Oncocytoma is the most common benign solid renal tumor and constitute 3-7% of all primary renal neoplasms. [1, 2, 3, 4] Oncocytoma affects mainly older patients, often in the seventh decade of life. Oncocytomas are most often discovered incidentally by imaging during a workup for other conditions. About one third of patients have signs and/or symptoms, with flank/abdominal pain, hematuria, and/or a flank mass being the most common. Rare familial cases have been described, and many of these patients have Birt-Hogg-Dubé syndrome. [5, 6]
The differential diagnosis of oncocytoma includes other neoplasms with eosinophilic or oncocytic cytoplasm, primarily chromophobe renal cell carcinoma (RCC), but also clear-cell RCC with eosinophilic cytoplasm and oncocytic variant of papillary RCC. Infrequently, oncocytoid RCC occurring after neuroblastoma and epithelioid angiomyolipoma ought to be included in the differential. [7, 8, 9, 10, 11, 12, 2, 13]
Because onocytoma is a benign disease, differentiating it from RCCs is important in that it can often be treated conservatively. [14, 9] . Size of RCCs, lesion enhancement in the excretory phase, and heterogeneous patterns can help distinguish RCCs from oncocytomas. [12, 13] The central stellate scar can help differentiate renal oncocytomas, but this scar may also be seen in cases of RCCs, thereby limiting its accuracy as a differentiating feature. 
Oncocytosis is a rare condition characterized by multiple oncocytic tumors with histologic features including renal oncocytoma, hybrid oncocytic tumor, and, occasionally, chromophobe RCC. Oncocytic nodules may be present in one or both kidneys, usually with one dominant nodule. [6, 4, 15]
Computed tomography (CT) scanning of the abdomen, combined with intravenous administration of iodinated contrast medium, is the examination of choice and the best modality for the evaluation of a solid renal mass. This technique assists in the detection and localization of the tumor, and CT scanning may help in characterizing the mass, especially if fat-containing lesions (eg, angiomyolipomas) are present. Additionally, staging of the tumor can be performed to classify the extent of the lesion, regional lymphadenopathy, vascular involvement, and metastases. CT scanning also helps in the detection of calcifications and in the differentiation of a complex cyst from a solid neoplasm. [7, 16, 17, 8, 14, 8, 14, 10, 11, 12, 2]
The imaging characteristics of oncocytomas and RCCs overlap, and differentiating an oncocytoma from an RCC and other solid renal neoplasms is not always possible with ultrasonography, CT scanning, or magnetic resonance imaging (MRI). The presence of a central scar on CT scans or MRIs and a spoke-wheel pattern of vessels on angiograms are often suggestive of oncocytoma but are not entirely specific. [8, 14, 10, 11, 12, 2]
Because of the overlap in imaging features and histologic appearances between oncocytomas and RCCs, accurate differentiation on preoperative imaging or percutaneous biopsy remains difficult. The diagnosis is often retrospectively established by means of gross pathology and microscopy with special stains. [8, 14, 10, 11, 12, 2]
The degree of confidence is low for detecting oncocytomas with radiographs. Plain radiographic findings are nonspecific, and images may demonstrate a large, soft-tissue mass in the renal area with displacement of the fat planes. Calcification is rare. Excretory urography shows a large mass with a renal-contour abnormality and compression of the collecting system. False-positive results may arise with any lesion that causes a renal-contour abnormality. Examples include renal cysts, any renal mass, and focal infections. In addition, any retroperitoneal tumor may have a similar appearance. False-negative findings are due to the small size of the tumor and the presence of overlapping bowel.
 On nonenhanced CT scans, oncocytomas appear isoattenuating or slightly hyperattenuating relative to the kidney parenchyma. On contrast-enhanced CT scans that are obtained during the nephrographic phase, the mass appears less attenuating than the renal parenchyma. (See the images below.) [2, 8, 10, 11, 12, 16, 17, 18]
Oncocytomas are well encapsulated and have distinct margins, a smooth contour, and a homogeneous appearance. The tumors may range in size from 3-10 cm, and in symptomatic patients, the lesions are most often larger than 5 cm.
A central hypoattenuating scar may be observed in 33% of cases, but this scar cannot be differentiated from the central necrosis commonly found in renal cell carcinoma (RCC). With the advent of multisection CT scanning, high-resolution thin sections through the kidneys may improve detection of the central scar.
