British Journal of Radiology (2003) 76, 271-277
© 2003 British Institute of Radiology
doi: 10.1259/bjr/33081866
High resolution ultrasound assessment of the parotid gland
D C Howlett, MRCP (UK), FRCR
Consultant Radiologist, Radiology Department, Eastbourne District General Hospital, Kings Drive, Eastbourne, East Sussex BN21 2UD, UK
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Abstract
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A variety of neoplastic and non-neoplastic conditions may present with parotid swelling. It is rarely possible to predict the nature of a swelling on clinical grounds alone and accurate imaging is necessary for further assessment. The parotid glands are superficial structures and are readily amenable to high resolution ultrasound examination. Ultrasound is able to: differentiate possible benign from malignant neoplasms; demonstrate whether a palpable lesion arises within the parotid gland, or is periparotid in location; and identify those entities that may not need surgical intervention. In this article the sonographic features of a range of parotid pathologies are reviewed and examples illustrated and the role of ultrasound in parotid assessment is discussed.
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Normal parotid anatomy
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The normal parotid gland appears homogeneous and of increased echogenicity relative to adjacent muscle on ultrasound. This increased echogenicity is related to the fatty glandular tissue composition of the gland [1]. The parotid gland is divided into superficial and deep lobes by a plane at the level of the facial nerve. The facial nerve is not routinely visualized with ultrasound [2] although its position can be inferred as it lies just lateral to the main intraparotid vessels which are readily identified. The retro-mandibular vein lies deep to the facial nerve with the external carotid artery, which is larger, running medially alongside the vein [2]. These vessels are constant ultrasound findings and their identification is important as this allows compartmentalization of focal lesions (Figure 1
). The deep lobe is poorly visualized with ultrasound, as it is obscured by the mandible.
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Figure 1. Longitudinal section through the right parotid gland which demonstrates the homogeneous and hyperechoic nature of the gland texture. The intraparotid vessels are well demonstrated with the retro-mandibular vein (long arrow) lying superficial to the external carotid artery (curved open arrow). Note the mandible (arrowhead).
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Normal intraparotid nodes are frequently observed during ultrasound examination, most commonly in a pre-auricular location or in the tail of the gland [2]. These nodes appear elliptical and hypoechoic with a hyperechoic, fatty, central hilum (Figure 2). Normal intraparotid ducts may be visualized as highly-reflective linear structures (Figure 2).
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Figure 2. Transverse section through the tail of the left parotid gland demonstrates a typical normal, intraparotid node (short arrow). Note the prominent central, hyperechoic hilum and also linear, hyperechoic intraparotid ducts (long arrow).
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Benign tumours
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Pleomorphic adenoma
The majority of parotid tumours are benign and of these 85–90% of lesions are pleomorphic adenomas, which are thought to arise from myoepithelial cells [2]. These tumours typically present as slow growing, painless masses in middle-aged patients and the majority arise in the superficial lobe of the gland. Pleomorphic adenoma typically appears rounded, well circumscribed and hypoechoic on ultrasound and has associated distal acoustic enhancement (Figure 3). Using colour Doppler ultrasound, pleomorphic adenomas may demonstrate a peripheral, "basket-like" pattern of flow [2]. As pleomorphic adenomas enlarge they may develop more atypical characteristics with internal heterogeneity and cystic changes (Figure 4) and lesion margins may become poorly defined [2] making characterization difficult. Long-standing pleomorphic adenomas may develop internal calcification. Ultrasound may be used to guide fine-needle aspiration biopsy or core biopsy [3].
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Figure 3. Longitudinal section through the left parotid gland demonstrates a typical pleomorphic adenoma (callipers) which is homogeneous, hypoechoic and well circumscribed with distal acoustic enhancement. This can be seen to lie superficial to the retro-mandibular vein (arrow) and hence in the superficial lobe.
