Parotid Gland and Other Salivary Gland Tumors

Abstract

Parotid gland masses are most frequently benign tumors. In order to determine the most appropriate therapy, MR imaging is the best imaging tool to evaluate tumor topography, locoregional extension, relationship with the facial nerve and even its nature, benign or malignant. MR diffusion-weighted images and dynamic contrast-enhanced sequences are helpful to differentiate benign from malignant tumors, and in case of atypical tumors. This chapter reviews the imaging characteristics of parotid gland and other salivary glands tumors.

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1 Introduction

The parotid gland is the largest salivary gland. It is located in the parotid space. The parotid gland can be affected by a variety of pathologic processes, especially neoplastic. Parotid tumors represent less than 3% of all head and neck tumors and are most frequently benign. These tumors require surgery in most cases and imaging is essential in the workup of these lesions.

2 Anatomy

The parotid space is a paired lateral suprahyoid neck space surrounded by the superficial layer of the deep cervical fascia. This space extends from the external auditory canal and the mastoid tip superiorly to the angle of the mandible below. It contains the parotid gland, intra- and extra-parotid lymph nodes. The gland contains about 20 intraglandular lymph nodes which are considered normal if their transverse diameter is less than 8 mm. The gland also contains extracranial branches of the facial nerve, and vessels: the external carotid artery and the retromandibular vein just behind the mandibular ramus (Fig. 1).

Fig. 1

Anatomy of the parotid space The parotid space (surrounded in red) is made of the parotid gland, the retromandibular vein (blue circle), the external carotid artery (red circle) and intra- and extra-parotid lymph nodes. It also contains the facial nerve, which is not directly seen on imaging; it is used as a reference plane (green line) to separate the parotid gland into an external superficial lobe and an internal deep lobe

The facial nerve exits the skull base via the stylomastoid foramen and continues within the parotid gland from its posterior and superior part to its anterior and inferior part, lateral to the retromandibular vein. It then divides into superior temporo-facial branches and inferior cervical branches. By convention, the facial nerve is used as a reference plane within the gland to separate the external superficial lobe and the internal deep lobe, but actually there is no true anatomic division. The facial nerve is not seen with imaging and its course can only be estimated (Fig. 1; Harnsberger 2004).

The parotid space is directly lateral to the anterior part of the parapharyngeal space (prestyloid space), without real anatomic division between these two spaces. The deep portion of the parotid gland bulges in the prestyloid compartment in which deep lobe tumors can extend (Fig. 2). The prestyloid compartment also contains fat tissue, accessory salivary glands and a prestyloid branch of the mandibular nerve (V3).

Fig. 2

Lateral and internal to the parotid space is the anterior part of the parapharyngeal space (or prestyloid compartment—both surrounded in red); there is no real anatomic division between these two spaces. The most internal part of the deep parotid lobe bulges in the prestyloid compartment. Posterior (retrostyloid) part of the parapharyngeal space, also called the carotid space (surrounded in green), separated from the anterior parapharyngeal space by the styloid process (indicated in yellow) as well as muscles and a fascial layer originating from it (see also “Parapharyngeal Space Neoplasms”). Masticator space (surrounded in blue)

Anterior to the parotid space is the masticator space or infratemporal fossa, which contains the pterygoid muscles.

Posterior to the prestyloid parapharyngeal space is the carotid space or retrostyloid space (Fig. 2).

There is an anatomic division between these different spaces (the masticator space, the carotid space and the anterior parapharyngeal space). Benign tumors will respect these anatomic limits whereas they will be infiltrated by malignant tumors.

The parotid gland also contains salivary ducts. The main parotid duct, or Stensen’s duct, emerges from the anterior part of the parotid gland, runs over the masseter muscle and in the superficial cervical fascia and then abruptly courses medially to pierce the buccinator muscle, forming a nearly 90° angle with this muscle, terminating in the buccal mucosa at the level of the second upper molar (Fig. 3).

Fig. 3

The main parotid salivary duct canal or Stensen’s duct (in red) emerges from the anterior part of the parotid gland, runs over the masseter muscle and in the superficial cervical fascia, then forms a 90° angle (arrow), pierces the buccinator muscle to terminate in the oral cavity at level of the second upper molar

3 Imaging Issues

When a patient presents with a palpable mass of the parotid space, the radiologist has to answer several key questions, which are essential to the head and neck surgeon in order to determine the best therapy (Harnsberger 2004; Shah 2002; Vogl et al. 1999):

• Is the mass intra or extraparotid?

  Small, intraparotid masses are easy to identify. For large and deep lobe masses, the knowledge of the different cervical spaces is essential. The pattern of displacement of the prestyloid parapharyngeal space has to be analyzed.

• Is the parotid space mass single or multiple? Unilateral or bilateral?

  Multiple lesions are suggestive for specific tumors: for example, bilateral tumors are suggestive of Warthin tumor; multiple cystic formations suggest Sjögren syndrome or benign lymphoepithelial lesions possibly related to HIV.

• Does the tumor show benign or malignant characteristics?

  The surgical approach will depend on these characteristics. If malignancy is suspected, is there evidence of perineural spread along the facial nerve or branches of the trigeminal nerve? In that case, the therapeutic attitude will be different.

• Is the tumor limited to the superficial lobe of the parotid?

