Renal oncocytoma is one of the most common benign renal neoplasms, accounting for around 3–7% of all renal tumours, and up to 18% of all small renal masses (SRMs) measuring less than 4cm [1, 2].
Most oncocytomas are asymptomatic and identified incidentally on imaging performed for other reasons. Large oncocytomas may present with flank pain, a palpable abdominal mass, haematuria and occasionally refractory hypertension. There is a higher prevalence in men and peak incidence is in the seventh decade of life [3].
Gross appearance of oncocytoma is usually a tan-brown or mahogany-coloured mass, similar in colour to the normal renal parenchyma, which is well circumscribed. This contrasts with the golden-yellow colour of clear cell renal cell carcinoma (RCC) [4]. The mass is usually in the cortex, but large masses can extend into the surrounding fat or even the renal vein. Haemorrhage may be present but there is usually no necrosis [5].
A central stellate scar is characteristic of oncocytoma but is neither specific for oncocytoma nor is it always present and may be present in as low as 30% of cases [6]. The central stellate scar may also be present in other slow-growing neoplasms and malignant chromophobe renal cell carcinoma (ChRCC). ChRCC has the most morphological overlap with oncocytoma and presents a diagnostic challenge [7].
In rare cases, oncocytomas may overlap with malignant tumours. This includes so-called “other oncocytic renal neoplasms,” which represent a heterogeneous group of tumours that are generally of low malignant potential, as well as hybrid oncocytic-chromophobe tumours (HOCTs), which exhibit overlapping features of oncocytoma and chromophobe renal cell carcinoma (ChRCC). HOCTs can occur sporadically but are a characteristic feature of Birt-Hogg-Dubé syndrome, an autosomal dominant disorder caused by mutations in the folliculin gene on chromosome 17p11.2, which confers susceptibility to benign and malignant renal tumours, pulmonary cysts and fibrofolliculomas [8].
Figure 1: Plain CT (top) and MIBI (bottom) appearances of oncocytoma (left) and RCC (right).
The red crosshair marks the location of the lesion in both.
Diagnosis
Imaging
Radiological diagnosis of oncocytoma is challenging. Conventional ultrasound (USS) is poor at differentiating between solid benign and solid malignant lesions. Oncocytomas have variably been described as hypoechoic, isoechoic or mildly hyperechoic compared to the surrounding renal parenchyma, and when present, a central stellate scar may be visible along with a spoke-wheel vascular pattern [6]. Likewise cross-sectional imaging, such as CT and MRI, has similar limitations with central scar being present in only a minority of cases and other characteristic features, such as perinephric fat or renal vein invasion showing no significant difference in prevalence between oncocytoma and ChRCC [9].
99mTc-sestamibi SPECT/CT (MIBI) is a relatively new and promising new nuclear medicine tool in the diagnosis of renal oncocytomas. Compared to RCCs, oncocytomas have a high density of mitochondria, and Sestamibi is a lipophilic cation which has been shown to accumulate in cells with a high density of mitochondria. Oncocytomas will therefore show good uptake of the sestamibi tracer and appear “hot” on the MIBI scan, similar to the surrounding renal parenchyma, whereas RCCs will show poor uptake and appear “cold” [10].
Biopsy
Renal mass biopsy (RMB) is increasingly used in the diagnosis of renal masses, including suspected oncocytomas. On biopsy the predominant cells will be oncocytes – large, eosinophilic cells rich in mitochondria. However, concordance between biopsy results and final resection pathology is variable, ranging from as low as 64.5% to as high as 92.2%, and some guidelines caution again making a definitive diagnosis of oncocytoma on biopsy [11,12].
Management
Resection
For decades, the management of renal masses, including suspected oncocytomas, has been surgical resection, either partial nephrectomy or radical nephrectomy, depending on size, for both diagnostic and therapeutic purposes. Minimally invasive, including robotic, techniques have improved surgical outcomes and reduced morbidity, however, there remains substantial risk associated with resection [13].
