Beyond Ultrasound: Multimodal Cross-Sectional Imaging for Preoperative Imaging of Parotid Gland Tumors: A Primer for Radiology Trainees

: Even if the management of parotid gland tumors depends on the histopathological subtype, preoperative imaging of parotid gland tumors is clinically relevant. Preoperative imaging gives insight into the differentiation between benign and malignant tumors, which might potentially decrease the number of unnecessary aggressive surgeries. Characteristic imaging findings on cross-sectional imaging, such as computed tomography (CT) and magnetic resonance imaging (MRI), can help narrow the differential diagnosis and guide the further management of patients presenting with parotid masses. While MRI is imperative for the determination of perineural spread, which is frequently encountered with malignant parotid tumors, CT is important for the evaluation of osseous invasion. Furthermore, multi-parametric MRI protocols provide insights into the tumor behavior and internal composition, which is helpful in the case of benign mixed tumors and others. While distant metastasis is uncommon with parotid neoplasms, PET/CT provides a valuable tool for the improved evaluation of loco-regional and distant metastatic disease. This article discusses the imaging features of common benign and malignant parotid tumors.


Introduction
Most salivary gland neoplasms develop within the parotid glands [1].Generally, −70-85% are benign, while the rest are malignant [2].The surgical management of parotid gland tumors differs greatly for benign and malignant neoplasms [3].While the former are treated with extracapsular dissection or superficial parotidectomy, the latter need more aggressive surgical approaches, such as total parotidectomy and neck dissection with potential adjuvant therapy [3].
The classic presentation of a slowly growing painless preauricular swelling or mass is not conclusive for either benign or malignant parotid gland neoplasms.Facial nerve palsy is probably the most robust clinical symptom suggesting malignancy [4].Therefore, preoperative imaging is critical in surgical planning to assess tumor extension and evaluate imaging criteria that might predict malignancy [5].Such information helps surgeons to determine the most appropriate therapeutic procedure and surgical approach.
Cross-sectional imaging modalities such as ultrasound, CT, MRI, and PET/CT are helpful for the imaging work-up for the evaluation of patients presenting with parotid masses.Given its feasibility, lack of radiation exposure, and the superficial position of the parotid glands, ultrasound is the first-choice imaging modality for the evaluation of mass morphology including borders and margins [1,2].Benign masses are typically well defined with smooth margins and regular borders, while malignant masses are typically ill defined with irregular borders.The identification of the external carotid artery and retromandibular vein at the posterior mandibular angle represents a line of demarcation between the superficial and deep lobes of the parotid glands.Ultrasound is sufficient for the evaluation of masses within the superficial lobe of the parotid gland, yet masses within the deep lobe or with aggressive features such as osseous invasion or perineural spread will require further evaluation with CT or MRI for better assessment [1,[4][5][6].
In the current article, we review the imaging features of the most common parotid tumors using cross-sectional imaging with computed tomography (CT), magnetic resonance (MR) imaging, and fluorodeoxyglucose (FDG)-positron emission tomography (PET)/CT.Though ultrasound is the initial imaging modality for the evaluation of parotid masses, ultrasound imaging characteristics of parotid tumors are beyond the scope of this review.The distinction between benign and malignant parotid tumors based on imaging features is not always obvious, given the substantial overlap in imaging appearances.Nonetheless, it is useful for radiologists to be familiar with the various pathologies and typical imaging features of each type to decrease the rate of unnecessary aggressive interventions.

Benign Parotid Gland Tumors
Benign parotid gland tumors encompass a wide histopathological variability with several tumor-like conditions [3].The most common imaging characteristics are welldefined masses with sharp margins, a hypointense rim on T2 WIs, and a lack of invasion of the surrounding structures or perineural extension on CT and MRI [6].Advanced MRI techniques can help to differentiate benign from malignant parotid neoplasms (Figure 1) [7].Previous studies found a potential role of diffusion-weighted imaging and diffusion tensor imaging with cutoff values in differentiating benign and malignant parotid tumors for ADC and fraction anisotropy (FA) of 1.02 × 10 −3 mm 2 /s and 0.24, respectively [7].We will focus on the commonly encountered parotid gland tumors, as shown in Table 1.
parotid glands, ultrasound is the first-choice imaging modality for the evaluation of mass morphology including borders and margins [1,2].Benign masses are typically well defined with smooth margins and regular borders, while malignant masses are typically ill defined with irregular borders.The identification of the external carotid artery and retromandibular vein at the posterior mandibular angle represents a line of demarcation between the superficial and deep lobes of the parotid glands.Ultrasound is sufficient for the evaluation of masses within the superficial lobe of the parotid gland, yet masses within the deep lobe or with aggressive features such as osseous invasion or perineural spread will require further evaluation with CT or MRI for better assessment [1,[4][5][6].
In the current article, we review the imaging features of the most common parotid tumors using cross-sectional imaging with computed tomography (CT), magnetic resonance (MR) imaging, and fluorodeoxyglucose (FDG)-positron emission tomography (PET)/CT.Though ultrasound is the initial imaging modality for the evaluation of parotid masses, ultrasound imaging characteristics of parotid tumors are beyond the scope of this review.The distinction between benign and malignant parotid tumors based on imaging features is not always obvious, given the substantial overlap in imaging appearances.Nonetheless, it is useful for radiologists to be familiar with the various pathologies and typical imaging features of each type to decrease the rate of unnecessary aggressive interventions.

