Branchial Cleft Cyst

First branchial cleft cysts (BCCs) classically occur in the parotid gland or around the external auditory canal (Box 15-2). Several classifications of first BCCs have been developed, including Arnot type I (intraparotid cyst) and Arnot type II (cyst in the anterior neck that may drain with a tract through the deep portion of the parotid gland to get to the external auditory canal) (Figs. 15-3 and 15-4). They may drain into the external auditory canal and are of the same density and intensity as cerebrospinal fluid (CSF) unless otherwise infected or traumatized. Because second BCCs are so much more common, they probably outnumber first BCCs in the periparotid location (Table 15-2). Adjacent inflammation may be present, and the whole complex may simulate an infiltrative process. Fistulization to the bone-cartilage junction of the external ear may occur with first branchial cleft anomalies.

Figure 15-3 First branchial cleft cyst (BBC). This intraparotid cyst with a sharply defined wall is very bright on T2-weighted imaging. There is no way of knowing whether this is an inflammatory cyst, a posttraumatic sialocele, or a BCC.

Figure 15-4 First branchial cleft cysts (BCC), Arnot types. A, Type I first branchial cleft anomaly (FBA). The cyst is located in the parotid gland. There is no communication with the external auditory canal (EAC). B, Type II FBA. The proximal portion of the anomaly communicates with the EAC. The cyst tract typically extends inferiorly within the deep lobe of the parotid gland. The main portion of the cyst is usually located inferior to the parotid gland. Consequently, these masses may present as submandibular masses.

(From Mukherji SK, Fatterpekar G, Castillo M, et al: Imaging of congenital anomalies of the branchial apparatus, Neuroimaging Clin North Am 10:76–77, 2000.)

Table 15-2 Branchial Cleft Anomalies

Simple Cyst

Simple congenital cysts, unassociated with the branchial apparatus, also occur in the salivary glands. Solitary lymphoepithelial cysts are found only in the parotid gland because of its encapsulation of lymphoid tissue; more often they are seen multiply in patients positive for human immunodeficiency virus (HIV).

Venous Vascular Malformations

Venous vascular malformations (formerly called hemangiomas) may occur in the submandibular space as well as in other spaces of the head and neck. On computed tomography (CT) they may show calcification, but invariably one identifies a lobulated, heterogeneously enhancing mass. Areas of enhancement that are as extensive as that of the neighboring jugular vein are not unusual. If you perform a fine-needle aspiration of this lesion you will be faced with the bane of the aspirator’s existence—recurrent samples of “nothing but blood.” Interestingly enough, there is a 20% coincidence rate of developmental venous anomalies (venous angiomas) of the brain in those subjects with head and neck venous malformations.

True capillary hemangiomas are neoplasms of infancy that have growth and involutional phases. They usually present in the first few years of life, grow for a few years, then start to regress, and disappear by adolescence. They can be encouraged to regress by steroid, interferon, laser, or cryotherapy. Their most common location is in the skin or subcutaneous tissue. The lesion enhances brightly and is very hyperintense on T2-weighted images (T2WI). Cutaneous hemangiomas may coexist. Congenital capillary hemangiomas represent 90% of parotid gland tumors in neonates. Lymphangiomas are the second most common benign lesion to affect the pediatric parotid gland. The third most common benign tumor of the pediatric parotid gland is the pleomorphic adenoma.

Calculous Disease

Calculous disease is the most common benign condition to affect the salivary glands. Because the submandibular gland secretes a more mucinous, viscous, alkaline saliva and the wider duct must drain in an uphill direction with greater possibility of stasis, calculi occur most commonly in the submandibular gland and duct (Fig. 15-5). In fact, submandibular gland calculi outnumber those in the parotid gland by a “calculation” of four to one. Sublingual gland calculi and minor salivary gland calculi are extremely uncommon. Although most of the calculi associated with the salivary glands are radiopaque, a small percentage (20%) may not be so radiodense as to be visible on plain films.

Figure 15-5 Submandibular sialadenitis from sialolithiasis. A, A large stone with a dilated Wharton’s duct is seen in the left sublingual space. B, Note the swollen gland (arrow) with the effaced sublingual space fat planes.