Active surveillance
Active surveillance (AS) has been proposed as an alternative management option for patients with biopsies suggesting oncocytoma, to reduce overtreatment and avoid the risks associated with resection. AS is not universally accepted, in part owing to fear of poor concordance between renal biopsy and final resection pathology and the risk of mis-labelling malignant ChRCC as oncocytoma. However, it could be argued that the risk of metastasis of a small ChRCC is sufficiently low as to still warrant AS in cases of presumed oncocytoma [12].
AS of oncocytoma appears to be safe, avoiding the risks associated with resection. A recent systematic review found no reports of metastases or disease-related death in patients managed with AS, and no deaths recorded to oncocytoma on post-mortem [14]. Features of aggressiveness, such as perinephric fat or vascular invasion, when associated with oncocytoma, continue to display non-malignant behaviour and have excellent prognosis [15].
Oncocytomas appear to be slow-growing tumours, with average annual growth rates of between 1.55mm/year to 2mm/year; however, their behaviour is heterogeneous, ranging from size regression and stability to growth rates of up to 5mm/year. Local progression appears to be at a similar rate to that of RCC and no variables, including initial tumour size, seem to correlate with growth rate [16,17].
AS appears to have no detrimental effect on renal function, with a mean change per year in eGFR of −0.99 mL/min/1.73 m2/year, which compares favourably to an average drop of −1 to −1.2 mL/min/1.73m2/year in the healthy population, increasing with age. Compared to patients undergoing partial nephrectomy, AS patients have similar long-term renal function outcomes, although AS patients tend to be older and have a lower baseline renal function. Initial tumour size also does not seem to correlate with renal function [18,19].
The proportion of patients undergoing conversion to active treatment after a period of AS has been variably reported as between 5% and 40% [16]. The most common reasons given for conversion to active treatment include rapid tumour growth, patient preference, onset of symptoms, change in tumour shape and independent review of biopsy suggesting raised possibility of ChRCC. Crucially, a recent study found that patients who underwent delayed radical nephrectomy following a period of AS had lesions that were not amenable to partial nephrectomy at time of initial diagnosis, meaning that no patient lost the opportunity to undergo nephron-sparing surgery by virtue of being on AS [18].
Previous studies had suggested that oncocytomas measuring more than 5cm should be definitively treated; however, more recent evidence suggests that even larger oncocytomas of up to 7cm can be safely managed on AS [18].
Suggested protocol
There are few if any published protocols for active surveillance of renal oncocytoma, but existing protocols for small renal masses (SRMs) could be adapted.
A proposed protocol is as follows:
Eligibility
- Biopsy-proven renal oncocytoma with classical features
- Size less than 7cm
- Patient asymptomatic
- Patient accepts AS and understands the small but nevertheless present risk of misclassification on biopsy.
Triggers for conversion to treatment
- Sustained growth of more than 5mm/year on CT/MRI
- Size approaching the limit of nephron sparing approaches
- Worsening renal function, when combined with imaging which suggests debulking may help preserve renal parenchyma
- Patient preference
- Symptomatic disease
- Distant metastases or other evidence to suggest the lesion may not be a pure oncocytoma.
Figure 2: Proposed guidelines for management of oncocytoma, including active surveillance.
Conclusion
Oncocytomas are common and benign renal tumours which are often asymptomatic and have excellent prognosis; however, diagnosis is challenging and therefore resection has traditionally been the mainstay of treatment. Active surveillance has been proposed more recently and appears to be increasingly safe, even for larger oncocytomas. Growth rates are low, metastasis is exceptionally rare, there are no detrimental effects on renal function and even delayed treatment does not carry any increased morbidity. However, there is still lack of universal acceptance of AS, and published protocols are few and far between.
References
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[All links last accessed December 2025]
Declaration of competing interests: None declared.