Benign Parotid Gland Tumors
Benign parotid gland tumors encompass a wide histopathological variability with several tumor-like conditions [3].The most common imaging characteristics are well-defined masses with sharp margins, a hypointense rim on T2 WIs, and a lack of invasion of the surrounding structures or perineural extension on CT and MRI [6].Advanced MRI techniques can help to differentiate benign from malignant parotid neoplasms (Figure 1) [7].Previous studies found a potential role of diffusion-weighted imaging and diffusion tensor imaging with cutoff values in differentiating benign and malignant parotid tumors for ADC and fraction anisotropy (FA) of 1.02 × 10 −3 mm 2 /s and 0.24, respectively [7].We will focus on the commonly encountered parotid gland tumors, as shown in Table 1.

Pleomorphic Adenoma
Pleomorphic adenomas (PAs) represent the most common benign salivary gland neoplasms, accounting for 50-75% of all salivary gland neoplasms as well as 70-80% of benign tumors [8].Due to the variable contents within their matrix, they are also referred to benign mixed tumors of salivary glands.They are particularly prevalent in the parotid gland [9].PAs are most frequently observed in the 3rd-6th decades, with a slightly higher incidence in females than in males [10].
The CT and MRI features of PAs depend on their size (Figure 2).Small tumors tend to be well defined, with multilobulated or bosselated margins and a homogeneously hyperintense signal on T2 WIs and a hypointense signal on T1 WIs relative to the muscles of mastication.They also exhibit solid intense enhancement after contrast administration [1].On occasions, they might show a thin rim of T2 hypointensity representing a fibrous capsule.Larger tumors tend to show exophytic outgrowth with a heterogeneous signal and enhancement due to internal necrosis and hemorrhagic changes [11].PAs usually have increased diffusivity with high ADC values due to their high content of chondroid and myxoid matrix (Figure 2) [12][13][14].The range of ADC values of PAs varies among different studies (1.5-2.2 × 10 −3 mm 2 /s) [7].

Pleomorphic Adenoma
Pleomorphic adenomas (PAs) represent the most common benign salivary gland oplasms, accounting for 50-75% of all salivary gland neoplasms as well as 70-80% of nign tumors [8].Due to the variable contents within their matrix, they are also referre benign mixed tumors of salivary glands.They are particularly prevalent in the par gland [9].PAs are most frequently observed in the 3rd-6th decades, with a slightly hig incidence in females than in males [10].
The CT and MRI features of PAs depend on their size (Figure 2).Small tumors t to be well defined, with multilobulated or bosselated margins and a homogeneously perintense signal on T2 WIs and a hypointense signal on T1 WIs relative to the muscle mastication.They also exhibit solid intense enhancement after contrast administration On occasions, they might show a thin rim of T2 hypointensity representing a fibrous c sule.Larger tumors tend to show exophytic outgrowth with a heterogeneous signal enhancement due to internal necrosis and hemorrhagic changes [11].PAs usually h increased diffusivity with high ADC values due to their high content of chondroid myxoid matrix (Figure 2) [12][13][14].The range of ADC values of PAs varies among diffe studies (1.5-2.2 × 10 −3 mm 2 /s) [7].The most concerning prognostic consideration with PA is potential post-surgica currence, which has been reported to vary between 1% and 50% [15].This variation pends upon the initial surgical approach [16].PAs have the potential for malignant tr formation into malignant mixed tumor (carcinosarcoma) (Figure 3) and carcinoma exomorphic adenoma [17].Metastasizing pleomorphic adenoma is histologically ben similar to pleomorphic adenoma, with local or distant metastatic lesions, particularly lowing multiple reopeerations [16,17].The most concerning prognostic consideration with PA is potential post-surgical recurrence, which has been reported to vary between 1% and 50% [15].This variation depends upon the initial surgical approach [16].PAs have the potential for malignant transformation into malignant mixed tumor (carcinosarcoma) (Figure 3) and carcinoma expleomorphic adenoma [17].Metastasizing pleomorphic adenoma is histologically benign, similar to pleomorphic adenoma, with local or distant metastatic lesions, particularly following multiple reopeerations [16,17].

Warthin Tumor
Warthin tumors (WTs) are the second most common benign parotid gland tumors in adults [18], accounting for 4-25% of all salivary gland tumors.The tail of the parotid gland is the most common location [19].WTs typically contain dense lymphoid parenchyma; therefore, they are also known as papillary cystadenoma lymphomatosum or adenolymphoma [18].WTs are the most common parotid tumor to present as multifocal bilateral nodules, which occurs in up to 10% of cases [17].They often appear as a slowly progressive parotid gland swelling in older male subjects (typically >60 years) [20][21][22].
WTs present as well-circumscribed mixed cystic and solid mass lesions on CT (Figure 4) or MRI [1,23].On T2 WIs, the solid components of WTs have a hypointense signal with a relatively poor contrast enhancement on post-contrast series (Figure 4) [23].This helps to differentiate WT from PA. WTs also have restricted diffusivity with low ADC values due to the propensity of internal lymphoid tissue.This tumor is to be considered in the differential diagnosis of multifocal parotid masses, along with metastases, metastatic intraparotid lymph nodes, lymphoma, benign lymphoepithelial lesions, and other inflammatory disorders (Figure 4) [3].

Warthin Tumor
Warthin tumors (WTs) are the second most common benign parotid gland tumors in adults [18], accounting for 4-25% of all salivary gland tumors.The tail of the parotid gland is the most common location [19].WTs typically contain dense lymphoid parenchyma; therefore, they are also known as papillary cystadenoma lymphomatosum or adenolymphoma [18].WTs are the most common parotid tumor to present as multifocal bilateral nodules, which occurs in up to 10% of cases [17].They often appear as a slowly progressive parotid gland swelling in older male subjects (typically >60 years) [20][21][22].
WTs present as well-circumscribed mixed cystic and solid mass lesions on CT (Figure 4) or MRI [1,23].On T2 WIs, the solid components of WTs have a hypointense signal with a relatively poor contrast enhancement on post-contrast series (Figure 4) [23].This helps to differentiate WT from PA. WTs also have restricted diffusivity with low ADC values due to the propensity of internal lymphoid tissue.This tumor is to be considered in the differential diagnosis of multifocal parotid masses, along with metastases, metastatic intraparotid lymph nodes, lymphoma, benign lymphoepithelial lesions, and other inflammatory disorders (Figure 4) [3].