  A superficial parotidectomy is sufficient for a benign, well-circumscribed, superficial lobe lesion. On the other hand, a superficial lesion extending in the deep lobe requires a total parotidectomy (O’Brien 2003).

• What is the relationship of the mass to the facial nerve?

  The facial nerve is not seen on imaging but its intraparotid course can be estimated (plane between the stylomastoid foramen and the lateral border of the retromandibular vein).

• Is it possible to determine the histologic type of a benign tumor?

  A pleomorphic adenoma of the parotid gland will require surgery, whereas Warthin tumor in elderly patients may be followed up clinically and by imaging.

In order to answer all these key questions, MR imaging is the primary modality of choice for parotid gland tumors (Joe and Westesson 1994; Shah 2004).

The classical sequences used are: axial and coronal turbo spin-echo T2-weighted and spin-echo T1-weighted sequences with a slice thickness of 2–3 mm and a spin-echo T1-weighted sequence after contrast administration. A sequence with fat saturation after contrast administration is useful to better visualize potential perineural extension along the facial nerve or intracranial extension. This sequence is also useful to analyze correctly the tumoral enhancement.

A sequence to analyze the cervical lymph nodes is necessary if a malignant tumor is suspected. MR sialography can be useful if a pseudo-tumoral pathology such as Sjögren syndrome is suspected.

Diffusion-weighted sequence with calculation of an apparent diffusion coefficient (ADC) map may be interesting to better characterize non-specific tumors (Ikeda et al. 2004) or to improve the distinction between benign and malignant tumor, high cellularity in malignant tumors coming along with a decreased ADC value (Eida et al. 2007). To improve the reproductibility, calculation of the ADC tumor/ADC normal parotid gland ratio is recommended (Tassart et al. 2010).

Dynamic contrast-enhanced MR imaging (DCE-MRI) is a technique recently introduced for the evaluation of salivary glands tumors: a T1-weighted sequence is repeated every 30 s for 5 min during and after bolus injection of gadolinium. Two parameters are analyzed: the time to peak enhancement (T _peak) and the washout ratio (WR), which are determined from time–signal intensity curves (TICs) (Yabuuchi et al. 2003). Four types of curves can be described (Table 1). This dynamic technique is helpful to differentiate benign from malignant tumors (Yabuuchi et al. 2003), and for further tissue characterization in benign tumors (Yabuuchi et al. 2003; Ikeda et al. 2004; Alibek et al. 2007; Hisatomi et al. 2007; Tassart et al. 2010).

Table 1 Different types of dynamic contrast enhancement of parotid tumors

4 Benign Parotid Tumors

Benign tumors of the parotid gland represent about 85% of all parotid tumors (Okahara et al. 2003).

4.1 Benign Mixed Tumor or Pleomorphic Adenoma

4.1.1 General Description

This is the most common parotid gland tumor and it represents 70–80% of all tumors of the parotid gland. The lesion is usually solitary. Of all pleomorphic adenomas, 90% occur in the superficial lobe. If the lesion originates from the deep lobe, it can become large, extending in the anterior parapharyngeal space without causing symptoms.

There is a female predominance (sex ratio: 2/1). Facial nerve paralysis is uncommon.

4.1.2 Histological Findings

Pleomorphic adenoma is encased in a capsule that may be incomplete. It contains epithelial, myoepithelial and stromal (mucoid, myxoid, chondroid) cellular components. Calcifications are rare. Sites of cystic changes and hemorrhage may be present, especially if the tumor is large.

The lesion may show multicentric outgrowth through its capsule. The risk of recurrence is very high when only an enucleation of the tumor is performed. Therefore, in some centers, total parotidectomy is routinely performed for pleomorphic adenoma, in order to limit the risk of recurrence as much as possible.

4.1.3 Imaging Findings

• Typically, pleomorphic adenoma is a solitary, well-circumscribed mass. It may be lobulated. This tumor has low T1-weighted and high T2-weighted signal intensities. It may demonstrate a low T2-weighted signal intensity capsule. It shows homogeneous enhancement on T1 contrast-enhanced images, well illustrated on delayed contrast-enhanced T1-weighted images with fat saturation (Figs. 4, 5; Ikeda et al. 1996).

  Fig. 4

  

  Typical pleomorphic adenoma, or benign mixed tumor, of the superficial lobe of the left parotid gland: a lobulated, well-circumscribed lesion with high T2-weighted (arrow, a, b) and low T1-weighted signal intensity (c). It shows clear enhancement on contrast-enhanced T1-weighted images (d), appearing more homogeneous on delayed contrast-enhanced images (e)

  Fig. 5

  

  Large pleomorphic adenoma of the right parotid gland extending to the deep lobe of the parotid gland (arrow). The tumor appears well-circumscribed and lobulated with high T2-weighted (a), low T1-weighted signal intensity (b) and heterogeneous enhancement on contrast-enhanced images (c)

  Imaging findings may be less typical in case of larger tumors, but the lesion remains lobulated and well-circumscribed. Areas of hemorrhage appear as regions of high-signal intensity on both T1- and T2-weighted images (Fig. 6). Myxoid degeneration appears as heterogeneous and intermediate T2-weighted signal intensity (Fig. 7). In these large tumors, signal intensity can be heterogeneous on early contrast-enhanced images but it can appear more homogeneous on delayed contrast-enhanced, fat-suppressed T1-weighted images (Fig. 4).

