DOI: 10.1148/rg.263055024
RadioGraphics 2006;26:745-763
© RSNA, 2006
US of the Major Salivary Glands: Anatomy and Spatial Relationships, Pathologic Conditions, and Pitfalls1
Ewa J. Bialek, MD, PhD,
Wieslaw Jakubowski, MD, PhD,
Piotr Zajkowski, MD, PhD,
Kazimierz T. Szopinski, MD, PhD and
Antoni Osmolski, MD, PhD
1 From the Department of Diagnostic Imaging, Second Faculty of Medicine, Medical University of Warsaw, ul. Kondratowicza 8, 03-242 Warsaw, Poland (E.J.B., W.J., P.Z., K.T.S.); and the Department of Otolaryngology, Center of Postgraduate Medical Education, Warsaw, Poland (A.O.). Recipient of an Excellence in Design award for an education exhibit at the 2004 RSNA Annual Meeting. Received February 8, 2005; revision requested March 18 and received November 8; accepted November 9. All authors have no financial relationships to disclose.
Address correspondence to E.J.B. (e-mail: ewajbmd{at}go2.pl).
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Abstract
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Ultrasonography (US) is useful for differential diagnosis of diseases of the salivary glands. In acute inflammation, salivary glands are enlarged and hypoechoic with increased blood flow; they may contain multiple small, oval, hypoechoic areas. In chronic inflammation, salivary glands are normal sized or smaller, hypoechoic, and inhomogeneous. Sialolithiasis appears as markedly hyperechoic lines or points with distal acoustic shadowing. Sialosis appears as enlarged hyperechoic glands without focal lesions or increased blood flow. The US features of advanced Sjögren syndrome include inhomogeneous salivary glands with scattered small, oval, hypoechoic or anechoic areas, usually well defined, and increased parenchymal blood flow. Pleomorphic adenomas are usually hypoechoic, well-defined, lobulated lesions with posterior acoustic enhancement that may contain calcifications; Warthin tumors are usually oval, hypoechoic, well-defined lesions that often contain anechoic areas and are often hypervascularized. Malignant neoplasms of the salivary glands may have irregular shapes, irregular borders, blurred margins, and a hypoechoic inhomogeneous structure or may have a benign appearance. Salivary gland cysts have well-defined margins, anechoic contents, posterior acoustic enhancement, and no internal blood flow. However, US appearances of some diseases may overlap, thus producing diagnostic pitfalls.
© RSNA, 2006
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Introduction
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The algorithm proposed in the United States for imaging of salivary glands includes nonenhanced and contrast-enhanced computed tomography (CT), nonenhanced and contrast-enhanced magnetic resonance (MR) imaging, and sialography (also MR sialography), applied in a different order depending on clinical data (1,2). In general, CT is considered the best single method for assessment of inflammatory diseases and MR imaging is considered the best single method for assessment of salivary gland tumors (1–3). According to Yousem et al (2) ultrasonography (US) is underused in most North American sites, but in experienced hands it may supplant both CT and MR in imaging of superficial salivary gland lesions.
In Europe and Asia, US is widely accepted as the first imaging method for assessment of lymph nodes and soft-tissue diseases in the head and neck, including major salivary glands (4–7). Results of the US examination alone may suggest the final diagnosis or supply important differential diagnostic data. As the head and neck region has a complex anatomic structure, a sound knowledge of sonographic anatomy and spatial relationships is crucial for reliable performance of the examination. Also, knowledge of the sonographic features of the most common diseases in this area is a requisite.
It is sometimes not possible to visualize examined lesions completely at US because of their location, penetrating to the deep lobe of the parotid gland or behind the acoustic shadow of the mandible. In these situations, performance of further imaging examinations—CT or MR imaging—is warranted. Also, in cases of suspected malignant lesions, further diagnostic methods (ie, CT or MR imaging) should be applied to assess possible infiltration of bones or deep structures invisible at US (the base of the skull, parapharyngeal space) and to evaluate deep-lying lymph nodes (1,3,8,9). On the other hand, dynamic scintigraphy is still the method of choice in functional evaluation of the salivary glands (10,11).