Warthin Tumor
Warthin tumors (WTs) are the second most common benign parotid gland tumors in adults [18], accounting for 4-25% of all salivary gland tumors.The tail of the parotid gland is the most common location [19].WTs typically contain dense lymphoid parenchyma; therefore, they are also known as papillary cystadenoma lymphomatosum or adenolymphoma [18].WTs are the most common parotid tumor to present as multifocal bilateral nodules, which occurs in up to 10% of cases [17].They often appear as a slowly progressive parotid gland swelling in older male subjects (typically >60 years) [20][21][22].
WTs present as well-circumscribed mixed cystic and solid mass lesions on CT (Figure 4) or MRI [1,23].On T2 WIs, the solid components of WTs have a hypointense signal with a relatively poor contrast enhancement on post-contrast series (Figure 4) [23].This helps to differentiate WT from PA. WTs also have restricted diffusivity with low ADC values due to the propensity of internal lymphoid tissue.This tumor is to be considered in the differential diagnosis of multifocal parotid masses, along with metastases, metastatic intraparotid lymph nodes, lymphoma, benign lymphoepithelial lesions, and other inflammatory disorders (Figure 4) [3].The mass indents the underlying left sternomastoid muscle with a clear line of cleavage.Axial contrast-enhanced CT image (B) of a left parotid WT in a 32-year-old male shows a well-circumscribed rim-enhancing mass within the posterior margin of the superficial lobe of the left parotid gland (arrow).The mass is partly exophytic and abuts the overlying skin.Axial contrast-enhanced CT image (C) of bilateral parotid WTs in a 60-year-old male demonstrates multifocal well-circumscribed mildly enhancing masses within the parotid glands bilaterally (long arrows).The mass within the right deep parotid lobe appears hypodense with peripheral enhancement (short arrow).Coronal contrast-enhanced CT image (D) of a left parotid WT in a 62-year-old male demonstrate a wellcircumscribed multi-loculated peripherally enhancing mass within the postero-inferior margin of the left parotid gland (wide arrow).Diffusion-weighted image (E) and corresponding ADC map (F) of a right parotid WT in a 64-year-old male demonstrate restricted diffusivity within the small enhancing focus within the posteromedial margin of the mass (arrows).

Lipoma
Parotid gland lipoma is uncommon and accounts for 0.6-4.4% of parotid lesions [24,25].Lipomas appear as oval-shaped well-circumscribed masses within the parotid gland.Lipomas are characterized by their fat content, being easily recognized on CT and MRI, secondary to the typical hypodense CT attenuation values (generally HU −50 to −150) and fat signal intensities on all MR pulse sequences [26].

Oncocytoma
Oncocytomas represent benign epithelial parotid gland tumors that most frequently occur between the sixth and eighth decades of life with a slight female predominance [27].Oncocytomas are rare and account for only 1% of all salivary gland tumors [28][29][30][31].
Parotid oncocytomas appear as well-defined sharply marginated masses on CT with non-enhancing clefts, as suggested by Tan et al. (Figure 5) [1,29].Large oncocytomas are deformable and distorted when extended into the stylomandibular groove [29].They have been called vanishing parotid tumors as they appear isointense to adjacent normal parotid glands in MRI [30].Oncocytomas share imaging characteristics with WTs, and both have an iso-to hypointense signal on T2 WIs [32].In addition, both demonstrate avid FDG activity on PET/CT and accumulate Tc 99 m pertechnetate, possibly due to their high content of metabolically active mitochondria [30].While oncocytomas appear hyperintense on fat-saturated contrast-enhanced T1 and are more enhanced than adjacent normal parotid glands, WTs have mild enhancement in these sequences, which may help in the differentiation between the two tumors [1].Several studies have investigated the value of DWI in oncocytomas, revealing low ADC values (1.06 ± 0.06 × 10 −3 mm 2 /s) [6].The mass indents the underlying left sternomastoid muscle with a clear line of cleavage.Axial contrast-enhanced CT image (B) of a left parotid WT in a 32-year-old male shows a well-circumscribed rim-enhancing mass within the posterior margin of the superficial lobe of the left parotid gland (arrow).The mass is partly exophytic and abuts the overlying skin.Axial contrast-enhanced CT image (C) of bilateral parotid WTs in a 60-year-old male demonstrates multifocal well-circumscribed mildly enhancing masses within the parotid glands bilaterally (long arrows).The mass within the right deep parotid lobe appears hypodense with peripheral enhancement (short arrow).Coronal contrast-enhanced CT image (D) of a left parotid WT in a 62-year-old male demonstrate a wellcircumscribed multi-loculated peripherally enhancing mass within the postero-inferior margin of the left parotid gland (wide arrow).Diffusion-weighted image (E) and corresponding ADC map (F) of a right parotid WT in a 64-year-old male demonstrate restricted diffusivity within the small enhancing focus within the posteromedial margin of the mass (arrows).

Lipoma
Parotid gland lipoma is uncommon and accounts for 0.6-4.4% of parotid lesions [24,25].Lipomas appear as oval-shaped well-circumscribed masses within the parotid gland.Lipomas are characterized by their fat content, being easily recognized on CT and MRI, secondary to the typical hypodense CT attenuation values (generally HU −50 to −150) and fat signal intensities on all MR pulse sequences [26].