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Figure 4. Longitudinal section through the left parotid gland demonstrates a lobulated mass in the superficial lobe of the gland distorting the capsule. There is associated distal acoustic enhancement. Note marked heterogeneity of internal architecture. Biopsy confirmed this lesion to represent a pleomorphic adenoma. The mandible is indicated by arrowheads.
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Warthin's tumour (cystadenolymphoma)
These are the second most common benign tumours of the parotid gland and arise from heterotopic parotid tissue within parotid lymph nodes. They are most common in elderly males and tend to occur in the superficial lobe near the angle of the mandible. On ultrasound, Warthin's tumours appear rounded or lobulated, are well circumscribed and internal cystic changes with septations are common (Figure 5). Lesions are multiple or bilateral in 10–15% of patients (Figure 6) [4].
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Figure 5. Longitudinal section through the right parotid gland demonstrates a Warthin's tumour overlying the retro-mandibular vein. This is lobulated with internal cystic elements and hyperechoic septation is demonstrated (arrow).
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Figure 6. Longitudinal section through the left parotid gland demonstrates multifocal Warthin's tumour with a smaller nodule (curved arrow) adjacent to a larger lesion.
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Oncocytoma
These are rare tumours, comprising 1% of parotid neoplasms, which arise from oncocytes derived from striated duct cells. They present as slow growing masses usually in the superficial lobe. They have similar sonographic features to pleomorphic adenoma (Figure 7).
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Figure 7. Longitudinal section through the left parotid gland demonstrates a well circumscribed, hypoechoic solid mass in the superficial lobe near the angle of the mandible. Biopsy was performed (the needle is arrowed) and confirmed this lesion to represent an oncocytoma.
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Lipoma
These are compressible, oval or elliptical masses with regular margins and a typical striped or feathered internal echotexture (Figure 8). Usually no flow signals are detected with colour Doppler examination [2]. MRI is useful for further evaluation of symptomatic, suspected parotid lipoma and will confirm the fatty nature of this lesion.
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Figure 8. Transverse section through the right parotid gland demonstrates a lipoma at its lower pole (callipers). This is well circumscribed with a striped internal echotexture. Note normal parotid tissue (arrow) and the lesion abuts the mandible.
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Retention cysts
Retention cysts occur secondary to parotid tumour, calculus or inflammation. They tend to present with painless swelling, although they may be painful if infected. These lesions are well characterized by ultrasound, appearing thin-walled and anechoic, with distal acoustic enhancement (Figure 9). Multiple parotid cysts are associated with human immunodeficiency virus infection [5].
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Figure 9. Longitudinal section through the left parotid gland demonstrates a retention cyst—anechoic and thin walled with distal acoustic enhancement. Note normal intraparotid vessels delineating superficial from deep lobes (retro-mandibular vein–arrow, external carotid artery–curved open arrow).
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Fatty infiltration
Fatty infiltration causes diffuse, usually bilateral and homogeneous parotid enlargement sonographically. Fatty infiltration can make ultrasound assessment of the parotid difficult due to beam attenuation obscuring deeper parts of the gland.
Other benign lesions
Haemangiomas of the parotid gland are more common in children and appear hypoechoic on ultrasound with prominent internal vascular structures [1]. Neurogenic tumours may also occur and tend to appear well circumscribed and hypoechoic.
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Malignant tumours
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Primary malignancy
Mucoepidermoid carcinoma is the most common primary parotid malignancy accounting for over 80% of cases [4]. The ultrasound features depend upon the histological grade of the tumour [6]. Lower grade, smaller lesions may appear well defined and not dissimilar to pleomorphic adenoma. More aggressive lesions have more typically malignant features appearing irregular and poorly defined with heterogeneous internal architecture (Figure 10). Increased tumoural resistance on colour Doppler ultrasound examination is indicative of malignancy [7]. If a suspected malignant lesion is identified the cervical regions should be assessed for associated adenopathy.