In this article, we present the anatomy of the major salivary glands and neighboring structures as seen at US, as well as the US features of the most common pathologic conditions affecting the parotid and submandibular glands. These conditions include inflammatory diseases, sialolithiasis, sialosis, Sjögren syndrome, neoplasms, cysts, and trauma, as well as the effects of irradiation.
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Technique
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The examination should be carried out with the highest-frequency transducer possible. Usually, 5–12-MHz wide-band linear transducers (median frequency, 7–7.5 MHz or more) are used (9). In assessment of large tumors and lesions located in deep portions of the glands, 5–10-MHz transducers may be useful (12). Probes with a median frequency above 10 MHz may be useful in evaluation of the internal structure of salivary glands (12,13).
Entire salivary glands and all lesions have to be evaluated in at least two perpendicular planes during a US examination. The whole neck should also be scanned to assess lymph nodes and search for concomitant or related disease.
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Anatomy
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Parotid Gland
The parotid gland is located in the retromandibular fossa, anterior to the ear and sternocleidomastoid muscle. Parts of the superficial lobe cover the ramus of the mandible and the posterior part of the masseter muscle (Fig 1).
The border between the superficial and deep parotid lobes is created by a plane in which the facial nerve and its branches are located. Branches of the facial nerve are not visible at US. Parts of the trunk of this nerve may be demonstrated only with high-frequency probes (above 10 MHz) (13). Therefore, the retromandibular vein, which usually lies directly above the trunk of the facial nerve (14), is used as a US landmark separating the superficial and deep lobes of the parotid gland (Figs 2–4). Although the extracranial portion of the facial nerve may be visualized on high-resolution MR images (15), the retromandibular vein is commonly used as an anatomic landmark in preoperative CT and MR imaging examinations of parotid neoplasms (16). The deep parotid lobe can be visualized only partially at US. Some areas of glandular parenchyma and possible lesions may be hidden in the acoustic shadow behind the mandibular ramus (Fig 4).
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Figure 2. Drawing shows the major blood vessels in the area of the salivary glands. 1 = retromandibular vein, 2 = external carotid artery, 3 = facial artery and vein, 4 = lingual artery and vein, 5 = external carotid artery, 6 = internal jugular vein, 7 = external jugular vein.
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Figure 3a. Transverse (a) and longitudinal (b) US images show the normal anatomy of the left parotid gland. The positions of the US probe are shown in the inset diagrams. 1 = retromandibular vein, 2 = external carotid artery, 3 = echo from the surface of the mandible, 4 = parotid gland, 5 = masseter muscle.
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Figure 3b. Transverse (a) and longitudinal (b) US images show the normal anatomy of the left parotid gland. The positions of the US probe are shown in the inset diagrams. 1 = retromandibular vein, 2 = external carotid artery, 3 = echo from the surface of the mandible, 4 = parotid gland, 5 = masseter muscle.
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Figure 4a. Transverse US image of the right parotid gland (a) and corresponding diagram (b) show the border between the superficial and deep lobes of the gland. The position of the US probe is shown in the inset diagram.
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Figure 4b. Transverse US image of the right parotid gland (a) and corresponding diagram (b) show the border between the superficial and deep lobes of the gland. The position of the US probe is shown in the inset diagram.
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The normal echogenicity of all major salivary glands, including the parotid gland, is generally homogeneous and varies from very bright and markedly hyperechoic to only slightly hyperechoic in comparison to adjacent muscles. The echogenicity of the parotid gland depends on the amount of intraglandular fatty tissue. Salivary glands with high fat content are hyperechoic in comparison to surrounding muscles and markedly suppress ultrasound waves, so that the deep lobe is inaccessible for US assessment and sometimes even large vessels crossing the parotid gland—the retromandibular vein and external carotid artery—are barely visible or not visible at all on gray-scale images (Fig 5).
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Figure 5. Transverse panoramic US image of the left parotid gland (arrows) and cheek shows that the gland has a high fat content. The parenchyma is hyperechoic with marked suppression of ultrasound waves, and no vessels are visible. The position of the US probe is shown in the inset diagram. 1 = masseter muscle.
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After leaving the parotid gland, the main excretory duct (Stenon duct) lies on the masseter muscle, about 1 cm below the zygomatic arch, then crosses the buccal muscle and has its orifice in the parotid papilla at the level of the upper second molar. The length of the Stenon duct usually varies between 3 and 5 cm. A nondilated duct is usually not visible during US examination (Fig 6). However, some authors report showing intraglandular nondilated parts of the Stenon duct with high-resolution US (13).