Oncocytoma
Oncocytomas represent benign epithelial parotid gland tumors that most frequently occur between the sixth and eighth decades of life with a slight female predominance [27].Oncocytomas are rare and account for only 1% of all salivary gland tumors [28][29][30][31].
Parotid oncocytomas appear as well-defined sharply marginated masses on CT with non-enhancing clefts, as suggested by Tan et al. (Figure 5) [1,29].Large oncocytomas are deformable and distorted when extended into the stylomandibular groove [29].They have been called vanishing parotid tumors as they appear isointense to adjacent normal parotid glands in MRI [30].Oncocytomas share imaging characteristics with WTs, and both have an iso-to hypointense signal on T2 WIs [32].In addition, both demonstrate avid FDG activity on PET/CT and accumulate Tc 99 m pertechnetate, possibly due to their high content of metabolically active mitochondria [30].While oncocytomas appear hyperintense on fatsaturated contrast-enhanced T1 and are more enhanced than adjacent normal parotid glands, WTs have mild enhancement in these sequences, which may help in the differentiation between the two tumors [1].Several studies have investigated the value of DWI in oncocytomas, revealing low ADC values (1.06 ± 0.06 × 10 −3 mm 2 /s) [6].
Figure 5. Bilateral parotid oncocytomas.Axial contrast-enhanced CT image (A) of bilateral parotid oncocytomas in a 77-year-old male demonstrates an ill-defined heterogeneously enhancing mass Figure 5. Bilateral parotid oncocytomas.Axial contrast-enhanced CT image (A) of bilateral parotid oncocytomas in a 77-year-old male demonstrates an ill-defined heterogeneously enhancing mass within the posterior portion of the left parotid gland (wide black arrow).Another ovoid mildly enhancing mass is noted within the left parapharyngeal space (wide white arrows).Other smaller multifocal scattered masses are seen within right parotid gland (short narrow arrows).Axial contrast-enhanced CT image (B) in a 78-year-old male shows a large dominant lobulated mildly enhancing mass within the left deep parotid gland (wide black arrow).The mass extends into the left parapharyngeal space with mild deformation of its outline.There is an additional large ovoid-shaped mass within the right parotid deep lobe, extending into the right parapharyngeal space (wide white arrow).Other smaller multifocal scattered masses are also seen within parotid glands (long narrow arrows).

Basal Cell Adenoma
Basal cell adenomas (BCAs) are uncommon, comprising around 1-2% of salivary gland neoplasms [27].They tend to occur in the parotid glands [33].BCAs share histopathological similarities with adenoid cystic carcinomas (AdCCs) and basal cell adenocarcinomas.The lower number of mitoses, basal layer integrity, slow growth, and lack of vascular or neural invasion suggest its benign nature.Some authors state that BCAs are the benign homologue of AdCCs [33,34].
On CT, BCAs manifest as masses with intratumoral cystic changes, linear bands, and stellate-like non-enhancing components [35].Cystic BCAs are more frequent.Solid basal cell adenomas typically have a hypointense signal on T2 and restricted diffusion [36].This is similar to a WT, yet the enhancement pattern can be used to differentiate the entities.Solid portions of BCAs are intensely enhanced on fat-suppressed contrast-enhanced T1 images, whereas WTs are only mildly so [37,38].Few studies have focused on the role of DWI in depicting the basal adenoma and found mean ADC values of 1.24 ± 0.18 × 10 −3 mm 2 /s) [7].

Facial Nerve Schwannoma
Facial nerve schwannomas (FNSs) are uncommon benign neoplasms which arise from Schwann cells surrounding the facial (VII) nerve.Schwannomas may arise within the intraparotid facial nerve, resembling pleomorphic adenomas [39].The majority of FNSs develop within the intratemporal segments of the facial nerve.FNSs on the extratemporal course of the facial nerve are rare and represent only 9% of cases [40].These lesions present clinically with unilateral facial palsy, weakness, and/or paralysis.Multiple bilateral schwannomas are usually associated with neurofibromatosis type 2 (NF-2) and schwannomatosis [2].
Intraparotid facial nerve schwannomas present as well-defined round or oval enhancing masses on CT [40].Typical MR findings include a well-circumscribed mass of an isointense signal on T1 WIs and a hyperintense signal on T2 WIs relative to the muscles, with strong enhancement after gadolinium administration.Larger masses usually have internal cystic degeneration with subsequent heterogeneous enhancement [41].

Benign Lymphoepithelial Lesions
Benign lymphoepithelial lesions (BLELs) are rare and usually found in patients with HIV or Sjögren syndrome [42].These lesions occur within the 4th-7th decades, with a female predominance, and involve both parotid glands [43].These lesions typically manifest clinically as painless parotid glands with swelling and enlargement.They have variable presentations, which include cystic and/or mixed solid and cystic masses [2].
The radiologic characteristics of BLELs usually overlap with those of WTs since both frequently present as bilateral cystic and solid parotid masses [44].CT will differentiate between the cystic components via a thin enhancing rim surrounding the heterogeneously enhancing solid components [45].The cystic portions of BLELs typically appear with T1 hypointense and T2 hyperintense signals, while the solid portions have variable signal intensity and levels of enhancement [1].In addition, hypertrophy of Waldeyer's lymphatic ring is strongly associated with BLEL in patients with HIV [46,47].
Other benign parotid gland tumors like myoepithelioma, sebaceous adenoma, cystadenoma, and lymphadenoma are extremely rare and do not have unique imaging characteristics [48].