  Fig. 6

  

  Pleomorphic adenoma of the right parotid gland: a well-delineated, lobulated tumor showing a typically high T2-weighted signal intensity and a low-signal intensity “capsule” (arrow, a). On the plain T1-weighted images (b), the tumor has a low-signal intensity with small regions of high T1-weighted signal intensity, corresponding to hemorrhagic areas (arrow)

  Fig. 7

  

  Pleomorphic adenoma of the left parotid gland with a high T2-weighted signal intensity and small heterogeneous areas of lower signal intensity, related to myxoid degeneration (arrow). The capsule is well identified (arrowhead)

  An extension to the deep lobe of the parotid gland must be looked for and reported.

  Large tumors of the deep lobe are usually mixed tumors (Figs. 8, 9).

  Fig. 8

  

  Pleomorphic adenoma extending to the left anterior parapharyngeal space with typical high signal on T2-weighted images

  Fig. 9

  

  Large pleomorphic adenoma of the deep lobe of the right parotid gland with non-homogeneous, high-signal intensity on this T2-weighted image. Larger tumors typically show slightly heterogeneous signal intensity

  Pleomorphic adenoma presents no restricted diffusion with high ADC value and ADC tumor/normal parotid gland ratio >1.2 (nearly two most frequently). These characteristics can be useful to differentiate pleomorphic adenomas from all other types of parotid gland tumors (Yerli et al. 2007).

• Dynamic contrast-enhanced MRI typically shows a progressive contrast enhancement of the tumor with a delayed time to peak enhancement (T _peak >  120 s) and a poor WR (<10% at 5 min), corresponding to an A curve on the TICs patterns (Yabuuchi et al. 2003; Hisatomi et al. 2007).

• Non-typical forms of pleomorphic adenoma are rare. These lesions have a low T2-weighted signal intensity which corresponds to fibrous tissue (Fig. 10). The correct diagnosis may be difficult but a solitary, well-circumscribed, encapsulated lesion is suggestive of benign tumor (Harnsberger 2004). Diffusion-weighted sequence and dynamic contrast-enhanced MRI can be helpful in this case showing respectively a typical high ADC value, and a characteristic intensity time curve type A.

  Fig. 10

  

  Atypical pleomorphic adenoma of the right parotid gland, with low heterogenous T2-weighted signal intensity (a, b), and low T1-weighted signal intensity (c). This tumor shows however benign characteristics: a well-delineated, sharply outlined mass with homogeneous enhancement on post-contrast images (d) and a typical A curve on the dynamic contrast-enhanced sequence with a progressive contrast enhancement (T _peak >  120 s)

• Calcified lesions are uncommon; these calcifications occur after a long evolution.

• Recurrent pleomorphic adenoma tends to be multifocal and more aggressive (Fig. 11). To improve the detection of nodular recurrences, a T2-weighted sequence with fat suppression (fat sat), or STIR sequence, is very useful (Tassart et al. 2010).

  Fig. 11

  

  Multifocal recurrent pleomorphic adenoma after partial parotidectomy (arrows)

• Malignant degeneration within a pleomorphic adenoma is rare but exists in 3–5% (Tassart et al. 2010). The most common type of malignancy associated with pleomorphic adenoma is a carcinoma ex pleomorphic adenoma, or sometimes an adenocarcinoma. Malignant characteristics are then present on imaging studies. The hypercellularity due to the malignant degeneration results in low-signal intensity on T2-weighted sequence, and low ADC values (Kato et al. 2008) with an ADC tumor/ADC normal parotid gland ratio lower than 1.

4.1.4 Differential Diagnosis

• Warthin tumor or papillary cystadenoma lymphomatosum

  Especially in case of non-typical imaging findings of pleomorphic adenoma (see Sect. 6).

• Malignant tumor

  Features suggesting malignancy are heterogeneous enhancement, an infiltrating mass with irregular margins, perineural tumor spread or infiltration of adjacent fat tissue.

• Non-Hodgkin lymphoma of the parotid

  The clinical presentation may be suggestive (see “Neck Lymphoma”).

• Parotid nodal metastasis (often from skin carcinoma or melanoma, sometimes systemic metastasis).

4.2 Warthin Tumor or Papillary Cystadenoma Lymphomatosum

4.2.1 General Description

This is the second most frequent benign tumor arising in the parotid gland and it represents 10–25% of all parotid tumors.

It is more common in males than in females (sex ratio: 3/1). The mean age for presentation is 60 years. About 90% of patients with this tumor are cigarette smokers. This diagnosis should not be suggested before age of 40 years (Harnsberger 2004).

It arises almost exclusively in the lower portion of the superficial lobe of the parotid gland. There is a bilateral involvement in 15–20% of patients, which presents simultaneously or metachronously, the contralateral location may be discovered on imaging only.

These are slow-growing tumors and malignant transformation is uncommon, occurring in less than 1% of Warthin tumors.

The recommended treatment is superficial parotidectomy, sparing the intraparotid facial nerve. Considering the slow-growing nature of this tumor and its very low risk of degeneration, a clinical and radiological follow-up can be proposed especially in elderly people in whom the diagnosis of Warthin tumor is suspected on imaging.

4.2.2 Histological Findings

The papillary cystadenoma lymphomatosum is encapsulated. It arises within the lymphoid tissue of the parotid gland and is composed of lymphoid stroma and epithelium. The epithelial component is characterized by cystic spaces; it can show hemorrhagic areas. For these reasons, Warthin tumors tend to be nonhomogeneous on imaging.

