Case 66 - Discussion

Uploaded: 2007-07-22, Updated: 2007-11-19

CK7, 20× CD117, 20×
Vimentin, 40× Colloidal iron staining,, 60×
EM, 5000× EM, 5000×


The tumor cells are arranged in a glandular, cord, sheet or nested pattern in a capillary-rich background. There are focal areas with abundant myxoid stroma, especially in the center of the specimen (Figure 2A).  The tumor cells are large, polygonal with abundant bright pink granular cytoplasm that contains numerous finely, eosinophilic granules (Figure 2B).  The tumor cells have one smooth round nucleus with clumped chromatin and a centrally placed nucleolus.  The tumor cells have no marked atypia or polymorphism, and no vascular, nerve or capsular invasion is noted.  Multiple benign simple cysts are also noted microscopically (Figure 2C). A Hale’s colloid iron staining shows focally weak luminal positivity in the tumor nests. Immunohistochemistry stains demonstrate that the tumor cells have diffuse strong positivity of CD117 (Figure 2D) and scattered strong reaction to CK7 (Figure 2E) but negative for RCC, CD10 and vimentin. Ultrastructurally, the tumor consists of large polygonal cells with abundant cytoplasm that is filled with numerous enlarged, rounded mitochondria (Figure 2F). All these features support a diagnosis of benign oncocytoma.


Oncocytoma associated with acquired cystic kidney disease (ACKD)



Related cases: renal mass 1, renal mass 2, renal mass 3, renal mass 4


The Key Features

  • Central scar, central loose hypocellular fibrous stroma;

  • Uniform granular eosinophilic cytoplasm;

  • CK7 scattered +, CD117+; Vim-, CD10-, RCC-;

  • EM: numerous mitochondria.

Clinical Futures

  • Most common benign solid renal tumor;

  • Originate from the intercalated cells of the collecting duct;

  • 3-7% of all renal tumor, 2-12% multifocal, and 4-14% bilateral;

  • M:F= 2-3:1;

  • Mean patient age is 62-68;

  • 10% of cases, oncocytoma and chromophobe RCC may coexist.

Gross Findings

  • Spherical and are large (average size, 7 cm) with a pseudocapsule or no capsule;

  • Cut sections, homogeneously tan-pale yellow, mahogany color;

  • Characteristic fleshy/stellate central scar (33-54%);

  • Necrosis, hemorrhage and calcification may be present.

Microscopic Findings

  • Large cells, granular eosinophilic cytoplasm;

  • Well-defined small nests, no sheet-like arrangement;

  • Nuclei: round/oval/smooth and round, minimal atypia, large nucleoli;

  • Some areas, the nests are separated by a loose hypocellular fibrous stroma.

Differential Diagnosis




Chromophobe RCC


Mahogany brown +/- central scar

Pale tan-brown, +/- central necrosis


Closely packed nests (periphery) and nests in loose hypocellular stroma; no trabeculae/sheets

Closely packed nests; +/-broad trabeculae/sheets


Uniform, limited atypia

Classic and eosinophic cells, more atypia


Granular, acidophilic

Granular, acidophilic with perinuclear halos


Uniform, round, degenerative pleomorphism

More pleomorphism, "raisinoid"


None or rare


Colloidal iron

Focal positivity, in the lumen.

Strongly diffuse positivity


Positive,CK7-scattered strong positive

Positive,CK7-diffusely strong positive





Numerous mitochondria with lamellar cristae;

no microvesicles

Numerous mitochondria with tubulovesicular cristae; interspersed microvesicles

Immunohistochemistry Staining

  • Cathepsin H +, EMA+, CK7-scattered strong positive, CD117+

  • RCC-, Vimentin-, CD10-

  • Colloid iron staining: focal positive

Election Microscopy

  • Abundant abnormal mitochondria


  • Losses of chromosome 1 and X chromosomes, deletion of chromosome 14 and a balanced translocation involving 11q13.

Treatment and Prognosis

  • Benign tumors, and the prognosis after total or partial nephrectomy is excellent.