Malignant Parotid Tumors
Malignant parotid neoplasms usually present as palpable, very firm, solid masses, often with facial nerve symptoms (Table 1).Typical imaging features of malignant parotid tumors on conventional CT and MRI include an infiltrative mass with ill-defined borders, unclear-unsharp margins and evident invasion of the surrounding structures or perineural invasion [49].Additionally, most high-grade malignant parotid tumors have a hypointense signal on T2 WIs with heterogeneous enhancement on the post-contrast series [49].Advanced MR imaging techniques might provide further information to help in the differentiation between benign and malignant parotid tumors [6].Malignant parotid tumors have restricted diffusion with low ADC [6].

Mucoepidermoid Carcinomas
Mucoepidermoid carcinomas (MECs) of the parotid gland represent the most common malignant tumors in adults and the pediatric age population [50,51].They are believed to arise from pluripotent reserve cells of the excretory ducts.MECs have a wide range of histological features owing to their cellular heterogeneity and different cellular compositions: mucinous, intermediate, and epidermoid [52,53].MECs are classified as high-, intermediate-, or low-grade, correlating with their clinical behavior [50,54].
Low-grade tumors are generally well circumscribed, and the high-grade ones tend to have ill-defined margins with infiltration of the surrounding tissues [54,55].Intermediateand high-grade mucoepidermoid carcinomas have a characteristic iso-hypointense signal on T1/T2 WIs reflective of high cellularity.Low-grade tumors tend to have a hyperintense signal on T2 WIs due to their higher content of mucin-secreting cells [55].High-grade tumors usually show ill-defined margins; however, low-grade MECs can present with similar margins as well, secondary to peri-tumoral inflammatory reaction [55].
MECs can metastasize, with common sites being local lymph nodes and, more distantly, the lung and bone.Perineural tumor invasion is a common feature along the intraparotid facial nerve to its mastoid portion within the temporal bone [2].

Adenoid Cystic Carcinoma
Adenoid cystic carcinomas (AdCCs) mainly occur in the parotid gland.When they develop within the minor salivary glands, they have a worse prognosis [56][57][58][59].AdCCs represent approximately 1% of all oral and maxillofacial malignant tumors, and 21.9% of all salivary gland malignancies [60].They often manifest as an infiltrative mass on CT and MRI with a high propensity for perineural spread [61].AdCCs typically demonstrate a heterogeneous hyperintense signal on T2 WIs with heterogeneous enhancement and unrestricted diffusion on DWI owing to internal cystic changes.Perineural invasion is common and usually appears as 'skip' lesions along the course of an apparently normal nerve.Perineural invasion is a major poor prognostic indicator due to the potential risk of extension into the skull base and cranial cavity as well as the high probability of local recurrences, making surgical resection difficult [62].

Acinic Cell Carcinomas
Acinic cell carcinomas (ACCs) are uncommon salivary gland tumors which occur most often during the 5th-6th decades of life, with a slight female predominance [63].
Ultrasound is the initial imaging modality used to evaluate parotid masses.ACCs typically present as masses with suspicious sonographic criteria [64].CT is better for evaluating tumor size, involvement, and relationship with surrounding structures, as well as distant metastasis (Figure 6).MRI is the imaging modality of choice for ACC evaluation [56].ACCs usually appear as masses with a hypointense signal on T1 WIs and T2 WIs, reflecting internal areas of fibrosis, hemosiderin deposition, and calcification.ACCs typically have mild homogeneous enhancement [63].These tumors typically present with facial nerve perineural spread, manifesting as focal nodular thickening and enhancement along the course of the nerve.Perineural spread is best captured on contrast-enhanced T1 fat-suppressed images [65,66].[56].ACCs usually appear as masses with a hypointense signal on T1 WIs and T2 WIs, reflecting internal areas of fibrosis, hemosiderin deposition, and calcification.ACCs typically have mild homogeneous enhancement [63].These tumors typically present with facial nerve perineural spread, manifesting as focal nodular thickening and enhancement along the course of the nerve.Perineural spread is best captured on contrast-enhanced T1 fat-suppressed images [65,66].

Salivary Duct Carcinoma
This is an uncommon, and highly aggressive, malignant parotid gland tumor which has a male predilection [67].Histologically, salivary duct carcinoma resembles mammary ductal carcinoma with an intra-ductal and infiltrating pattern [68].These tumors are characterized by local recurrence, loco-regional and early distant metastasis, with significant overall mortality [69].
Salivary duct carcinomas do not have any characteristic imaging findings on CT and MRI.Like most aggressive tumors, they present as infiltrative mases with ill-defined margins and frequent internal necrosis [1,70].Intratumoral calcification is frequently seen in this tumor.Some authors have proposed that a T2 signal could be correlated with the tumor grade, since high-grade tumors tend to have a hypointense signal on T2 WIs [24,71].

Polymorphous Adenocarcinomas
Polymorphous adenocarcinomas (PACs) most commonly occur in the oral cavity and oropharyngeal minor salivary glands [72].They were previously known as polymorphous low-grade adenocarcinomas prior to the new WHO classification of salivary gland tumors

Salivary Duct Carcinoma
This is an uncommon, and highly aggressive, malignant parotid gland tumor which has a male predilection [67].Histologically, salivary duct carcinoma resembles mammary ductal carcinoma with an intra-ductal and infiltrating pattern [68].These tumors are characterized by local recurrence, loco-regional and early distant metastasis, with significant overall mortality [69].
Salivary duct carcinomas do not have any characteristic imaging findings on CT and MRI.Like most aggressive tumors, they present as infiltrative mases with ill-defined margins and frequent internal necrosis [1,70].Intratumoral calcification is frequently seen in this tumor.Some authors have proposed that a T2 signal could be correlated with the tumor grade, since high-grade tumors tend to have a hypointense signal on T2 WIs [24,71].