4.2.3 Imaging Findings

Warthin tumor has typically a heterogeneous appearance on imaging, due to cystic and hemorrhagic changes, with overall benign characteristics: a well-circumscribed lesion measuring 2–4 cm in diameter.

The tumor presents low T1-weighted signal intensity, with small areas of high-signal intensities due to accumulation of proteinous fluid, cholesterol crystals or hemorrhagic changes. Such areas are seen in about 60% of the cases and are typical of the diagnosis of Warthin tumor (Figs. 12, 13; Ikeda et al. 2004).

Fig. 12

Typical Warthin tumor in the superficial lobe of the right parotid gland showing a low T1-weighted signal intensity with small high-signal intensity areas (arrow, a), a low T2-weighted signal intensity (b), and a mild enhancement more pronounced in the periphery of the mass (arrow, c)

Fig. 13

Well-defined tumor of the left inferior parotid gland with low-signal intensity on both T2-weighted images (a, b) and T1-weighted images (c). Post-contrast T1-weighted images demonstrate mild contrast enhancement (d). DCE-MRI shows a typical B curve with an early enhancement (T _peak < 120 s) and a high washout pattern (e); diffusion-weighted sequence confirms an important diffusion restriction, with a high-signal intensity at b factor of 1,000 s/mm² and a very low ADC value of 0.58 (f). These features are typical of a Warthin Tumor

The tumor has intermediate and high T2-weighted signal intensities. The high-signal intensities areas correspond to cystic foci.

Warthin tumor shows a mild enhancement on post-contrast-enhanced T1-weighted images. Cystic spaces show no enhancement. A typical aspect of the tumor is a ring enhancement (Fig. 12). Fat-suppressed, contrast-enhanced T1-weighted images illustrate better the heterogeneous enhancement of the tumor. DCE-MRI shows typical cases a curve with early enhancement (T _peak < 120 s) and high washout pattern (WR >  30% at 5 min), corresponding to a B curve on TICs patterns (Yabuuchi et al. 2003; Hisatomi et al. 2007) (Fig. 13). This high washout, as well as the presence of cystic portions explain that these tumors show mild enhancement on post-contrast T1-weighted sequences.

Multiple lesions in one parotid gland or bilaterally have to be looked for. Bilateral lesions in the lower parts of the parotid glands are virtually pathognomonic (Fig. 14).

Fig. 14

Typical bilateral Warthin Tumor with low T1-weighted (a) and T2-weighted (b) signal intensity. Note the small spontaneous hyperintense areas (due to hemorrhagic changes and cholesterol crystals) on the T1-weighted sequence (a). Cystic areas show high T2-weighted signal intensity within the left lesion (b)

Warthin tumor may extend to the deep lobe of the parotid gland. On the other hand, a primary location in the deep lobe is very rare.

On diffusion-weighted images, the tissular compartment of Warthin tumors typically presents a very low ADC value (Ikeda et al. 2004; Yerli et al. 2007).

4.2.4 Differential Diagnosis

• Atypical pleomorphic adenoma.

• Benign lymphoepithelial lesions in HIV-positive patients.

• Malignant tumor with few malignant characteristics on imaging.

4.3 Other Benign Tumors

4.3.1 Lipoma

It represents 1% of all parotid gland tumors. CT and MR findings are characteristic: well-circumscribed lesion with negative H.U.-values on CT, showing high T1-weighted and high T2-weighted signal intensities, without enhancement on contrast-enhanced T1-weighted images. The high T1-weighted signal intensity disappears on fat-suppressed sequences (Fig. 15).

Fig. 15

Lipoma of the left parotid gland extending to the deep lobe and the anterior parapharyngeal space, showing a spontaneous high T1-weighted (a) and a high T2-weighted signal intensity (b)

4.3.2 Neurogenic Tumor

A schwannoma of the intraparotid portion of the facial nerve may occur but remains rare in this extratemporal localization. Facial nerve palsy is exceptional.

Imaging findings are nonspecific but show characteristics of a benign tumor, centered on the facial nerve plane, with low T1-weighted, high T2-weighted signal intensities and homogeneous strong enhancement. Some authors describe a higher signal intensity surrounding the periphery of the tumor on T2-weighted images as a suggestive sign for schwannoma (Shimizu et al. 2005). When large in size, cystic changes may occur, making the diagnosis on imaging more difficult. Extension through the stylomastoid foramen and along the third portion of the facial nerve has to be looked for very carefully, this aspect being very suggestive for the diagnosis of schwannoma (Shimizu et al. 2005). The differential diagnosis is perineural extension of a malignant tumor (in particular in adenoid cystic carcinoma), but in such a case the tumor is ill-defined, with low-signal intensity on T2-weighted sequence (Tassart et al. 2010).

4.4 Congenital Tumors

4.4.1 Lymphangioma

Lymphangioma is rare in adults, more common in children. It presents as lobulated, multiloculated, cystic mass with septations. Some of the cystic areas can show spontaneous high-signal intensity due to hemorrhagic changes. Sometimes fluid–fluid levels are seen. Cystic foci do not enhance. Cystic lymphangioma often occurs in the lower portion of the parotid gland (Fig. 16).

