What is the malignant potential of renal oncocytoma?

Malignant Potential of Renal Oncocytoma

Renal oncocytoma is classified as a benign tumor, but should be considered as having very low rather than no malignant potential, as rare cases demonstrate metastatic behavior and invasive features. 1, 2

Benign Classification with Important Caveats

Renal oncocytoma is officially designated as a benign renal tumor with distinctive morphologic characteristics including eosinophilic cytoplasm, regular nuclei with prominent central nucleoli, and nested architecture. 1 When strict diagnostic criteria are applied, these tumors demonstrate excellent prognosis with minimal risk of aggressive clinical behavior. 1

Features That Do NOT Alter Benign Behavior

The following features can be present in true oncocytomas without changing their benign classification: 1

• Vascular or renal vein involvement
• Involvement of perinephric adipose tissue
• Small cells ("oncoblasts")
• Localized collections of pleomorphic cells (degenerative-type atypia)

Features That Should Exclude the Diagnosis

The presence of these features should deter from diagnosing oncocytoma: 1

• Coagulative necrosis
• Well-formed papillary formations
• Brisk or atypical mitotic activity

Evidence of Malignant Potential

Despite the benign classification, documented cases of metastatic disease exist, though they are exceptionally rare. 2

Metastatic Disease Data

A systematic review of 412 renal oncocytomas revealed: 2

• Four patients (1%) died from their disease
• Six patients (1.5%) experienced disease progression
• Metastatic sites included liver, bone, lung, lymph nodes, and local recurrence

Invasive Histopathologic Features

Some oncocytomas demonstrate concerning features including: 2, 3

• Perinephric fat invasion
• Renal sinus fat invasion
• Renal capsular invasion
• Vascular invasion

Critically, even when these aggressive features are present, the prognosis remains atypically good, suggesting these features do not reliably predict malignant behavior. 2

Growth Characteristics

Approximately 80% of renal oncocytomas demonstrate growth on active surveillance, with an estimated average growth rate of 0.16 mm monthly. 4 This growth rate is similar to renal cell carcinoma, meaning absence of tumor growth is not a robust indicator of benign histology. 4

Diagnostic Challenges and Borderline Cases

The most challenging clinical scenario involves tumors with overlapping features between oncocytoma and chromophobe renal cell carcinoma. 1

Recommended Diagnostic Approach

For tumors that don't fit strict criteria for either oncocytoma or chromophobe RCC: 1

1. Apply very strict definitions for prototypical oncocytoma
2. Use the term "oncocytic renal neoplasm of low malignant potential, not further classified" for borderline sporadic cases
3. Reserve "hybrid oncocytic tumor" terminology for hereditary cases (Birt-Hogg-Dubé syndrome) with multiple/bilateral tumors

Collective experience from large renal cancer centers strongly suggests these "difficult to classify" tumors have exceedingly low risk for developing metastatic disease. 1

Clinical Management Implications

No imaging technique can reliably distinguish oncocytoma from malignant lesions, requiring tissue diagnosis through biopsy or resection. 5

Biopsy Considerations

Exercise caution when diagnosing oncocytoma on core biopsy due to regional tumor heterogeneity—chromophobe RCC may have foci indistinguishable from oncocytoma. 1 An alternative approach is to diagnose such biopsies as "oncocytic renal neoplasm" with a comment noting that if the sample is representative, appearances would be consistent with oncocytoma. 1

Immunohistochemical Profile

Typical oncocytoma demonstrates: 1

• CK7: Restricted to rare, scattered cells
• KIT (CD117): Diffuse positive reactivity
• Vimentin: Negative

Molecular Features

Oncocytomas harbor limited chromosomal abnormalities compared to chromophobe RCC, ranging from normal karyotype to loss of chromosome 1 and/or Y, or rearrangement of 11q13. 1, 6

Bottom Line for Clinical Practice

Renal oncocytoma should be viewed as having very low malignant potential rather than being truly benign. 2, 3 While the vast majority behave in a benign fashion, rare metastatic cases exist. Active surveillance of tissue-proven oncocytoma appears safe in the short term at centers where biopsy can be performed safely and accurately, with alternative management including partial nephrectomy or minimally invasive approaches. 4