Renal oncocytoma is a type of relatively benign renal tumour. The main clinical importance of this lesion is the difficulty in pre-operatively distinguishing it from renal cell carcinomas, as epidemiology, presentation, imaging and even histology can be very similar.
Renal oncocytomas account for approximately 5% of resected primary adult epithelial renal neoplasms. They typically presents sporadically in the 6th to 7th decades with a peak incidence at 55 years of age. There is 2:1 male predilection 10,13. Unfortunately these demographics are similar to renal cell carcinoma.
Up to three quarters of patients with a renal oncocytoma are asymptomatic. In cases where the mass is large, then a flank or abdominal mass may be the presenting complaint. Occasionally hypertension, haematuria or pain may be the presenting complaint 13.
Macroscopically oncocytomas are tan in colour, similar to the renal cortex, or darker brown (mahogany brown). A rim of compressed normal renal parenchyma is sometimes seen, forming a pseudocapsule 12-13. Macroscopic haemorrhage is seen in a minority of patients (~20%) 13.
Oncocytomas are believed to originate from intercalated tubular cells of the collecting tubules and are composed of large, swollen eosinophilic cells of protuberant mitochondrial components 2,12-13. Biopsy is usually unhelpful in distinguishing an oncocytoma from a renal cell carcinoma, as the latter may also have oncocytic elements 9. Necrosis is usually absent 12.
Unfortunately the imaging appearance of oncocytomas is difficult to distinguish from renal cell carcinoma, and as such they are usually resected 9. They appear as sharply demarcated lesions of variable size, but often large at presentation. A potentially helpful sign is the presence of a sharp central stellate scar, which although characteristic of oncocytomas, is only seen in a third of cases 4. Additionally renal cell carcinomas may also demonstrate a central scar 9.
The only reliable feature is evidence of metastasis or aggressive infiltration into adjacent structures, in which case the diagnosis of renal cell carcinoma can be safely made.
It is important to carefully evaluate both kidneys, as up to 13% of patients have multiple oncocytomas (renal oncocytosis), and up to 32% have concurrent renal cell carcinomas 13.
Oncocytomas appear as sharply demarcated, often large exophytic masses with enhancement during the nephrographic phase.
Ultrasound demonstrates a well circumscribed mass with echogenicity similar to the rest of the kidney. Occasionally a central scar may be visible 9.
Generally tend be large well demarcated tumours at presentation.
- if less than 3 cm - homogenous attenuation
- if more than 3 cm - heterogenous attenuation
- perinephric fat-stranding may be present due to oedema
- calcification may be present
- small tumours may enhance homogeneously, but usually enhancement is heterogenous and the mass is larger
- central stellate non-enhancing scar is seen in approximately a third of cases 4
- renal vein thrombosis may be present, but tumour thrombus is absent
Typical signal characterisitics include
- T1: hypointense compared to renal cortex 14
- hyperintense compared to renal cortex
- may demonstrate hypointense central stellate scar
- T1 C+ (Gd): usually demonstrates homogeneous enhancement
May demonstrate a spoke wheel pattern, of peripheral circumferential vessels penetrating towards the center of the lesion, which again, although characteristic of oncocytomas, may also be seen in renal cell carcinomas.
During the capillary phase the tumour demonstrates a homogenous blush, similar to normal renal parenchyma, lacking the 'bizarre' vascularity typically seen in RCCs 7,9,12. A lucent avascular rim may be seen due to the compressed pseudocapsule 12.
Microaneuryms have occasionally been reported 12.
Treatment and prognosis
Although renal oncocytomas are benign lesions and metastases are extremely uncommon 6,13, as they cannot be confidently pre-operatively distinguished from renal cell carcinomas, they are surgically resected. If the diagnosis is suspected pre-operatively then renal sparing surgery can be performed.
General imaging differential considerations include
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