DISCUSSION
Clinical features of oncocytomas resemble those of other benign and low-grade salivary gland tumours which makes clinical diagnosis challenging.
Patients typically present with a solitary,
slow-growing,
painless parotid mass,3,
4,
6,
8,
9,
16,
19,
20 similar to what was observed in our series.
All 10 cases in our series were diagnosed in adults in their 5th to 8th decades with a female preponderance.
Complete surgical excision of oncocytomas in the form of superficial or total parotidectomy depending on the location of the tumour is the treatment of choice,
with radiotherapy not indicated as oncocytes are radio-resistant.3 Local recurrence for oncocytomas following surgery is uncommon,
although recurrence rates of 20-22% have been reported in the literature.4,
7-9,
23 Malignant forms of oncocytomas or oncocytic carcinomas,
characterised by cytomorphologically malignant oncocytes or evidence of metastasis,
are occasionally reported and account for less than 1% of all salivary gland tumours.1,
3,
6,
14,
24,
25 These are usually associated with a pre-existing oncocytoma but may arise de novo.3 No histological features of malignancy were detected in our series and none of the 7 cases 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 few case reports in the published literature on MRI imaging of parotid oncocytomas describe these tumours as demonstrating T1 and T2 hypointensity with homogenous contrast enhancement.19-21 Özcan et al.
reported an oncocytoma in the deep lobe of the right parotid gland which showed T1 hyperintensity,
T2 hypointensity and heterogeneous contrast enhancement.15 Kasai et al described multiple bilateral oncocytomas which were isointense on STIR images and hyperintense on DWI with corresponding low ADC values.
The tumours also demonstrated early enhancement with early washout on dynamic contrast-enhanced images.21
The ultrasound features of parotid oncocytomas are non-specific and include a hypoechoic mass with well-defined margins,
not unlike other benign parotid tumours such as pleomorphic adenomas.2 Parotid oncocytomas have shown focal uptake of technetium-99m pertechnetate but not with Gallium 67m scintigraphy.19-21 These tumours have also been reported to demonstrate uptake of FDG during PET scanning.26,
27
The common CT findings of parotid oncocytomas described in the reviewed literature are that of a well-defined parotid mass showing homogenous enhancement.
The important differential diagnoses for a well-defined enhancing parotid tumour seen on CT include a Warthin’s tumour and basal cell adenoma.
Warthin’s tumours show enhancement in the early phase post-contrast scan but decreased enhancement in the delayed phase.30 Basal cell adenomas show similar increased enhancement in the early phase post-contrast scan,
with Yerli et al.
describing gradual washout of contrast in the delayed phase.31,
32 Pleomorphic adenomas,
the most common parotid tumour which usually occurs in adults over 40 years with a slight female predominance,2,
26 are considered a less likely differential diagnosis as these demonstrate minimal or no enhancement in the early post-contrast scan but progressive enhancement in the delayed scan.29,
30 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 tumour.2,
17,
18,
30
In addition,
we have reported the features of a non-enhancing 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 non-enhancing 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 histological finding previously associated with oncocytomas.7,
9 These imaging features have not been previously described in parotid oncocytomas,
although Chawla et al.
reported similar ‘linear bands’ and ‘cystic areas’ in parotid basal cell adenomas.31 Shellenberger et al.
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 which is categorically distinct from an oncocytoma.16 Cyst formation has also been commonly associated with Warthin’s tumours.2,
17,
30
The parotid oncocytomas in all of our cases had sharp margins which conveyed benignity,
in contrast to malignant salivary gland tumours which usually demonstrate ill-defined margins.2,
17,
31 The contours of the parotid oncocytomas in 4 of our cases were lobulated,
a feature seen mainly with pleomorphic adenomas but also in Warthin’s tumours and basal cell adenomas.2,
17,
31,
33 Three of our cases showed large tumours which extended to the parapharyngeal space through the stylomandibular gap.
The contours of these tumours were distorted by the surrounding anatomical structures,
giving the appearance of ‘deformable’ tumours which is similar to the CT finding in the case report of Shellenberger et al.16 Rare cases of large parotid deep lobe tumours extending to the parapharyngeal space have been described with Warthin’s tumours and pleomorphic adenomas.34,
35
Warthin’s tumors are usually diagnosed in elderly men,
with 10-15% showing synchronous bilateral disease.
17,
25,
36 There is tendency for oncocytomas to present with synchronous bilateral,
multifocal disease as evidenced by 5 out of 10 cases in our series,
with the reported incidence of synchronous bilateral oncocytomas in the reviewed literature ranging from 7-15%.3,
8,
23
There is therefore overlap of radiological features between oncocytomas and other benign parotid tumours such as Warthin’s tumours,
basal cell adenomas and to less 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 tumours which demonstrate a non-enhancing curvilinear cleft.
These imaging findings will be atypical for pleomorphic adenomas due to their lack of enhancement in the early post-contrast phase.
Bilateral and multifocal parotid tumours on CT also render pleomorphic adenomas and basal cell adenomas as less likely differential diagnoses as these tend to present as unilateral,
solitary tumours.
Although Warthin’s tumours are commonly associated with cyst formation,
they are usually seen in elderly men and have not been reported to demonstrate a non-enhancing curvilinear cleft correlating histologically to a central scar.
Large parotid oncocytomas in our series which extended to the parapharyngeal space through the stylomandibular gap exhibited contour distortion by the surrounding anatomical structures.
The ‘deformable’ appearance of these tumours has only been rarely reported with other benign parotid tumours in the reviewed literature and may therefore be useful in distinguishing parotid oncocytomas on CT.
There are several limitations in our study.
The CT imaging 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 where the CT imaging features of parotid oncocytomas were reported based on consensus by a group of blinded independent observers would have resulted in a reduction in observer bias.
Further studies which examine the imaging features of parotid oncocytomas in conjunction with other parotid tumours (both benign and malignant) are necessary to assess the specificity and positive predictive value of the non-enhancing curvilinear cleft and ‘deformable’ appearance of parotid oncocytomas which 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 tumours,
particularly malignant neoplasms.
Although there are several reports in the published literature which describe the high sensitivity,
specificity and accuracy of ultrasound-guided core-needle biopsy in establishing the histopathological diagnosis for both benign and malignant parotid tumours,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 should malignancy become a concern.
Although an equal number of 2 parotid oncocytomas with non-enhancing curvilinear clefts were detected by each of the 4-slice and 64-slice Toshiba systems using comparable time delays in our study,
the authors recognise that the difference in the speed of image acquisition between the two imaging systems may result in the representation of different phases of tumour enhancement and alter the conspicuity of the non-enhancing curvilinear cleft.
In a small series of major salivary gland tumours which included a single submandibular oncocytoma,
Kei et al.
also described how weak tumour enhancement in an early phase post-contrast CT scan could potentially produce attenuation values similar to that of the surrounding native parotid gland parenchyma and make the tumours inconspicuous.41 It is suggested that a further study be undertaken to examine the conspicuity of parotid oncocytomas and the non-enhancing curvilinear cleft described in our series with dynamic contrast-enhanced CT imaging.
CONCLUSION
A combined clinical,
radiological and pathological assessment of a patient who presents with a parotid mass is essential in establishing an accurate diagnosis.
The CT findings of a non-enhancing curvilinear cleft and ‘deformable’ appearance of parotid oncocytomas described in this largest imaging series to date are potentially helpful in distinguishing these benign lesions from other parotid tumours in clinical scenarios which preclude surgical resection or when biopsy results are non-diagnostic.
Further studies are however advocated to validate the specificity and positive predictive value of these imaging features.