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Figure 6a. (a) Diagram shows the location of the Stenon duct. 1 = parotid gland, 2 = Stenon duct, 4 = masseter muscle, 5 = surface of the mandible, 6 = buccal muscle, large arrow = retromandibular vein and external carotid artery. (b) Panoramic US image shows a dilated Stenon duct in a patient with sialolithiasis and inflammation. 1 = inflamed left parotid gland, 2 = dilated Stenon duct, 3 = stone, 4 = masseter muscle, 5 = surface of the mandible, 6 = buccal muscle, large arrow = retromandibular vein and external carotid artery.
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Figure 6b. (a) Diagram shows the location of the Stenon duct. 1 = parotid gland, 2 = Stenon duct, 4 = masseter muscle, 5 = surface of the mandible, 6 = buccal muscle, large arrow = retromandibular vein and external carotid artery. (b) Panoramic US image shows a dilated Stenon duct in a patient with sialolithiasis and inflammation. 1 = inflamed left parotid gland, 2 = dilated Stenon duct, 3 = stone, 4 = masseter muscle, 5 = surface of the mandible, 6 = buccal muscle, large arrow = retromandibular vein and external carotid artery.
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Along the course of the Stenon duct in the soft tissues of the cheek, an accessory parotid gland may be found, unilaterally or bilaterally. The accessory parotid gland may also be the site of salivary gland tumors, benign or malignant (17,18).
In the parenchyma of the parotid gland, lymph nodes may be found (19). They are localized mainly in the area of the upper and lower poles of the gland. Normal intraparotid lymph nodes may be oval or have a longitudinal shape (Fig 7). Almost 60% of parotid nodes have a short axis–to–long axis ratio greater than 0.5. The presence of a hyperechoic hilum is one of the important criteria for the normality of parotid lymph nodes (Fig 7). Their short axis should not exceed 5–6 mm in the normal state (6,7). With the application of sensitive power Doppler US, central vessels may be seen in normal parotid lymph nodes.
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Figure 7. Three-dimensional US images show a normal intraparotid lymph node (arrows), which is oval with a homogeneous cortex and a central hyperechoic hilum. The hilum is connected to surrounding connective tissue (arrowhead).
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Submandibular Gland
The submandibular gland lies in the posterior part of the submandibular triangle. The sides of the submandibular triangle are created by the anterior and posterior bellies of the digastric muscle and the body of the mandible. The space anterior to the submandibular gland is occupied by connective tissue and lymph nodes. Generally, the shape of the submandibular gland in longitudinal and transverse sections is close to a triangle (Fig 8). The submandibular gland may be connected with the parotid or sublingual gland by the glandular processes.
The facial artery may cross the parenchyma of the submandibular gland in its tortuous course (Fig 9). The facial vein runs along the anterosuperior part of the submandibular gland. In its posterior portion, a branch connecting with the retromandibular vein may be found (Fig 2). Medially to the submandibular gland run the lingual artery and vein.
The submandibular excretory duct (Wharton duct) runs from the area of the submandibular gland hilum at the level of the border of the mylohyoid muscle, then bends around the free part of the mylohyoid muscle and extends to its orifice at the sublingual caruncle along the medial part of the sublingual gland. In general, a nondilated duct is not visible at US, but sometimes in slim individuals it may be visible (Fig 10).
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Figure 10a. (a) US image shows a nondilated Wharton duct (arrow) in a slim patient. Arrowheads = submandibular gland, 1 = mylohyoid muscle. (b) Diagram shows the course of the Wharton duct (arrow). Arrowheads = submandibular gland, 1 = mylohyoid muscle, 2 = sublingual gland.
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Figure 10b. (a) US image shows a nondilated Wharton duct (arrow) in a slim patient. Arrowheads = submandibular gland, 1 = mylohyoid muscle. (b) Diagram shows the course of the Wharton duct (arrow). Arrowheads = submandibular gland, 1 = mylohyoid muscle, 2 = sublingual gland.
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In some patients (obese patients, those who have undergone neck irradiation), the submandibular parenchyma may suppress ultrasound waves to such an extent that it is not possible to show not only deeper-lying structures but also the lower outline of the submandibular gland.