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Figure 10. Longitudinal ultrasound of the left parotid gland demonstrates an irregular, poorly defined mass in the superficial lobe with heterogeneous internal architecture. Biopsy confirmed this to represent mucoepidermoid carcinoma. Note extension of tumour superficially through the parotid capsule (arrow).
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Adenoid cystic carcinoma is the second most common primary parotid malignancy and has a tendency for perineural and perivascular extension, often presenting with pain and evidence of facial nerve involvement. These tumours, as with mucoepidemoid carcinoma, when small often appear circumscribed but develop more overtly malignant ultrasound features as they enlarge (Figure 11). Contrast-enhanced MRI is required to demonstrate perineural extension, which is not well visualized on ultrasound.
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Figure 11. Longitudinal ultrasound through the right parotid gland demonstrates a large, biopsy-confirmed, adenoid cystic carcinoma. The margins are poorly defined and there is tumour extension into the deep lobe (arrow) and through the superficial aspect of the gland (curved open arrow) beneath the skin.
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Other primary parotid malignancies, including acinic cell carcinoma, adenocarcinoma and malignant change in pleomorphic adenoma, are rare and the different forms of primary malignancy are not readily differentiated using ultrasound.
Metastasis
Metastatic involvement of the parotid is usually lymphatic and lesions tend to appear hypoechoic, poorly defined and heterogeneous (Figure 12). Squamous carcinoma or melanoma of the head and neck represent the most common sites of primary tumours. Haematogenous spread from lung, breast or renal carcinoma may also occur.
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Figure 12. Longitudinal section through the right parotid gland demonstrates an irregular, poorly defined mass in the superficial lobe. Biopsy confirmed metastatic squamous cell carcinoma from a primary on the ipsilateral pinna.
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Lymphoma
Primary parotid lymphoma is rare and lymphoma usually involves the gland via haematogenous spread. There is an increased incidence of parotid lymphoma in patients with Sjögren's syndrome. The ultrasound features of parotid lymphoma are variable. Involved nodes may have a "pseudocystic" appearance (Figure 13), although a micronodular pattern and large masses may also occur [8].
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Figure 13. Longitudinal section through the right parotid gland demonstrates multiple enlarged, "pseudocystic" nodes in a patient with B-cell lymphoma. Biopsy confirmed lymphomatous nodal involvement.
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Parotid inflammatory disease
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Acute sialadenitis
Acute sialadenitis may be viral, when it is commonly secondary to mumps infection, or bacterial in origin. Bacterial infection usually relates to sepsis of adjacent structures, e.g. skin, ear, teeth, parotid calculi or systemic disease causing immunosuppression. On ultrasound, in acute inflammation, the gland enlarges, appears hypoechoic and of heterogeneous echotexture (Figures 14 and 15). In severe cases, abscess formation may occur (Figure 16). Ultrasound is highly sensitive in the detection of parotid calculus formation (Figure 17) and is the initial imaging modality of choice in patients with a history of recurrent swelling suggestive of calculus disease. In those patients with negative ultrasound where further assessment is required, or in those where intervention is being considered, sialography is then indicated. Recurrent infections may cause gland atrophy and fibrous tissue replacement of the normal gland.
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Figure 14. Longitudinal section through the right parotid gland in a patient with acute, painful parotid swelling. The gland is enlarged, hypoechoic and of a heterogeneous echotexture. The appearances are consistent with acute parotitis. Note an enlarged intraparotid reactive lymph node (curved open arrow).
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Figure 15. Longitudinal section through the left parotid gland in a patient with acute parotitis secondary to Escherichia coli. The gland is enlarged and hypoechoic. Multiple hyperechoic foci are present within the gland (long arrow) which represent air within intraglandular ducts secondary to sepsis. Note associated comet-tail artefacts. The normal facial nerve can be identified as a fibrillar structure (curved open arrow) lying lateral to the retro-mandibular vein.
