Clinical history: 59 year old female with a slow-growing renal mass.
What is the diagnosis?
Renal oncocytomas are neoplasms of the renal cortex that often are discovered incidentally by radiologic examinations. The male to female ratio is 2:1. Almost all cases have occurred in adults, mostly from 50 to 80 years old. Resection of the tumor is curative. Radiologically, a striking spoke and wheel appearance can be seen but it is not specific.
Grossly, the tumour is well circumscribed, unencapsulated, solid, homogenous, mahogany or brown-yellow (same as cortex), 3-6 cm, stellate central scar in larger lesions. Occasional foci of hemorrhage can be seen but necrosis is rare. Rarely, large numbers of small oncocytomas may be present in the cortices of both kidneys, a condition termed oncocytomatosis.
Microscopically, the tumour cells are arranged either in diffuse sheets or as cellular islands in a background of loose edematous connective tissue. The cytoplasm is intensely eosinophilic and finely granular. The nuclei are mainly round with small clumps of chromatin and nucleoli that may be visible. Occasional bizarre and enlarged nuclei may be present. Mitotic figures are absent or very rare. Because they are benign, oncocytomas are not graded. By electron microscopy, the cytoplasm is seen to be filled with mitochondria and microvilli are sparse. Extension into small veins is seen microscopically in a little more than 5% of cases and appears to have no adverse prognostic significance. Small extensions into perirenal fat are seen in almost 10% of cases and also appear to have no adverse effect.
Features Rare or Impermissible in Renal Oncocytoma
Clear or spindle cells
Positive colloidal iron stain or chromophobe-type vesicles seen by electron microscopy
Gross vascular invasion
Gross extension into perirenal fat
The principal consideration is the eosinophilic variant of chromophobe renal cell carcinoma (see above features). Immunohistochemical analysis can also be helpful because oncocytomas do not express vimentin and often have a characteristic punctate pattern of staining for cytokeratin.