Sublingual Gland
The sublingual gland lies between the muscles of the oral cavity floor: the geniohyoid muscle, intrinsic muscles of the tongue, and hyoglossal muscle (medially) and the mylohyoid muscle. Its lateral side is adjacent to the mandible. On transverse sections, the shape of the sublingual gland is close to an oval (Fig 11); on sections parallel to the body of the mandible, the shape is longitudinal and lentiform. Along its medial part runs the excretory duct of the submandibular gland.
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Inflammatory Diseases
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Inflammatory diseases are the most common diseases affecting the major salivary glands (1,3).
Acute Inflammation
Acute inflammation causes painful swelling of the salivary gland, often bilaterally. Viral salivary gland infections are the most common in children. A particular predilection for the salivary glands is shown by mumps virus and cytomegalovirus (20). Acute bacterial infections are usually caused by Staphylococcus aureus or oral flora (21).
In acute inflammation, salivary glands are enlarged and hypoechoic. They may be inhomogeneous; may contain multiple small, oval, hypoechoic areas; and may have increased blood flow at US (Figs 12, 13) (9,22–25). Enlarged lymph nodes with increased central blood flow may be observed in acute inflammation of salivary glands (26).
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Figure 12. Power Doppler US image shows an acutely inflamed right submandibular gland (arrows) containing a stone (arrowhead). The gland is enlarged and hypoechoic with rounded edges and increased blood flow.
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Figure 13. Gray-scale US image shows an acutely inflamed right parotid gland (arrows) in a 5-year-old child. The gland is enlarged and inhomogeneous with multiple small, oval, hypoechoic areas (arrowheads). The position of the US probe is shown in the inset diagram.
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Abscess
During acute sialadenitis, abscess formation may take place. Predisposing factors include dehydration and excretory duct obstruction caused by stones or fibrosis (27). At clinical examination, abscesses may be difficult to detect. They usually manifest as painful swelling of the salivary gland with skin reddening (28). The typical fluctuation sign may be absent in about 70% of cases (28).
At US, abscesses are hypoechoic or anechoic lesions with posterior acoustic enhancement and unclear borders (22,28). Central liquefaction may be distinguished as an avascular area or identified by means of moving debris (9). Hyperechoic foci due to microbubbles of gas may be seen within the abscess (19). Organized abscesses may be surrounded by a hyperechoic "halo" (22). US guidance is being used for therapeutic drainage (28,29).
Chronic Sialadenitis
Chronic sialadenitis is clinically characterized by intermittent swelling of the gland, often painful, that may or may not be associated with food (30). In chronic inflammation, salivary glands are normal sized or smaller, hypoechoic, and inhomogeneous and usually do not have increased blood flow at US (Fig 14) (9,22,24).
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Figure 14. Power Doppler US image shows chronic inflammation of the left submandibular gland (arrowheads). The gland is inhomogeneous with decreased parenchymal echogenicity but without increased blood flow. Arrows = stones.
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At US, chronic and sometimes acute sialadenitis in children (Fig 13), as well as acalculous submandibular gland sialadenitis in adults, have also been described as showing multiple small, round or oval, hypoechoic areas or lesions distributed throughout glandular parenchyma (23,25,31). The differential diagnosis in such cases includes sarcoidosis and other granulomatous diseases, Sjögren syndrome, disseminated lymphoma, hematogenous metastases, and benign lymphoepithelial lesions in human immunodeficiency virus (HIV)–positive patients (32–35).
Chronic Sclerosing Sialadenitis
A special form of chronic sialadenitis that may mimic a malignant lesion, both clinically and at imaging, is chronic sclerosing sialadenitis (Küttner tumor) (36,37). In Küttner tumor, diffuse involvement of the salivary gland (usually the submandibular gland) may occur, with multiple small hypoechoic foci scattered on a heterogeneous background of salivary tissue visible at US (36). Focal involvement may also be encountered, with a focal hypoechoic heterogeneous lesion within a normally shaped gland (36,37). In all doubtful cases, verification with fine-needle aspiration biopsy is recommended (38,39).
