Renal Oncocytoma

Description

Renal oncocytoma is a benign tumour arising from the intercalated cells of the collecting duct in the kidney, characterised by the proliferation of oncocytes, epithelial cells with a large number of mitochondria that give them an eosinophilic (pink) appearance on standard histopathological examination. It represents approximately 3-7% of all primary renal neoplasms. Despite being benign, it is often clinically and radiologically indistinguishable from renal cell carcinoma (RCC), necessitating careful evaluation and management.

Pathogenesis

The exact pathogenesis of renal oncocytoma is not well understood, but it is believed to originate from intercalated cells of the collecting ducts in the kidney due to a mitochondrial abnormality. There are no identified subtypes of renal oncocytoma. Some studies suggest a link between chromosomal abnormalities, particularly loss of the Y chromosome and mutations on chromosome 1.

Epidemiology, Risk Factors and Associations

• Renal oncocytomas are relatively rare, accounting for about 3-7% of all renal neoplasms.
• They are more prevalent in males and the incidence peaks in the 6th to 7th decade of life.
• Commonly seen in patients with Birt-Hogg-Dube syndrome (these patients develop a mean of 5.3 renal tumours in their lifetime, mostly chromophobe renal carcinoma and oncocytomas).

Clinical Features

Patients with renal oncocytoma are often asymptomatic, with the tumour discovered incidentally during imaging studies for unrelated conditions. When symptoms do occur, they are non-specific and can include flank pain, haematuria, or a palpable abdominal mass, similar to the clinical features of renal cell carcinoma.

Pathological Features

Histopathology

• Renal oncocytomas are composed of uniform, round, or polygonal cells with intensely eosinophilic (pink) granular cytoplasm due to the presence of mitochondria.
• The classic architectural pattern is nested or tubulocystic, often with a central scar.
• A central scar, which is a stellate area of coalescent central bands of fibrosis and compressed blood vessels, is present in a significant percentage of cases (33%)
• Small, round, benign-appearing nuclei without nucleoli.
• Perinuclear halos (clear areas around the nuclei) are often present.

Radiological Features

General Features

• Oncocytomas are typically well-defined, homogeneous solid masses that can occasionally be multifocal and bilateral.
• It usually demonstrates a sharp margin with normal renal parenchyma.
• May display a central hypoattenuating stellate scar which is seen in 33-54% of cases, typically in lesions larger than 5 cm. This scar can be mimicked by central necrosis of renal cell carcinoma (RCC).
• Rarely, calcification, necrosis, cystic change, and haemorrhage may be seen.

Ultrasound

Oncocytomas typically appear as a solid and hypoechoic mass compared to the renal cortex.

• Grayscale: Appear as well-defined, homogeneous, hypo-/isoechoic masses. The central scar, if present, is usually echogenic.
• Color Doppler: May be internal flow with central radiating vessels.
• Contrast-enhanced ultrasound: Enhancement patterns similar to CT, and the central scar in larger lesions may display decreased enhancement.

CT

• Non-contrast: Appear as well-defined, homogeneous soft tissue masses, either isodense or slightly hyperdense relative to the kidney.
• Post-contrast: The degree and timing of enhancement are variable due to variable cellularity, with several potential enhancement patterns. These include intense arterial enhancement with washout due to high cellularity, segmental enhancement inversion due to adjacent areas of high and low cellularity, and hypovascularity with gradual enhancement due to low cellularity and edematous stroma. Cystic changes and haemorrhage are also possible, but typically there are no malignant features such as adenopathy or vascular, collecting system, or perinephric invasion.
• Central stellate non-enhancing scar (33% of cases) can be characteristic, but not diagnostic (also seen in RCC)
• Renal vein thrombosis (bland) may be present

MRI

• T1: Typically display isointense to low signal.
• T2: Heterogenous signal. Intermediate signal from highly cellular components and increased signal from oedematous stroma, central scar.
  • Central scar: Central stellate area of T2 hyperintensity with lack of enhancement during the corticomedullary phase with or without enhancement during subsequent phases
• DWI/ADC: Increased signal in highly cellular components and increased apparent diffusion coefficient (ADC) values can be seen. However, these are not useful in differentiating from RCC.
• T1C+:
  • Segmental enhancement inversion:
    • Corticomedullary phase show components of relatively greater and less enhancement in a renal mass
    • Early excretory phase show inversion of these relative intensities
• Not typically seen/rare: cysts, subacute haemorrhage, haemosiderin, and microscopic lipid

NM

• FDG PET: Typically display less FDG uptake than RCCs, usually isointense to renal tissue.
• Tc-99m DMSA: A photopenic area displacing the cortex and collecting system may be seen.

Differential Diagnosis

• Renal Cell Carcinoma (RCC): It is the most common renal neoplasm and can be difficult to distinguish from oncocytoma based solely on imaging. However, RCC often presents with more heterogeneous enhancement on CT/MRI and may exhibit areas of necrosis or haemorrhage. Also, RCC is typically hyperintense on T2-weighted MRI images and often extends into renal veins or develops regional lymph node metastasis. RCC can also demonstrate a central scar.
  • Chromophobe Renal Cell Carcinoma (ChRCC): ChRCC shares some morphologic and genetic features with oncocytomas. On imaging, it can resemble oncocytomas but tend to be larger and more heterogeneous with a higher likelihood of containing necrotic or cystic components.
  • Clear Cell Renal Cell Carcinoma: Hyperenhancement relative to renal cortex during the corticomedullary phase that becomes less marked during the nephrographic phase is characteristic of clear cell RCC
• Angiomyolipoma: This benign tumour often contains macroscopic fat, which is not seen in oncocytomas. On CT, the presence of fat attenuation can aid in differentiating angiomyolipomas from oncocytomas.
• Metanephric adenoma: Although rare, this benign renal tumour can mimic oncocytoma on imaging. However, it is typically seen in a younger age group (median age around 41 years) compared to oncocytoma.
• Hydatid Cyst: In endemic areas, a hydatid cyst may be included in the differential diagnosis. On imaging, hydatid cysts show characteristic features such as daughter cysts and the hydatid sand.

Management

• The management of renal oncocytomas often involves surgical resection due to the difficulty in distinguishing these tumours from RCCs preoperatively.
• Options include radical nephrectomy (removal of the entire kidney), simple nephrectomy (removal of just the tumour and some surrounding tissue), or partial nephrectomy. If the diagnosis of oncocytoma can be confidently made preoperatively, active surveillance may be an option for patients with small tumours or those who are not candidates for surgery.
• Percutaneous biopsy is often not performed as oncocytomas and the closely related chromophobe renal cell carcinoma share common and overlapping histological findings.