Fig. 16

Cystic lymphangioma of the inferior pole of the left parotid gland. Typical aspect of a lobulated, septated, cystic mass showing a high T2-weigthed signal intensity

Mixed vascular malformations exist, such as the cavernous lymphangioma, composed of (enhancing) solid areas and dilated lymphatic spaces.

4.4.2 Infantile Hemangioma

This is the most frequent tumor of the parotid space in infants and young children. This tumor shows a 3-phase evolution: rapid growth until the age of about 10 months, then stabilization and finally regression with sometimes an incomplete involution with residual calcifications.

Imaging findings are a large solid mass which shows intermediate T1-weighted, high T2-weighted signal intensities, and intense and fast enhancement with multiple enlarged vascular signal voids (Fig. 17).

Fig. 17

Infantile capillary hemangioma: a large solid mass of the left parotid space showing intense enhancement and containing multiple enlarged vascular signal voids (arrow)

4.5 Cystic Tumors

4.5.1 Solitary Cystic Lesion

A solitary cystic lesion of the parotid gland is rare (Som et al. 1995). The cystic nature of the lesion has to be confirmed on imaging using specific sequences (in particular diffusion-weighted sequence). Actually, most of parotid gland tumors show high T2-weighted signal intensity, so this finding does not systematically correspond to a cystic lesion. A cystic formation shows low T1-weighted signal intensity without any enhancement on contrast-enhanced T1-weighted sequence. It has a high T2-weighted signal intensity including T2-weighted sequence with long echo time (Fig. 18). Cystic lesion appears hypointense on diffusion-weighted sequence at b factor of 800 or 1,000 s/mm².

Fig. 18

Solitary simple cystic lesion of the left parotid gland with a high T2-weighted (a) and a low T1-weighted signal intensity without any contrast enhancement (b)

A cystic lesion of the parotid gland is suggestive of first branchial cleft abnormality and a fistulous connection in or around the external auditory canal must be looked for (Van der Goten et al. 1997). If the cyst appears to be unilocular and non-communicating, the differential diagnosis is between a non-communicating branchial cleft cyst and a simple cyst of the parotid gland.

4.5.2 Dermoid Cysts

Dermoid cysts may occur in the parotid space. Their heterogeneous structure (fat, bone, skin adnexa) is highly suggestive of this diagnosis.

4.5.3 Epidermoid Cysts

Rarely epidermoid cysts occur in the parotid gland. Signal intensity is typical on MRI: low T1-weighted signal intensity, high T2-weighted signal intensity and no enhancement on contrast-enhanced T1-weighted images. Diffusion-weighted sequence is very helpful because epidermoid cysts show high-signal intensity at b factor of 800 or 1,000 s/mm², with a very low ADC value (around 0.5 × 10⁻³ mm²/s).

These are slow-growing tumors and a simple follow-up imaging study can be proposed.

4.5.4 Multiple Intraparotid Cystic Lesions

Multiple intraparotid cystic lesions must evoke the possibility of benign lymphoepithelial lesions, as may be seen in HIV-positive patients (Holliday et al. 1998).

The etiology remains unknown, probably from inflammatory origin.

Imaging findings are multiple, bilateral cystic and solid masses, enlarging both parotid glands, associated with tonsillar hyperplasia and bilateral cervical adenopathy.

These lesions are well-circumscribed, sometimes heterogeneous, with low T1-weighted and high T2-weighted signal intensities (Fig. 19).

Fig. 19

Multpile bilateral cystic lesions in a HIV-positive patient, corresponding to benign lymphoepithelial lesions

The clinical profile should help in the diagnosis.

5 Malignant Parotid Tumors

Malignant tumors represent 15% of all parotid tumors. Clinical symptoms suggestive for malignancy are: facial nerve paralysis, pain, skin infiltration, a rapidly enlarging lesion and cervical adenopathy. Prognosis depends on histologic grade.

5.1 Histologic Classification

• Adenocarcinoma, squamous cell carcinoma and undifferentiated carcinoma

  These malignant tumors have a predilection for men and most cases occur over the age of 60 years. They are most frequently rapidly growing tumors, presenting with facial nerve paralysis and early infiltration of the infratemporal fossa. The prognosis of these cancers is very bad.

• Adenoid cystic carcinoma

  This malignant tumor occurs more commonly in the submandibular gland and the minor salivary glands. Adenoid cystic carcinoma is a slow-growing, widely infiltrative tumor with a tendency for perineural spread along the facial nerve and trigeminal nerve (auriculotemporal nerve and mandibular nerve). A significant number of patients develop late intracranial recurrence due to perineural infiltration. Distant metastases develop in 40–50%.

• Mucoepidermoid carcinoma

  It has a female predominance and is most frequently seen in the 40- to 60-year age group. Prognosis depends on the histologic grade. Low-grade mucoepidermoid carcinoma may have a clinical and radiological presentation of a benign tumor. High-grade tumors have a high risk of local recurrence and distant metastases.

• Non-Hodgkin lymphoma

  It develops from intraparotid lymph nodes and is frequently associated to a systemic lymphoma with cervical lymphadenopathy and other lymphoma localizations. Patients suffering Sjögren syndrome have an increased risk of developing parotid lymphoma.

• Intraparotid metastases

  The vast majority of parotid metastases of known primary tumors originate from the skin in the head and neck region.

• Liposarcoma and rhabdomyosarcoma

  These are rare parotid neoplasms.