Granulomatous Sialadenitis
Granulomatous sialadenitis occurs only rarely (20). US features of granulomatous sialadenitis are nonspecific: single or multiple hypoechoic areas in an enlarged or normally sized gland or diffuse low echogenicity (9,12,19,40–43). Blood flow may be increased (40).
Mycobacterial disease of major salivary glands may manifest as a salivary gland mass, clinically indistinguishable from a neoplasm (44). In the parenchymal type of tuberculosis, Chou et al (45) described focal, intraparotid, nearly anechoic zones that might have a cavity or cavities within them. In necrotic caseous cavities, which appear very hypoechoic, no color flow signals can be detected at US, in contrast to most salivary tumors (45). Salivary gland actinomycosis may mimic a malignant tumor at US; it may manifest as a hypoechoic area with ill-defined margins (46).
Lymph Nodes in Sialadenitis
In acute or chronic inflammation, lymph nodes may be enlarged; however, their normal echo-structure (homogeneous cortex and hyperechoic central hilum) is preserved. Central blood vessels or short vessel segments may be visible. Increased central blood flow in lymph nodes may be observed in acute inflammation (26).
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Sialolithiasis
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Salivary stones are most often located in the submandibular gland (60%–90% of cases) and may be multiple (47–50). Parotid glands are affected in about 10%–20% of cases (51).
On classic radiographs, intraglandular and small stones may be missed, and only about 20% of sialoliths are radiopaque (52). CT allows visualization of large stones but without their precise localization and without the possibility of assessment of the ducts (53). The standard technique for imaging of the submandibular duct and the intraglandular ductal system remains digital sialography (54). A novel, noninvasive, promising method appears to be MR sialography, which also gives very good results in detection of sialoliths (2,54,55). US is a noninvasive method, well-established in cases of clinical suspicion of sialolithiasis, and is used as a primary modality, particularly in Europe (51,54). Although some authors claim that sialoliths smaller than 2–3 mm may be overlooked because of the absence of acoustic shadow, these articles are from the 1980s and currently used machines have better resolution and detection possibilities (56,57).
Sialolithiasis causes partial or total mechanical obstruction of the salivary duct, which results in recurrent swelling of a salivary gland during eating and may be complicated by bacterial infection (20,48).
Sialoliths in the distal part of the submandibular duct (Wharton duct) may be palpable in the floor of the mouth. However, sialoliths in the proximal portion of the duct or in the parenchyma of salivary glands may be demonstrated only radiologically.
US features of sialolithiasis include strongly hyperechoic lines or points with distal acoustic shadowing, which represent stones (Fig 15) (22). In symptomatic cases with duct occlusion, dilated excretory ducts are visible (22).
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Figure 15. US image obtained obliquely relative to the mandible shows a sialolith (arrowheads) in the inflamed parenchyma of the right submandibular gland (dashed line), which appears hypoechoic and inhomogeneous. The intraglandular excretory duct (arrows) above the stone is dilated. T = tongue.
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When sialolithiasis of the submandibular gland is suspected, US may demonstrate whether the stone is located in the glandular parenchyma or in the Wharton duct (Figs 15, 16) (58). This distinction is essential for choosing the method of treatment.
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Figure 16. US image shows a stone (arrows) in the dilated Wharton duct (arrowheads) near its orifice at the sublingual caruncle. M = acoustic shadow behind the surface of the body of the mandible. The position of the US probe is shown in the inset diagram.
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In chronic ductal sialolithiasis complicated by chronic or recurrent inflammation, the gland may lose its function. At this stage of disease, stones located in a nondilated duct may be difficult to demonstrate.
Stones located near the duct orifice or in the middle part of the Wharton duct may sometimes be better demonstrated when additional pressure is administered from inside the oral cavity during US examination.
In about 50% of patients, sialolithiasis coexists with inflammation (23). Hyperechoic bubbles of air mixed with saliva may mimic stones in the Wharton duct and thus be a diagnostic pitfall (Fig 17) (19).
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Figure 17. US image shows hyperechoic linear structures (arrows), which may be mistaken for sialoliths in the Wharton duct. These structures represent air bubbles in the oral cavity. Note the "dirty" (not purely anechoic) shadow behind the hyperechoic lines and points. Arrowheads = submandibular gland.