Studies have discussed the use of segmental enhancement inversion to help identify small renal oncocytomas. Segmental enhancement inversion is present when the renal oncocytoma is divided into 2 differently enhanced segments on corticomedullary phase images (30–40 seconds after contrast injection) that demonstrate reversal of the relative degree of enhancement on the nephrographic phase images (120-180 seconds after contrast injection). Some studies have found that segmental enhancement inversion was often present in oncocytomas smaller than 3 cm, while central scars were more often seen in oncocytomas larger than 2.5 cm. [19, 8]
Calcification, necrosis, and hemorrhage are rare with oncocytomas. Typically, features of a malignant tumor—such as invasion or infiltration into the perinephric fat, collecting system, or vessels—are absent. Likewise, regional lymphadenopathy and metastases are not encountered in patients with oncocytoma. Occasionally, multifocal or bilateral tumors may be found.
The degree of confidence for detecting this tumor with CT scanning is high. However, the degree of confidence in differentiating an oncocytoma from an RCC is low. Differentiating an oncocytoma from an RCC and other solid renal tumors is not always possible. The presence of a central scar may help, but necrosis in RCC may mimic this finding. [8, 14, 10, 11]
In a study of 43 patients with 53 oncocytomas and 123 patients with 128 renal cell carcinomas who underwent biphasic contrast-enhanced CT, a combination of CT attenuation values and tumor texture (ie, heterogeneity and skewness) helped differentiate oncocytomas from RCCs, including clear cell and other subtypes. 
Using 4-phase CT to differentiate between RCC and oncocytomas in 97 patients, lesion size larger than 4 cm, lesion enhancement in the excretory phase in relation to unenhanced phase, and heterogeneous enhancement patterns helped differentiate the tumors. The RCCs were larger and often more enhanced heterogeneously. 
Magnetic Resonance Imaging
On nonenhanced T1-weighted MRIs, oncocytomas are well-defined, homogeneous masses. They may appear isointense to hypointense relative to the renal cortex. On T2-weighted images, the tumors are typically isointense to slightly hypointense; however, slight T2 hyperintensity has also been reported.  For MRIs of oncocytoma, see the images below.
When present, the tumor's scar may be seen as a hypointense, stellate area in the center of the lesion on T1- and T2-weighted MRIs. However, tumor necrosis, a common feature of malignant masses, appears hypointense on T1-weighted images and hyperintense on T2-weighted images. Rarely, the central scar may appear bright on T2-weighted images.
After the intravenous administration of gadopentetate dimeglumine contrast material, oncocytomas show homogeneous enhancement, with a nonenhancing central scar.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
The degree of confidence with MRIs for detecting this lesion is high, but this technique's degree of confidence is low for making a specific diagnosis. Differentiating an oncocytoma from an RCC and other solid renal tumors is not always possible.
In a study of relative enhancement of multiphasic MRI to differentiate RCCs from oncocytomas, relative signal intensity in the corticomedullary phase differentiated clear cell RCC from oncocytomas with an AUC of 0.90 and an accuracy of 84%, sensitivity of 90%, and specificity of 63%. 
Ultrasonography has low sensitivity and specificity in the detection and characterization of solid renal masses. However, this modality is useful for differentiating a solid mass from a complex cystic mass. On ultrasonography images, oncocytomas appear as well-defined, homogeneous, and hypoechoic to isoechoic masses. The central scar cannot be confidently identified on sonograms; however, when the scar is seen, especially in large lesions, it may appear echogenic. Color Doppler ultrasonography may show central radiating vessels. [21, 22] Small isoechoic lesions may be missed on sonograms. Larger lesions cannot be differentiated from other renal masses. However, RCCs often demonstrate more heterogeneous echogenicity and irregularly shaped and poorly demarcated margins with the adjacent tissue. 
Scintigraphy is not routinely performed in the evaluation of renal tumors. On technetium-99m (99m Tc) dimercaptosuccinic acid (DMSA) scans, the oncocytoma appears as a photopenic area that displaces the renal cortex and collecting system.
On fluorodeoxyglucose (FDG) positron emission tomography (PET) scans, oncocytomas usually have less FDG uptake than RCCs. The amount of uptake is usually isointense relative to the renal parenchyma. However, oncocytomas can occasionally have uptake in the range similar to that of RCC uptake.
Degree of confidence
The degree of confidence with nuclear medicine is low.
False-positive results are due to renal cysts or other photopenic renal masses. False-negative results occur if the mass is small or if the lesion does not cause a renal-contour abnormality.
With the advent of CT scanning, routine angiography is not performed to diagnose renal masses. However, the classic angiographic findings for oncocytomas include a spoke-wheel arrangement of tumoral vessels, homogeneous tumoral contrast during the capillary phase, sharp demarcation from the kidney and surrounding areas, and a peritumoral halo (lucent-rim sign). Bizarre neoplastic vessels are conspicuously absent, which is in contrast to RCC.
Degree of confidence
The degree of confidence with angiography is low.
Hypovascular lesions may result in false-negative results, and hypervascular lesions may mimic RCCs.