5.2 Imaging Findings

5.2.1 Parotid Cancer

The aim of imaging is to identify the malignant features of a tumor in order to initiate the best therapy as soon as possible. Subtle signs of malignancy (Figs. 20, 21, 22, 23; Freling et al. 1992) that have to be scrutinized include the following:

• Ill-defined margins, indicating an invasive mass, including infiltration of the adjacent fat tissue, which can be an early and only sign suggesting malignancy, best depicted on precontrast T1-weighted images;

• cervical lymphadenopathy;

• extension to the adjacent deep facial spaces (Fig. 24) such as the masticator and parapharyngeal space;

  Fig. 20

  

  Malignant tumor of the left parotid gland, with ill-defined margins, infiltrating the adjacent subcutaneous fat tissue (arrow), well appreciated on pre-contrast T1-weighted images (a, b)—Adenocarcinoma

  Fig. 21

  

  Malignant tumor of the right parotid gland, with poorly defined margins and infiltrating the subcutaneous adjacent fat tissue (arrow) showing a low T1-weighted (a), a low T2-weighted signal intensity (arrow, b) and mild enhancement on the contrast-enhanced T1-weighted image (arrow, c)—Mucoepidermoid carcinoma

  Fig. 22

  

  Small malignant tumor of the inferior pole of the left parotid gland (a). The ill-defined margins, low T2-weighted signal intensity (b) and infiltration of the adjacent fat tissue suggestive of malignancy (arrow)—Mucoepidermoid carcinoma

  Fig. 23

  

  Malignant tumor in the superficial lobe of the left parotid gland, with low T2-weigthed signal intensity (arrow, a). The infiltration of the adjacent fat tissue is well identified on pre-contrast T1-weighted image (arrow, b), as well as on the contrast-enhanced T1-weighted images with fat saturation (arrow, c)—Adenocarcinoma

  Fig. 24

  

  Adenoid cystic carcinoma of the right parotid gland extending to the parapharyngeal space and the infratemporal fossa (arrow)

• perineural spread along the facial nerve or trigeminal nerve, better depicted on contrast-enhanced images with fat saturation, is typically seen in adenoid cystic carcinoma (Fig. 25; Harnsberger 2004; Som and Curtin 1996);

  Fig. 25

  

  Adenoid cystic carcinoma of the deep lobe of the right parotid gland, showing perineural spread along the mandibular branch of the trigeminal nerve (arrow) toward the foramen ovale

• low T2-weighted signal intensity may indicate malignancy (Som and Biller 1989), but may also be seen in some benign tumors;

• the hypercellularity of malignant tumors results in low ADC values (Kato et al. 2008) with an ADC tumor/ADC normal parotid gland ratio lower than 1; in such a case, the only differential diagnosis is a Warthin tumor;

• DCE-MRI depicts an early peak of enhancement (but slower than in a Warthin tumor) and a low washout pattern, inferior to 30% at 5 min (curve type C).

Therefore, combination of diffusion-weighted imaging and DCE-MRI is useful to differentiate a malignant tumor from a Warthin tumor (Yabuuchi et al. 2003; Hisatomi et al. 2007).

If a malignant tumor is suspected on imaging, a fine needle aspiration may be performed. This allows differential diagnosis with a benign tumor in 85–90% of the cases (Zbaren et al. 2001).

The typical treatment of these malignant tumors is wide surgical resection, lymph node dissection and radiation therapy.

5.2.2 Non-Hodgkin Lymphoma

Non-Hodgkin lymphoma often has more specific imaging features: multiple homogeneous nodal lesions with intermediate T1-weighted and T2-weighted signal intensities and moderate enhancement, rarely necrotic. Usually, there is also cervical lymphadenopathy showing the same signal intensity on all MR sequences (Fig. 26). The submandibular gland as well as the lacrimal glands may also be involved. In lymphoma, the ADC value is particularly low with a ratio often inferior to 0.5 (Tassart et al. 2010).

Fig. 26

Multiple homogeneous nodular lesions of the left parotid gland (a, b, arrow), associated with bilateral lymphadenopathy (c, arrows) showing the same intermediate signal as the parotid lesions on all sequences, corresponding to lymphoma

Parotid lymphomas are uncommon but a history of autoimmune disease, especially Sjögren syndrome, previous lymphoma, HIV with a painful, rapidly enlarging parotid gland, is suggestive of this diagnosis (Loggins and Urquhart 2004 ).

Fine needle aspiration may confirm the diagnosis. However, the histologic diagnosis has to be confirmed by a larger tissue sample. The treatment consists of chemotherapy, possibly associated with radiotherapy.

6 Difficult Cases

• Real difficult cases in imaging parotid gland tumors are rare and represent less than 10% of the cases. However, it can be impossible to differentiate an atypical parotid benign tumor, such as atypical pleomorphic adenoma, from a malignant tumor showing few typical malignant characteristics.