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Sialosis
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Sialosis is a noninflammatory, nonneoplastic, recurrent, painless salivary gland swelling, usually bilateral, which most often concerns the parotid glands. Sialosis has been described in connection with endocrine diseases, malnutrition, hepatic cirrhosis, chronic alcoholism, or different deficiency diseases (eg, avitaminoses) (20). US reveals enlarged, hyperechoic salivary glands with a poorly visible deep lobe but without focal lesions or increased blood flow (9).
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Sjögren Syndrome
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Sjögren syndrome is a chronic autoimmune disease predominantly affecting women over 40 years of age. It is characterized by intense lymphocytic and plasma cell infiltration and destruction of salivary and lacrimal glands (59). Major clinical symptoms include a dry mouth and eyes. Advanced stages of Sjögren syndrome may be recognizable at US examination of the parotid and submandibular glands (60). The disease may affect all salivary glands.
US features of advanced Sjögren syndrome include inhomogeneous structure of the gland with scattered multiple small, oval, hypoechoic or anechoic areas, usually well defined, and increased parenchymal blood flow (Fig 18) (9,32,61). Hypoechoic or anechoic areas are believed to represent infiltration by lymphatic cells, destroyed salivary parenchyma, and dilated ducts.
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Figure 18a. Gray-scale (a) and power Doppler (b) US images show advanced-stage Sjögren syndrome in the parotid gland. The gland has an inhomogeneous structure with multiple small, oval, hypoechoic areas (arrowheads) and increased blood flow. The position of the US probe is shown in the inset diagram.
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Figure 18b. Gray-scale (a) and power Doppler (b) US images show advanced-stage Sjögren syndrome in the parotid gland. The gland has an inhomogeneous structure with multiple small, oval, hypoechoic areas (arrowheads) and increased blood flow. The position of the US probe is shown in the inset diagram.
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Sjögren syndrome is frequently associated with both reactive and neoplastic lymphoproliferative disease (62). Further US monitoring for early detection of possible lymphomatous change is required in patients with Sjögren syndrome (63,64). Biopsy is recommended for lesions exceeding 2 cm or fast-growing lesions (9). Differential diagnosis of Sjögren syndrome with disseminated lymphoma in salivary glands may be challenging. Non-Hodgkin lymphoma manifesting as small multiple nodular disseminations with hypervascularization in the salivary gland has been reported (34,65). In addition, bilateral inflammation (acalculous), granulomatous disease (eg, sarcoidosis), hematogenous metastases, and benign lymphoepithelial lesions in HIV-positive patients should be taken into consideration in cases of multiple hypoechoic areas scattered in salivary gland parenchyma (23,25,33,35,42,43).
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Neoplasms
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Salivary gland neoplasms are relatively rare. Most of them are benign (70%–80%) and found in the parotid glands (80%–90%). About 10%–12% of all salivary gland neoplasms are located in the submandibular glands, but almost half of these neoplasms may be malignant (3,66).
Benign Neoplasms
The most common benign neoplasms of major salivary glands are pleomorphic adenomas (mixed tumor) and Warthin tumors (adenolymphoma, cystadenolymphoma, papillary cystadenoma lymphomatosum). Clinically, they manifest as slowly growing painless masses (67). However, small lesions may be detected incidentally at US. When their US appearance is analyzed, many common features may be found, but definitive differential diagnosis is usually not possible with US even between benign and malignant tumors.
Pleomorphic Adenoma.—
Pleomorphic adenomas occur most often in the parotid gland (60%–90%) in people in the fourth and fifth decades of life but may arise at any age (3,66,68). There is a slight predominance in women (66). Pleomorphic adenomas are usually solitary and unilateral (3,66,68). They grow slowly and may be asymptomatic.
Nontreated pleomorphic adenomas may undergo malignant transformation after decades (3,68). In exceptional cases, pleomorphic adenomas may be clinically aggressive; they may metastasize and even be fatal (20,69,70).
At US, pleomorphic adenomas are hypoechoic, well-defined, lobulated tumors with posterior acoustic enhancement (Fig 19) and may contain calcifications (71–73). The feature of lobulated shape is being emphasized in differential diagnosis (73).
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Figure 19. Gray-scale US image shows the typical appearance of a pleomorphic adenoma (arrows). The lesion is hypoechoic and lobulated with distinct borders and posterior acoustic enhancement.
