CT Features of Parotid Gland Oncocytomas: A Study of 10 Cases and Literature Review

Discussion

The clinical features of oncocytomas resemble those of other benign and low-grade salivary gland tumors making clinical diagnosis challenging. Patients typically present with a solitary slow-growing painless parotid mass,^{3,4,6,8,9,16,19,20} similar to that observed in our series. All 10 patients in our series were adults in their fifth to eighth decades with a female preponderance.

Complete surgical excision of oncocytomas in the form of superficial or total parotidectomy, depending on the location of the tumor, is the treatment of choice, with radiation therapy not indicated because oncocytes are radioresistant.³ Local recurrence for oncocytomas following surgery is uncommon, though recurrence rates of 20%–22% have been reported in the literature.^4,7–9,23 Malignant forms of oncocytomas or oncocytic carcinomas, characterized by cytomorphologically malignant oncocytes or evidence of metastasis, are occasionally reported and account for <1% of all salivary gland tumors.^{1,3,6,14,24,25} These are usually associated with a preexisting oncocytoma but may arise de novo.³ No histologic features of malignancy were detected in our series, and none of the 7 patients who underwent surgery showed clinical features of disease recurrence following surgery.

Due to their low prevalence, only a few case reports on the CT imaging features of parotid oncocytomas are available in the published literature.^{15,16,19–21} The reports on MR imaging of parotid oncocytomas describe these tumors as demonstrating T1 and T2 hypointensity with homogeneous contrast enhancement.^19–21 Özcan et al¹⁵ reported an oncocytoma in the deep lobe of the right parotid gland, which showed T1 hyperintensity, T2 hypointensity, and heterogeneous contrast enhancement. Kasai et al²¹ described multiple bilateral oncocytomas that were isointense on short τ inversion recovery imaging and hyperintense on diffusion-weighted imaging, with corresponding low apparent diffusion coefficient values. The tumors also demonstrated early enhancement with early washout on dynamic contrast-enhanced images.

The sonography features of parotid oncocytomas are nonspecific and include a hypoechoic mass with well-defined margins, not unlike other benign parotid tumors such as pleomorphic adenomas.² Parotid oncocytomas have shown focal uptake of technetium Tc99m pertechnetate but not with gallium Ga67m scintigraphy.^19–21 These tumors have also been reported to demonstrate uptake of fluorodeoxyglucose during positron-emission tomography scanning.^26,27

The common CT finding of parotid oncocytomas described in the literature is a well-defined parotid mass showing homogeneous enhancement. The important differential diagnoses for well-defined enhancing parotid tumors seen on CT include the Warthin tumor and basal cell adenomas. Warthin tumors show enhancement in the early postcontrast scan phase but decreased enhancement in the delayed phase.²⁸ Basal cell adenomas show similar increased enhancement in the early postcontrast scan phase, and gradual washout of contrast in the delayed phase.²⁹ Pleomorphic adenomas, the most common parotid tumors, which usually occur in adults older than 40 years of age with a slight female predominance,^2,30 are considered a less likely differential diagnosis because these demonstrate minimal or no enhancement in the early post-contrast scan phase but progressive enhancement in the delayed phase.^28,31 A low-grade parotid malignancy is an important differential diagnosis and a major diagnostic pitfall in the imaging assessment of a well-defined enhancing parotid tumor.^2,17,18,28

In addition, we have reported the features of a nonenhancing curvilinear cleft and a cystic component in the 6 cases of parotid oncocytomas with heterogeneous enhancement in our series. We concluded, by correlating with histology findings, that the nonenhancing curvilinear cleft was most likely secondary to a central scar. The cystic component seen on CT probably corresponded to an area of cystic degeneration, a histologic finding previously associated with oncocytomas.^7,9 These imaging features have not been previously described in parotid oncocytomas, though Chawla et al³² reported similar linear bands and cystic areas in parotid basal cell adenomas. Shellenberger et al¹⁶ described the CT finding of a cystic parotid tail mass, which was confirmed histopathologically as nodular oncocytic hyperplasia, a multifocal process with diffuse oncocytic replacement of the parotid gland that is categorically distinct from an oncocytoma. Cyst formation has also been commonly associated with the Warthin tumors.^2,17,28