  Diffusion-weighted sequences are helpful in such situations (Table 2). The lesion’s signal intensity at b factor of 1,000 s/mm² and the ADC value have to be evaluated. Pleomorphic adenoma appears hypointense on diffusion-weighted sequence at b factor of 1,000 s/mm², with an ADC value >1.2 (Fig. 27). On the other hand, malignant tumors, including lymphoma, show a much lower ADC value <1.2. If the ADC value is >1.2, suggestive of a pleomorphic adenoma, surgery of the lesion can be proposed. If the ADC value is <1.2, fine needle aspiration can be done in order to first exclude malignancy. However, there is no consensus whether to perform fine needle aspiration or not. As said, this procedure allows differentiation of benign tumors from malignant tumors in 85–90% of all cases (Zbaren et al. 2001; Lim et al. 2003). On the contrary, a specific histologic diagnosis for benign tumors is possible only in 40% of cases (Hamilton et al. 2003), except for cystadenolymphoma for which a diagnosis is possible in 74% of cases (Parwani and Ali 2003).

  Table 2 ADC values in parotid gland tumor

  Fig. 27

  

  Pleomorphic adenoma of the deep lobe of the right parotid gland, appearing hyperintense on T2-weighted images (a), hyperintense on a diffusion-weighted image with b factor of 0 s/mm² (arrow, b) and hypointense with b factor of 800 s/mm² (arrow, c); the ADC value is >1.2

• It can also be very hard to differentiate an atypical pleomorphic adenoma showing low T2-weighted signal intensity from a Warthin tumor. In such situation, MR diffusion-weighted imaging is very helpful: a pleomorphic adenoma appears hypointense on diffusion-weighted sequence at b factor of 1,000 s/mm², with ADC value >1.2, whereas a Warthin tumor presents a high-signal intensity at b factor of 1,000 s/mm² with a low ADC value, lower than the one observed in case of malignant tumor (Ikeda et al. 2004; Habermann et al. 2009).

• Diffusion-weighted imaging also allows making the difference between a simple intraparotid cyst and an epidermoid cyst. An epidermoid cyst shows a high signal on diffusion-weighted images at b factor of 1,000 s/mm² with a very low ADC value (around 0.5), in favor of this diagnosis.

• Combination of diffusion-weighted sequence and dynamic contrast-enhanced MRI is very helpful in these difficult cases.

7 Pseudo-Tumors of the Parotid Gland

7.1 Sjögren Syndrome

This is a chronic systemic auto-immune disease that affects salivary and lacrimal glands.

The best diagnostic clue on imaging is bilateral enlargement of both parotid glands, usually asymmetric, with multiple small cystic and solid lesions; sometimes calcifications are present.

On MR imaging, a miliary pattern of small cysts throughout both parotids is diagnostic of Sjögren’s syndrome. The small collections show low T1-weighted, high T2-weighted signal intensities; the small cysts do not enhance, while the surrounding parenchyma shows mild enhancement. MR sialography suggests the diagnosis of Sjögren syndrome by showing multiple cystic lesions while the salivary ducts appear normal (Fig. 28; Ohbayashi et al. 1998; Izumi et al. 1996).

Fig. 28

Miliary pattern appearance of multiple cystic lesions throughout both enlarged parotid glands, showing high T2-weighted signal intensity, in a patient affected by Sjögren syndrome (a, b). These cystic lesions and the normal salivary ducts (arrow) are well seen on MR sialography (c)

The differential diagnosis includes benign lymphoepithelial cystic lesions in HIV-positive patients; cystic areas are usually larger in this pathology.

A solid tumoral mass, developing in the parotid gland in a patient with Sjögren syndrome, is suspect for lymphoma.

7.2 Sarcoidosis

Sarcoidosis is a systemic disease. The parotid glands are affected in 10–30% of patients. It can present with solid and cystic lesions involving both parotid glands, often associated with cervical adenopathy.

Differential diagnosis must be made with non-Hodgkin lymphoma, in which the imaging pattern commonly consists of bilateral, solid, nodal-appearing intraparotid masses. The clinical presentation is usually suggestive of the diagnosis.

7.3 Intraparotid Lymph Nodes

Presence of multiple intraparotid lymph nodes can be caused by lymphoid hyperplasia, lymphoma, sarcoidosis or metastasis from a cervicofacial tumor, in particular from a skin cancer in 60% (especially melanoma—Tassart et al. 2010). Intraparotid metastasis from extrafacial tumors are rare.

8 Tumors of the Other Salivary Glands

The paired submandibular and sublingual glands are referred to as major salivary glands, such as the parotid glands. The minor salivary glands are submucosal clusters of salivary tissue present in the oral cavity, particularly at the junction of hard palate and soft palate, pharynx, upper respiratory tract, as well as in the anterior parapharyngeal space. While parotid tumors are frequently benign, other salivary glands tumors are often malignant.

8.1 Minor Salivary Glands Tumors

• Adenoid cystic carcinoma and mucoepidermoid carcinoma are the most frequent malignancies. A tumor of the palate is suspicious of a minor salivary gland tumor. Perineural spread toward the maxillar branch of the trigeminal nerve (V2) is suggestive of adenoid cystic carcinoma. Cervical lymphadenopathy may be present. Treatment consists of wide surgical resection, often associated with radiotherapy. Mucoepidermoid carcinoma often mimics a benign tumor (a well-defined, non-infiltrative tumor) and has a better prognosis than adenoid cystic carcinoma (Fig. 29).