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Many authors add also a feature of homogeneity, but it seems to depend on the composition of the tumor; when high-resolution transducers are used, more and more internal inhomogeneities are being found (Fig 20) (71,73,74).
Vascularization in pleomorphic adenomas is often poor or absent (even when the sensitive power Doppler mode is used) (Fig 21) but may be abundant (71,73,74). After inadequate surgery, pleomorphic adenomas often recur, usually multifocally (75).
Warthin Tumor.—
Warthin tumor is the next most common benign salivary neoplasm (5%–10% of all benign salivary neoplasms) (66,68). It arises most often in men in the fifth and sixth decades of life (66,68,76). The relationship between smoking and development of Warthin tumors has been proved (67). Warthin tumor is usually solitary, unilateral, and slow growing. In about 10%–60% of cases, tumors may occur bilaterally or multifocally, sometimes metachronously, growing and manifesting clinically at different times (3,68,77,78). Sporadically, the epithelial component of Warthin tumor may undergo malignant transformation (68,79).
At US, Warthin tumors are oval, hypoechoic, well-defined tumors and often contain multiple anechoic areas (Figs 22, 23) (25,74,78,80). Warthin tumors are often hypervascularized (Fig 24) but may also contain only short vessel segments.
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Figure 22. Gray-scale US image shows the typical appearance of a Warthin tumor (arrows). The lesion, which is located in the lower pole of the parotid gland, is oval, well defined, hypoechoic, and inhomogeneous with multiple irregular anechoic areas (arrowheads) and posterior acoustic enhancement.
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Figure 23. Panoramic gray-scale US image shows two Warthin tumors (arrows) in the lower pole of the left parotid gland. The lesions are oval, well defined, hypoechoic, and inhomogeneous.
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Diagnosis of a Warthin tumor may be supported by results of technetium 99m scintigraphy, which reveals a "hot" tumor because of the increased uptake of the tracer by the tumor (81). However, some other parotid neoplasms, benign as well as malignant, may sporadically show uptake of the radionuclide.
Lobulated shape in pleomorphic adenomas and anechoic areas in Warthin tumors, although common, are not pathognomonic and may be found in many other lesions, including malignancies (73,74) (Fig 25). For example, macroscopic cystic structures, which appear as anechoic areas at US, may occur in other benign tumors (pleomorphic adenoma, basal cell adenoma), in malignant tumors (mucoepidermoid carcinoma, acinic cell carcinoma), and in an abscessed or necrotic metastatic node; in addition, benign lymphoepithelial lesions in HIV-positive patients may have the appearance of solid-cystic nodules (Fig 25) (3,35,72,74,82–86). Warthin tumor may also appear in the form of a simple cyst at US and thus require differentiation from cystic carcinomas (mucoepidermoid carcinoma, acinic cell carcinoma) and benign cysts (lymphoepithelial cysts) (43,73,82).
Other Benign Tumors.—
Other benign tumors (eg, oncocytoma, basal cell adenoma) occur less frequently in the salivary glands. Their differentiation is not possible with US. Among nonepithelial lesions, hemangiomas, lipomas (Fig 26), and neurinomas or schwannomas may be found in salivary glands (12,87–91).
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Figure 26. Panoramic gray-scale US image shows the typical appearance of a lipoma (arrowheads). The lesion, which is located in the left parotid gland (arrows), is hypoechoic with regularly distributed linear structures. 1 = echo from the surface of the ramus of the mandible, 2 = masseter muscle.
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Hemangiomas, the most frequent tumors in infants, may manifest as heterogeneous lesions with sinusoidal spaces and calcifications representing phleboliths (88). Lipomas are usually oval and hypoechoic with sharp margins and hyperechoic linear structures regularly distributed within the lesion in a striated or feathered pattern (Fig 26) (9,92). At color or power Doppler US, only single vessel segments may be found (9). In infants with hemangioma, US may show a homogeneous, mildly lobulated mass with a lobular structure, fine echogenic septa, and extremely high vascularization at color Doppler imaging (93). Other vascular lesions, such as pseudoaneurysms or arteriovenous fistulas, may also be encountered in the parotid gland, although they are rare (88). Neurogenic tumors often contain anechoic areas (25).