Polymorphous Adenocarcinomas
Polymorphous adenocarcinomas (PACs) most commonly occur in the oral cavity and oropharyngeal minor salivary glands [72].They were previously known as polymorphous low-grade adenocarcinomas prior to the new WHO classification of salivary gland tumors [73].PACs do not frequently develop de novo or from a pre-existing PA within the major salivary glands [72].
PACs present as indolent slow-growing lesions with an infiltrative growth pattern locally, along with perineural and perivascular invasive characteristics.They have fewer common nodal metastases with rare occurrences of distant spread [74].
The imaging criteria of PACs overlap with those for PAs and AdCCs.PACs typically exhibit irregular walls with heterogeneous enhancement on contrast-enhanced CTs [75].On MRI, PACs manifest as well-defined masses with T1 intermediate and T2 hyperintense signals as low as 0.95 ± 0.09 × 10 −3 mm 2 /s in terms of the ADC.MRI is helpful for the evaluation of the perineural invasion, while CT is crucial in evaluating for bone invasion [76,77].

Primary Squamous Cell Carcinomas
Squamous cell carcinomas (SCCs) are high-grade malignant tumors that account for 0.1-3.4% of all parotid gland tumors [78].They may be a primary de novo SCC or arise from a pre-existing PA [78,79].
Because these tumors are rare, typical imaging features have not been thoroughly described in the literature.However, their known aggressive behavior and high grade make these tumors appear as ill-defined masses with irregular margins [17].Infiltration and invasion of the surrounding structures are sometimes seen, with possible perineural invasion [3].Typical necrotic areas in solid tumors have been described and are best recognized with post-contrast CT imaging and fat-suppressed contrast-enhanced T1 MR images (Figure 7) [78].[73].PACs do not frequently develop de novo or from a pre-existing PA within the major salivary glands [72].
PACs present as indolent slow-growing lesions with an infiltrative growth pattern locally, along with perineural and perivascular invasive characteristics.They have fewer common nodal metastases with rare occurrences of distant spread [74].
The imaging criteria of PACs overlap with those for PAs and AdCCs.PACs typically exhibit irregular walls with heterogeneous enhancement on contrast-enhanced CTs [75].On MRI, PACs manifest as well-defined masses with T1 intermediate and T2 hyperintense signals as low as 0.95 ± 0.09 × 10 −3 mm 2 /s in terms of the ADC.MRI is helpful for the evaluation of the perineural invasion, while CT is crucial in evaluating for bone invasion [76,77].

Primary Squamous Cell Carcinomas
Squamous cell carcinomas (SCCs) are high-grade malignant tumors that account for 0.1-3.4% of all parotid gland tumors [78].They may be a primary de novo SCC or arise from a pre-existing PA [78,79].
Because these tumors are rare, typical imaging features have not been thoroughly described in the literature.However, their known aggressive behavior and high grade make these tumors appear as ill-defined masses with irregular margins [17].Infiltration and invasion of the surrounding structures are sometimes seen, with possible perineural invasion [3].Typical necrotic areas in solid tumors have been described and are best recognized with post-contrast CT imaging and fat-suppressed contrast-enhanced T1 MR images (Figure 7) [78].

Metastases
Parotid metastatic lesions should be taken into consideration in patients with a known malignancy presenting with a parotid mass [3].Dermal and epidermal malignancies of the scalp and face, like melanoma and squamous cell carcinoma represent the most frequent intraparotid metastatic lesions [80].Intraparotid metastases from breast or lung cancers are extremely uncommon [81].
The majority (approximately 90%) of intraparotid metastases result from supraclavicular neoplasms through the lymphatic route [54].They typically present as multifocal masses, however solitary metastatic deposits within the parotid gland have been also described [54,81].
It is hypothesized that intraparotid metastatic deposits are due to propensity of intraparotid lymphatics and lymph nodes, which drain the scalp, external ear, and face [82].Cutaneous malignancies for instance, squamous cell carcinomas, and melanoma are the most encountered tumors metastasizing to the parotid glands, hence careful clinical examination with high-risk of suspicion should be performed (Figure 8) [82].The imaging criteria lack significant sensitivity and specificity for intraparotid metastatic disease [51,83].DWI might help differentiate between benign and malignant lymph nodes through quantitative measurement of ADC [55].An ADC threshold of 0.94 × 10 −3 mm 2 /s was suggested by Thoeny et al. with a sensitivity of 84%, and a specificity of 94% [55].

Metastases
Parotid metastatic lesions should be taken into consideration in patients with a known malignancy presenting with a parotid mass [3].Dermal and epidermal malignancies of the scalp and face, like melanoma and squamous cell carcinoma represent the most frequent intraparotid metastatic lesions [80].Intraparotid metastases from breast or lung cancers are extremely uncommon [81].
The majority (approximately 90%) of intraparotid metastases result from supraclavicular neoplasms through the lymphatic route [54].They typically present as multifocal masses, however solitary metastatic deposits within the parotid gland have been also described [54,81].
It is hypothesized that intraparotid metastatic deposits are due to propensity of intraparotid lymphatics and lymph nodes, which drain the scalp, external ear, and face [82].Cutaneous malignancies for instance, squamous cell carcinomas, and melanoma are the most encountered tumors metastasizing to the parotid glands, hence careful clinical examination with high-risk of suspicion should be performed (Figure 8) [82].The imaging criteria lack significant sensitivity and specificity for intraparotid metastatic disease [51,83].DWI might help differentiate between benign and malignant lymph nodes through quantitative measurement of ADC [55].An ADC threshold of 0.94 × 10 −3 mm 2 /s was suggested by Thoeny et al. with a sensitivity of 84%, and a specificity of 94% [55].