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Figure 16. Longitudinal section through the right parotid gland in a middle-aged male who presented with acute onset of painful parotid swelling. The gland appears hypoechoic and inflamed and there is a poorly defined hypoechoic mass in the superficial lobe. The appearances are suggestive of parotid abscess formation and this was confirmed following ultrasound-guided aspiration of 5 ml of pus from the lesion from which viridans streptococcus was isolated.
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Figure 17. Ultrasound of the tail of the left parotid gland in an elderly male with a history of intermittent parotid swelling. This demonstrates a small intraglandular calculus (arrow) with associated distal acoustic shadowing.
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Chronic granulomatous parotitis
The parotid gland may be involved with infective and non-infective granulomatous disease with sarcoidosis and Sjögren's syndrome the principal non-infective causes.
Infective granulomatous parotitis
Infective causes of granulomatous parotitis include mycobacterial infection, actinomycosis and histoplasmosis. Tuberculous parotitis usually presents as painless enlargement of an involved intraparotid node, although diffuse enlargement of the whole gland may occur [9]. Tuberculous infection is generally secondary to intraglandular lymph node spread (Figure 18).
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Figure 18. Longitudinal section through the tail of the right parotid gland in a patient who presented with painless enlargement of a nodule. Ultrasound demonstrates an enlarged hypoechoic node (callipers) which is heterogeneous and the fatty hilum is displaced peripherally. Biopsy was performed and confirmed involvement with M. tuberculosis with caseating granulomata demonstrated.
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Sarcoidosis
Sarcoidosis involves the parotid glands in 6% of cases and may be associated with uveitis and facial paralysis (Heerfordt's disease). The ultrasound features are variable ranging from nodal enlargement to diffuse and hypoechoic enlargement of both parotid glands (Figure 19).
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Figure 19. Longitudinal section through the right parotid gland in a patient who presented with painless bilateral parotid and submandibular gland enlargement. The parotid gland is enlarged and hypoechoic and the texture is heterogeneous. Biopsy confirmed infiltration with sarcoid granulomata.
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Sjögren's syndrome
Sjögren's syndrome is an auto-immune disease presenting with a typical triad of keratoconjunctivitis sicca, xerostomia and auto-immune disorder [2]. In the early stages, the gland may appear normal on ultrasound but later the gland enlarges, appearing of heterogeneous echotexture and multiple 2–3 mm hypoechoic foci are observed (Figure 20) [10]. These hypoechoic areas represent areas of non-obstructive sialectasis and large, septated cysts may also develop (Figure 20) before atrophy supervenes in end-stage disease [2]. Ultrasound is used for monitoring patients with Sjögren's syndrome for lymphoma development to which they are prone.
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Figure 20. Longitudinal section through the right parotid gland in a patient with Sjögren's syndrome. The gland is enlarged and heterogeneous and small internal hypoechoic foci are identified (arrow) which represent areas of sialectasis. Note a larger septated cyst (curved open arrow).
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Conclusion
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Ultrasound represents the initial imaging modality of choice for the assessment of palpable abnormalities of the parotid gland and also of suspected parotid calculus disease. Ultrasound is able to demonstrate benign and malignant features of focal lesions and can be used to guide fine-needle aspiration biopsy or core biopsy to confirm their nature. Small, well-differentiated primary parotid gland malignancies may appear benign on ultrasound and all such lesions should be considered for biopsy to exclude malignancy. If ultrasound is able to compartmentalize a mass to the superficial lobe and biopsy confirms benign pathology then no further imaging is required. Ultrasound is able to guide the need for further imaging (CT or MRI) in those lesions with sonographically malignant features or large masses whose extent is difficult to assess with ultrasound, particularly if deep lobe involvement is suspected.
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Acknowledgments
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The author would like to thank Louise Pellett for typing the manuscript and Nick Taylor for preparation of the illustrations.
Received for publication June 28, 2002.
Revision received October 7, 2002.
Accepted for publication November 27, 2002.
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Abstract
Normal parotid anatomy