  Fig. 29

  

  Mucoepidermoid tumor of a minor salivary gland at the junction of hard palate and soft palate: an ovoid, well-demarcated lesion showing high, heterogeneous T2-weighted (a, b), and low T1-weighted signal intensity (c), with mild contrast enhancement (d)

• Pleomorphic adenoma of minor salivary gland origin in the anterior parapharyngeal space is rare (see “Parapharyngeal Space Neoplasms”). These are usually slow-growing, large tumors without much clinical symptoms, presenting with the MR characteristics of a pleomorphic adenoma: a well-defined, lobulated mass with low T1-weighted, high T2-weighted signal intensities and important enhancement. These lesions often appear heterogeneous (Fig. 8). The differential diagnosis is a pleomorphic adenoma arising in the deep lobe of the parotid gland (Fig. 9).

8.2 Submandibular Gland Tumors

• The role of imaging is to determine whether a submandibular lesion is either a salivary gland tumor or an adenopathy. In case of a salivary localization, the lesion appears to be malignant in 55% of patients (Kaneda et al. 1996).

• Adenoid cystic carcinoma and mucoepidermoid carcinoma are the most frequent submandibular gland tumors. On MR imaging, these tumors are usually poorly defined, with low T1-weighted, intermediate T2-weighted signal intensities and heterogeneous enhancement. The surrounding structures, such as the adjacent cervical deep spaces, muscles, mandible and cervical lymph nodes, have to be analyzed in order to rule out tumoral involvement. Treatment consists of a wide resection with neck dissection, and frequently adjuvant radiotherapy. Imaging follow-up must last a very long time because of the possibility of late recurrence.

• Benign mixed tumor is the most frequent benign tumor arising in the submandibular gland. MR imaging characteristics are similar to those of intraparotid pleomorphic adenoma. Typical appearance on CT is a hypodense, lobulated mass with heterogeneous enhancement related to cystic and hemorrhagic changes.

  Fine needle aspiration of submandibular gland tumors has poor sensitivity and specificity. Surgical resection of the submandibular gland may be proposed if the lesion is small, homogeneous, well-defined, showing MR characteristics of a benign mixed tumor. Wide surgical resection associated with neck dissection has to be performed if the lesion is larger, heterogeneous, ill-defined and suggestive of a malignant tumor.

• Chronic sialadenitis due to obstruction of the ductal system can clinically simulate a tumoral lesion of the submandibular space. Imaging is very useful to look for the obstructive calculus in Wharton’s duct and the inflammatory modifications of the submandibular gland. On imaging, the involved gland is usually enlarged, diffusely densely enhancing, associated with dilatation of the main salivary duct, often containing a calculus at its distal part. Lithiasis is well depicted on CT (Fig. 30).

  Fig. 30

  

  Chronic sialadenitis: multiple calcified lithiasis (arrows) within the left submandibular gland and Wharton’s duct

8.3 Sublingual Gland Tumors

• About 80% of the sublingual gland tumors are malignant. Again, the two main etiologies are adenoid cystic carcinoma and mucoepidermoid carcinoma (Sumi et al. 1999).

  On MR imaging, these tumors are more or less well-circumscribed and present low T1-weighted signal intensity, intermediate T2-weighted signal intensity and a heterogeneous enhancement (Fig. 31). Extension outside the sublingual space has to be looked for, such as extension to the mandible or to the cervical lymph nodes. These lesions are usually well-circumscribed in case of mucoepidermoid tumor (Fig. 32).

  Fig. 31

  

  Adenoid cystic carcinoma of the left sublingual gland: huge mass appearing hyperintense on T2-weighted image (arrow, a), hypointense on T1-weighted image (arrow, b), showing a moderate and heterogeneous enhancement on contrast-enhanced T1-weighted image (arrow, c)

  Fig. 32

  

  Small mucoepidermoid tumor of the left sublingual gland showing a high T2-weighted signal intensiy (arrow, a) and an important enhancement on contrast-enhanced T1-weighted images (arrow, b), associated with two small adjacent submandibular adenopathies (arrow, c)

  Treatment consists of a wide resection with neck dissection, most often associated with radiation therapy. Because of the risk of recurrence, a long-term follow-up is recommended.

• A ranula is a mucous retention cyst that originates from the sublingual gland. It most commonly results from trauma or inflammation. A ranula occurs in two forms. The first one, the simple ranula, is a retention cyst that remains located in the sublingual space (Fig. 33). The second type is the plunging ranula, which is a mucocele extending through the mylohyoid muscle, to the submandibular space.

  Fig. 33

  

  Cystic formation in the anterior part of the right sublingual space, compressing the genioglossal muscles and extending over the midline. Confirmed simple ranula (Courtesy Robert Hermans, MD, PhD, Leuven, Belgium)

  On MR imaging, the simple ranula is a cystic formation with low T1-weighted, high T2-weighted signal intensity and a thin enhancement ring on contrast-enhanced images. The plunging ranula appears as a multi-lobulated cystic formation; it sometimes shows spontaneous high T1-weighted signal intensity because of its protein concentration, high T2-weighted signal intensity with much more important enhancement of the cyst wall.

  On CT, a ranula has usually a low attenuation because of its high water content; the cyst wall may enhance after contrast administration.

The differential diagnosis includes epidermoid cysts and cystic hygromas of the sublingual space.

9 Conclusion

Most of parotid gland lesions are benign tumors. The best imaging approach to analyze parotid gland tumors is MR imaging. In many cases, the MRI findings are indicative of benign or malignant nature. In case of benign tumors, an etiologic diagnosis can sometimes be suggested on MRI. MR diffusion-weighted and dynamic contrast-enhanced sequences appear to be helpful in improving the diagnostic yield in case of atypical or difficult cases.