Primary Lymphoma
Non-Hodgkin lymphoma (NHL) of the salivary glands is uncommon, accounting for 5% of primary extra-nodal NHLs and 2% of salivary gland neoplasms [84].The parotid gland is the most frequently involved salivary gland and the site of about 75% of these cases [84].Extra-nodal marginal zone B-cell lymphoma (MZL) of the mucosa-associated lymphoid tissue (MALT) type, diffuse large B-cell lymphoma, and follicular B-cell lymphoma are the most commonly encountered NHL subtypes within parotid glands [85].
Primary parotid lymphomas have variable presentations, ranging from focal solid or solid-cystic masses to diffusely solid-cystic changes in the gland and multiple solid masses [84,86].The parotid gland is the most commonly involved salivary gland in secondary lymphoma, being seen in approximately 80% of cases.Patients with Sjogren's syndrome have a 44-times-greater risk of developing NHL compared with the general population [87].CT and MRI are equally effective for the evaluation of tumor characteristics,

Primary Lymphoma
Non-Hodgkin lymphoma (NHL) of the salivary glands is uncommon, accounting for 5% of primary extra-nodal NHLs and 2% of salivary gland neoplasms [84].The parotid gland is the most frequently involved salivary gland and the site of about 75% of these cases [84].Extra-nodal marginal zone B-cell lymphoma (MZL) of the mucosa-associated lymphoid tissue (MALT) type, diffuse large B-cell lymphoma, and follicular B-cell lymphoma are the most commonly encountered NHL subtypes within parotid glands [85].
Primary parotid lymphomas have variable presentations, ranging from focal solid or solid-cystic masses to diffusely solid-cystic changes in the gland and multiple solid masses [84,86].The parotid gland is the most commonly involved salivary gland in secondary lymphoma, being seen in approximately 80% of cases.Patients with Sjogren's syndrome have a 44-times-greater risk of developing NHL compared with the general population [87].CT and MRI are equally effective for the evaluation of tumor characteristics, yet CT is preferred due to its accessibility, lower cost, and fast results (Figure 9) [87]. 18F-FDG-PET/CT evaluation is essential for proper staging [88].
yet CT is preferred due to its accessibility, lower cost, and fast results (Figure 9) [87]. 18F-FDG-PET/CT evaluation is essential for proper staging [88].

Conclusions
Preoperative imaging evaluation of the parotid gland tumors is imperative to differentiate between benign and malignant neoplasms and decrease the rate of unnecessary interventional procedures.Multimodality imaging with CT and various MR sequences are usually required to suggest a diagnosis.Figure 10 shows a suggested approach for the evaluation of parotid masses with the proper imaging modality and the impact of preoperative imaging on management planning.

Conclusions
Preoperative imaging evaluation of the parotid gland tumors is imperative to differentiate between benign and malignant neoplasms and decrease the rate of unnecessary interventional procedures.Multimodality imaging with CT and various MR sequences are usually required to suggest a diagnosis.Figure 10 shows a suggested approach for the evaluation of parotid masses with the proper imaging modality and the impact of preoperative imaging on management planning.

Figure 2 .
Figure 2. Parotid pleomorphic adenoma in 3 different patients.Axial contrast-enhanced CT im (A) of a left parotid PA in a 90-year-old female shows a small, well-circumscribed enhancing m within the anteromedial portion of the left parotid tail (arrow).Axial contrast-enhanced CT im (B) of a right parotid PA in a 69-year-old female shows a small, well-circumscribed ovoid-sha enhancing mass within the posteromedial portion of the right parotid tail (dashed circle).A Short Tau Inversion Recovery (STIR) image (C) of a right parotid PA in a 69-year-old female sh a well-circumscribed lobulated mass within the deep lobe of the right parotid gland.The mass characteristic T2 hyperintensity (arrows).The left lateral retropharyngeal lymph node is part imaged.

Figure 2 .
Figure 2. Parotid pleomorphic adenoma in 3 different patients.Axial contrast-enhanced CT image (A) of a left parotid PA in a 90-year-old female shows a small, well-circumscribed enhancing mass within the anteromedial portion of the left parotid tail (arrow).Axial contrast-enhanced CT image (B) of a right parotid PA in a 69-year-old female shows a small, well-circumscribed ovoidshaped enhancing mass within the posteromedial portion of the right parotid tail (dashed circle).Axial Short Tau Inversion Recovery (STIR) image (C) of a right parotid PA in a 69-year-old female shows a well-circumscribed lobulated mass within the deep lobe of the right parotid gland.The mass has characteristic T2 hyperintensity (arrows).The left lateral retropharyngeal lymph node is partially imaged.

Figure 3 .
Figure 3. Left parotid, radiation-induced carcinosarcoma in a 51-year-old male.Axial contrast-enhanced CT image (A) shows a large heterogeneously enhancing mass involving the left parotid gland (white arrow), with internal areas of necrosis (arrowhead).Axial fat-saturated contrast-enhanced T1 image (B) shows a heterogeneously enhancing mass within the left parotid gland (arrows), with areas of necrosis at the posterolateral margins of the lesion (arrowhead).Axial FDG-PET/CT image (C) demonstrates avid FDG uptake of the enhancing solid components of the mass (wide arrow).The area of necrosis lacks FDG uptake (arrowhead).

Figure 3 .
Figure 3. Left parotid, radiation-induced carcinosarcoma in a 51-year-old male.Axial contrastenhanced CT image (A) shows a large heterogeneously enhancing mass involving the left parotid gland (white arrow), with internal areas of necrosis (arrowhead).Axial fat-saturated contrastenhanced T1 image (B) shows a heterogeneously enhancing mass within the left parotid gland (arrows), with areas of necrosis at the posterolateral margins of the lesion (arrowhead).Axial FDG-PET/CT image (C) demonstrates avid FDG uptake of the enhancing solid components of the mass (wide arrow).The area of necrosis lacks FDG uptake (arrowhead).