Malignant Neoplasms
The most common malignant neoplasms occurring in salivary glands are mucoepidermoid carcinoma and adenoid cystic carcinoma (94). Squamous cell carcinoma, acinic cell carcinoma, and adenocarcinoma are less common. Less than 30% of focal lesions in the parotid gland are malignant, whereas almost 50% of focal lesions in the submandibular gland are malignant (3,66).
Unlike benign salivary neoplasms, malignant tumors may grow rapidly, may be tender or painful at palpation, may be fixed to the background, and may cause facial nerve paresis or paralysis (3,20,94).
Mucoepidermoid carcinoma occurs mostly between 30 and 50 years of age. Mucoepidermoid carcinoma may show several levels of differentiation and thus different tendencies to infiltration, metastases, and progress; the poorly differentiated form is extremely aggressive (20). The macroscopic appearance of the tumor, and similarly its imaging features, depend mostly on the level of malignancy (8,20). Well-differentiated tumors may be similar to benign tumors at US (8). Adenoid cystic carcinoma, which is a slowly growing tumor, shows a particular tendency to nerve infiltration (and thus pain), and late metastases are frequent (20).
Classic US features of poorly differentiated or advanced malignant neoplasms of salivary glands are like those in other organs or tissues. US features of malignant salivary neoplasms include the following: an irregular shape, irregular borders, blurred margins, and a hypoechoic inhomogeneous structure (Figs 27, 28) (8,19,25,41,95,96). However, malignant tumors may also be homogeneous and well defined (18,73,96). The internal structure of a malignant tumor at US may be not only solid but also cystic or cystic with a mural solid nodule (85). Malignant tumors may have a lobulated shape, similar to that of pleomorphic adenomas (96).
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Figure 27. Panoramic gray-scale US image shows an acinic cell carcinoma (arrowheads) in the left parotid gland (solid arrows). The tumor is well defined and has regular margins; however, there are signs of mandibular destruction (open arrows), a finding that suggests malignancy.
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Figure 28. Panoramic gray-scale US image shows metastatic lymph nodes (open arrows), which are oval or round and inhomogeneous without hyperechoic hila. There is a primary adenocarcinoma (arrowheads) in the left parotid gland (solid arrows).
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Vascularization of malignant tumors is not pathognomonic, and assessment with color Doppler or power Doppler US does not allow reliable differentiation between benign and malignant salivary gland tumors (72,76). However, Schick et al (72) report that high vascularization and high systolic peak flow velocity should raise the suspicion of malignancy. On the other hand, Bradley et al (97) conclude that tumors demonstrating an increased intratumoral vascular resistance index have an increased risk of malignancy.
The presence of metastatic-appearing lymph nodes accompanying a tumor in the salivary gland strongly suggests a malignancy (Fig 28). Very rarely, malignant tumors may occur multifocally or bilaterally, sometimes metachronously, or may coexist with benign neoplasms (78,96,98).
An important problem in US is caused by small malignant neoplasms and metastases, less than 20 mm in diameter, and well-differentiated malignant neoplasms because they may demonstrate benign features: clear, even margins and homogeneous structure (Fig 29) (72,73). These tumors also cause similar diagnostic problems with other diagnostic methods (CT and MR imaging) (3).
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Figure 29. US image shows an oval, well-defined, homogeneous tumor with even margins (arrows) in the right submandibular gland; the parenchyma of the gland (arrowheads) has been changed by therapeutic neck irradiation. Despite its benign features, the tumor proved to be a metastasis from a squamous cell carcinoma at the base of the tongue.
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Metastases
Salivary glands are very uncommonly sites of metastases. Primary tumors metastasizing to salivary glands may be located in the head and neck region, as well as in more distant parts of the body. Melanoma (Fig 30), spinocellular cancer, breast cancer, and lung cancer may produce metastases to intraparotid lymph nodes (99–103). Extremely rare are metastases from renal cancer (100,104–106).
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Figure 30. Power Doppler US image shows a metastasis (arrowheads) to the superficial lobe of the parotid gland (arrows) from a melanoma. The tumor is lobulated, inhomogeneous, and virtually anechoic with posterior acoustic enhancement and chaotic, mainly peripheral vessel segments.
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At US, metastases may be well defined and oval (Fig 29) (9). It may be difficult to differentiate multiple metas
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Abstract
Introduction
