Discussion:

Benign renal epithelial neoplasms characterized by large round eosinophilic cells packed with mitochondria

5 - 9% of renal tumors

Benign

Usually asymptomatic; detected incidentally on imaging for unrelated reasons

Rarely presents with hematuria and dysuria

Diagnosis may be suggested by the presence of a central stellate scar on CT, MRI or US; however, this feature is not specific and can be present in any slow growing renal tumor

Well circumscribed mass in the cortex

Mahogany brown cut surface to tan or yellow

Central stellate scar commonly present; might be in an eccentric location

Hemorrhage might be present but should not show necrosis

Architecture is variable: typically, small solid nests in a loose connective tissue

Should not have a papillary architecture; more than focal intracystic papillations is not compatible with the diagnosis of oncocytoma

Composed of large round eosinophilic cells (oncocytes) with dense granular cytoplasm; nuclei are round and regular with even chromatin; small but conspicuous nucleoli are present

Bizarre pleomorphic hyperchromatic cells are commonly present and are thought to be degenerative in nature

Population of small cells (oncoblasts) has been occasionally described at the edge of the scar or nests

Mitotic figures are rare with no atypical mitosis

Hemorrhage can be present but no coagulative necrosis

Rarely have foci of dystrophic calcifications

Cytoplasmic clearing can be identified around scar areas

Some show perinephric fat or vascular invasion; this does not change the diagnosis of a benign oncocytoma

Positive stains: CD117, E-cadherin, S100A1, PAX8, CAM 5.2, pancytokeratin

Negative stains: CK7 negative or scattered rare positive cells, AMACR, CAIX, CK20, HMB45, melan A, vimentin negative but can be positive in cells around central scars or in oncoblasts

Differential diagnosis:

• Eosinophilic variant of chromophobe RCC: CK7 positive, nuclear membrane irregularities (wrinkling) and perinuclear halos
• Hybrid oncocytic/chromophobe tumors (HOCT): Overlapping features morphologically and immunophenotypically between oncocytoma and chromophobe renal cell carcinoma, low grade oncocytic tumors
• SDH deficient RCC: Classic cases show cytoplasmic inclusions with pale eosinophilic material, can show nuclear pleomorphism and high-grade tumors, other architectural patterns as a papillary or cystic morphology. Loss of SDHB staining. Negative or focally positive for keratin
• Oncocytic papillary RCC: Papillary morphology, negative for c-KIT and positive for racemase
• Eosinophilic clear cell RCC: Areas with clear cell morphology, higher grade tumors, positive for CAIX and negative for racemase
• Epithelioid angiomyolipoma: Negative keratins and negative PAX8, positive for melanocytic markers HMB45 and melan A; positive for cathepsin K
• MiTF associated RCC: Positive for TFE3and cathepsin K, negative or focally positive for keratin. Can be focally positive for HMB45 and MART1
• Low grade oncocytic tumors (LOT): Negative for KIT and positive for CK7
• Eosinophilic, solid and cystic RCC: Positive for CK20