but the presence of HCI posi tivity or microvesicles in an eosinophilic renal tumor does not rule out on cocytoma.