Figure 3 .
Figure 3. Left parotid, radiation-induced carcinosarcoma in a 51-year-old male.Axial contrast-enhanced CT image (A) shows a large heterogeneously enhancing mass involving the left parotid gland (white arrow), with internal areas of necrosis (arrowhead).Axial fat-saturated contrast-enhanced T1 image (B) shows a heterogeneously enhancing mass within the left parotid gland (arrows), with areas of necrosis at the posterolateral margins of the lesion (arrowhead).Axial FDG-PET/CT image (C) demonstrates avid FDG uptake of the enhancing solid components of the mass (wide arrow).The area of necrosis lacks FDG uptake (arrowhead).

Figure 4 .
Figure 4. Variable imaging presentations of Warthin tumors.Axial contrast-enhanced CT image (A) of left parotid WT in a 59-year-old male reveals a well-circumscribed ovoid-shaped heterogeneously

Figure 4 .
Figure 4. Variable imaging presentations of Warthin tumors.Axial contrast-enhanced CT image (A) of left parotid WT in a 59-year-old male reveals a well-circumscribed ovoid-shaped heterogeneously enhancing mass within the posterior margin of the superficial lobe of the left parotid gland (arrows).The mass indents the underlying left sternomastoid muscle with a clear line of cleavage.Axial contrast-enhanced CT image (B) of a left parotid WT in a 32-year-old male shows a well-circumscribed rim-enhancing mass within the posterior margin of the superficial lobe of the left parotid gland (arrow).The mass is partly exophytic and abuts the overlying skin.Axial contrast-enhanced CT image (C) of bilateral parotid WTs in a 60-year-old male demonstrates multifocal well-circumscribed mildly enhancing masses within the parotid glands bilaterally (long arrows).The mass within the right deep parotid lobe appears hypodense with peripheral enhancement (short arrow).Coronal contrast-enhanced CT image (D) of a left parotid WT in a 62-year-old male demonstrate a wellcircumscribed multi-loculated peripherally enhancing mass within the postero-inferior margin of the left parotid gland (wide arrow).Diffusion-weighted image (E) and corresponding ADC map (F) of a right parotid WT in a 64-year-old male demonstrate restricted diffusivity within the small enhancing focus within the posteromedial margin of the mass (arrows).

Figure 6 .
Figure 6.Right parotid acinic cell carcinoma in a 61-year-old female.Axial contrast-enhanced CT image demonstrates a large mildly irregular enhancing mass within the posteromedial margin of deep lobe of the right parotid gland (wide arrow).

Figure 6 .
Figure 6.Right parotid acinic cell carcinoma in a 61-year-old female.Axial contrast-enhanced CT image demonstrates a large mildly irregular enhancing mass within the posteromedial margin of deep lobe of the right parotid gland (wide arrow).

Figure 7 .
Figure 7. Left parotid squamous cell carcinoma in a 77-year-old male.Axial contrast-enhanced CT images (A,B) demonstrate a large non uniform peripherally enhancing necrotic mass within the superficial lobe of the left parotid gland (wide arrows).The mass slightly abuts the adjacent mandible without invasion.Small rim-enhancing necrotic left level 2 lymph node is seen (long narrow arrow).Axial FDG-PET/CT images (C,D) confirm intense peripheral FDG uptake within the left superficial parotid mass and left level 2 lymph node (wide arrow in image (C), and long narrow arrow in image (D)).

Figure 7 .
Figure 7. Left parotid squamous cell carcinoma in a 77-year-old male.Axial contrast-enhanced CT images (A,B) demonstrate a large non uniform peripherally enhancing necrotic mass within the superficial lobe of the left parotid gland (wide arrows).The mass slightly abuts the adjacent mandible without invasion.Small rim-enhancing necrotic left level 2 lymph node is seen (long narrow arrow).Axial FDG-PET/CT images (C,D) confirm intense peripheral FDG uptake within the left superficial parotid mass and left level 2 lymph node (wide arrow in image (C), and long narrow arrow in image (D)).

Figure 8 .
Figure 8. Left intraparotid metastatic melanoma in a 75-year-old female.Axial contrast-enhanced CT images (A,B) show a small ovoid-shaped mass within the superficial lobe of the left parotid gland (long white arrow).Note the mass effect and intimate distance to the left retromandibular vein.A large left level 1b metastatic lymph node can be seen in the image (B) (wide black arrow).

Figure 8 .
Figure 8. Left intraparotid metastatic melanoma in a 75-year-old female.Axial contrast-enhanced CT images (A,B) show a small ovoid-shaped mass within the superficial lobe of the left parotid gland (long white arrow).Note the mass effect and intimate distance to the left retromandibular vein.A large left level 1b metastatic lymph node can be seen in the image (B) (wide black arrow).

Figure 9 .
Figure 9. Right parotid lymphoma in a 60-year-old female.Axial contrast-enhanced CT image demonstrates a large micro-lobulated mildly enhancing mass involving most of the posterior right parotid gland (wide arrow).The lesion extends into subcutaneous fat with skin infiltration (short white arrows).

Figure 9 .
Figure 9. Right parotid lymphoma in a 60-year-old female.Axial contrast-enhanced CT image demonstrates a large micro-lobulated mildly enhancing mass involving most of the posterior right parotid gland (wide arrow).The lesion extends into subcutaneous fat with skin infiltration (short white arrows).

Figure 10 .
Figure 10.Expected impact of preoperative imaging on management of clinically presented parotid masses.ADC: apparent diffusion coefficient; CT: computed tomography; FNAC: fine-needle aspiration cytology; MRI: magnetic resonance imaging.

Table 1 .
Typical imaging features of common benign and malignant parotid tumors.