The parotid oncocytomas in all of our patients had sharp margins, which conveyed benignity, in contrast to malignant salivary gland tumors, which usually demonstrate ill-defined margins.^2,17 The contours of the parotid oncocytomas in 4 of our patients were lobulated, a feature seen mainly with pleomorphic adenomas but also in Warthin tumors and basal cell adenomas.^2,17,32,33 Three of our patients had large tumors that extended to the parapharyngeal space through the stylomandibular gap. The contours of these tumors were distorted by the surrounding anatomic structures, giving the appearance of deformable tumors, which is similar to the CT findings in the case report of Shellenberger et al.¹⁶ Rare cases of large parotid deep lobe tumors extending to the parapharyngeal space have been described with Warthin tumor and pleomorphic adenomas.^34,35

Warthin tumors are usually diagnosed in elderly men, with 10%–15% showing synchronous bilateral disease.^17,30,36 There is a tendency for oncocytomas to present with synchronous bilateral multifocal disease as evidenced by 5 of 10 patients in our series, with the reported incidence of synchronous bilateral oncocytomas in the reviewed literature ranging from 7% to 15%.^3,8,23

There is, therefore, overlap of radiologic features between oncocytomas and other benign parotid tumors such as Warthin tumors, basal cell adenomas, and, to a lesser degree, pleomorphic adenomas. However, when taken together, the diagnosis of a benign parotid oncocytoma is favored in a middle-aged or elderly woman who presents with CT findings of well-defined, enhancing bilateral, and multifocal parotid tumors that demonstrate a nonenhancing curvilinear cleft. These imaging findings will be atypical for pleomorphic adenomas due to their lack of enhancement in the early postcontrast phase. Bilateral and multifocal parotid tumors on CT also render pleomorphic adenomas and basal cell adenomas less likely differential diagnoses because these tend to present as unilateral solitary tumors.

Although Warthin tumors are commonly associated with cyst formation, they are usually seen in elderly men and have not been reported to demonstrate a nonenhancing curvilinear cleft correlating histologically to a central scar. Large parotid oncocytomas in our series that extended to the parapharyngeal space through the stylomandibular gap exhibited contour distortion by the surrounding anatomic structures. The deformable appearance of these tumors, to our knowledge, has only been rarely reported with other benign parotid tumors in the reviewed literature and may, therefore, be useful in distinguishing parotid oncocytomas on CT.

There are several limitations in our study. The CT findings of the 10 cases of parotid oncocytomas in our study were described by a single unblinded senior head and neck radiologist. The authors acknowledge that a retrospective study in which the CT imaging features of parotid oncocytomas were reported on the basis of consensus by a group of blinded independent observers would have resulted in a reduction in observer bias. Further studies that examine the imaging features of parotid oncocytomas in conjunction with other parotid tumors (both benign and malignant) are necessary to assess the specificity and positive predictive value of the nonenhancing curvilinear cleft and deformable appearance of parotid oncocytomas that were described in our imaging series. A high specificity and positive predictive value of these imaging features for parotid oncocytomas may potentially aid the reporting radiologist in excluding other parotid tumors, particularly malignant neoplasms.

Although there are several reports in the published literature that describe the high sensitivity, specificity, and accuracy of sonography-guided core-needle biopsy in establishing the histopathologic diagnosis for both benign and malignant parotid tumors,^37–40 the potential for misdiagnosis in the 3 patients who underwent needle biopsy exists, in particular with respect to differentiation from oncocytic carcinomas. Judicious follow-up of these 3 patients is suggested, with a view for further imaging and repeat biopsy if malignancy becomes a concern.

Although an equal number of 2 parotid oncocytomas with nonenhancing curvilinear clefts were detected by each of the 4- and 64-section Toshiba systems which used comparable time delays in our study, the authors recognize that the difference in the speed of image acquisition between the 2 imaging systems may result in the representation of different phases of tumor enhancement and alter the conspicuity of the nonenhancing curvilinear cleft. In a small series of major salivary gland tumors that included a single submandibular oncocytoma, Kei and Tan⁴¹ also described how weak tumor enhancement in an early-phase postcontrast CT scan could potentially produce an attenuation value similar to that of the surrounding native parotid gland parenchyma and make the tumors inconspicuous. We suggest that a further study should be undertaken to examine the conspicuity of parotid oncocytomas and the nonenhancing curvilinear cleft described in our series using dynamic contrast-enhanced CT imaging.