Patients who have calculous disease usually have painful glands, exacerbated by chewing foods that precipitate salivation. If the clinician suspects calculous disease, the usual workup includes plain films to evaluate for large radiopaque calculi. If no calculi are identified by plain films, or if the patient has a fever associated with a painful salivary gland and an abscess is suspected, unenhanced CT should be done because it is more sensitive for the detection of calcification and inflammatory masses (Fig. 15-6). Remember that magnetic resonance (MR) imaging is less sensitive to calcified or noncalcified tiny calculi; larger calculi can be seen as low-intensity areas on T2-weighted scans, but flow voids may simulate calculi. In the instance of a calculus that is not radiopaque on CT, one might be forced to perform conventional sialography (cannulation of the salivary duct with injection of contrast). MR sialography is a new technique that neuroradiologists have stolen from the MR cholangiopancreatography protocol of the body MR guys. One can use either a single-shot fast spin-echo heavily T2-weighted sequence that highlights the ducts alone or perform a high-resolution 3D fast spin-echo T2-weighted sequence that can be prescribed through the gland and duct. Sialography is useful in demonstrating strictures of the ducts after passage of a calculus and intraluminal filling defects from nonopaque stones. Ductal strictures may predispose to recurrent calculous disease. However, the use of sialography for diagnosing calculi has dramatically decreased because CT is so effective here.

Figure 15-6 Parotid sialolithiasis. That is a big stone in a small Stensen’s duct. Note how much larger the inflamed left parotid gland is when compared to the right.

Most of the sialograms that are performed at our institution are for chronic sialadenitis from autoimmune causes. These studies are done because patients are seen with hard glands, worrisome for masses (Kuttner tumor), but cross-sectional imaging shows no lesion. The sialogram suggests the diagnosis of autoimmune chronic sialadenitis if there are pruned, truncated main ducts with punctate/globular collections peripherally in the glandular parenchyma.

Treatment of sialolithiasis (a catchy seven-syllable word for salivary gland stones) generally consists of administering solutions to increase salivation with the hope of passing the calculi naturally. Transoral resection of sialoliths and sialodochoplasty can be performed for isolated distal duct (close to ampulla) sialoliths. Imaging may help define the location of isolated nonpalpable or multiple sialoliths. For proximal or glandular sialoliths the surgeon may decide to treat the patient with resection of the gland. This is often the preferred treatment with recurrent bouts of sialolithiasis with sialadenitis. A cervical (submandibular) approach may be taken with sialoliths that extend beyond the mylohyoid (in the proximal duct).

With a classic clinical history suggesting stones, treatment may proceed without antecedent imaging.

Sialosis

Sialosis is a painless enlargement of the parotid glands that has been associated with numerous causes, including (1) diabetes, (2) alcoholism, (3) hypothyroidism, (4) medications including phenothiazines and some diuretics, (5) obesity, (6) starvation, and (7) idiopathic causes. This usually is a bilateral and symmetric process that may resolve when the underlying cause has been removed. It should be noted, however, that the normal range of parotid gland size and consistency is varied and often it is difficult to state definitively that the glands are larger than normal. On imaging studies the glands with sialosis generally have a CT density and signal intensity on T2WI slightly greater than that of normal fatty parotid glands. The glands in sialosis usually are not as bright on T2WI as glands that are infected. You may see distortion of the facial contour by the enlarged glandular tissue.

Sialadenitis

Sialadenitis refers to glandular inflammation, whereas sialectasis refers to dilatation of ductal spaces. Sialadenitis is often associated with sialectasis, or dilatation of the ductal system. The most common cause of these conditions is calculous disease. Microabscesses within the parotid tissue may be seen in a person who has sialectasis or sialadenitis. They are identified on CT as areas of low density with peripheral rim enhancement and on MR as areas of very bright signal intensity on T2WI of the salivary glands. Microabscesses often are multiple and may be a source of painful parotid glands with fever. Abscesses may develop around the mandible, sublingual gland, or submandibular gland in association with dental infections (Fig. 15-7).

Figure 15-7 Abscess. An abscess in the sublingual space is usually a result of carious teeth or treatment for such. A pocket of air and fluid is marked by the arrow.

A wide variety of inflammatory conditions may affect the salivary glands. Although mumps may be the most common infection to affect the salivary glands (specifically the parotid glands), it is virtually never imaged; the diagnosis is a clinical one. On the other hand, inflammatory conditions that enlarge the parotid glands in the adult may be evaluated to rule out masses. Other viral etiologies include HIV, coxsackie, and influenza viruses. Bacterial infections are uncommon and are usually due to Streptococcus, Haemophilus, and Staphylococcus species.

Other etiologies of acute parotitis include granulomatous (tuberculosis, Candida, cat scratch fever) and idiopathic (postpartum parotitis) causes. Poor dental hygiene may contribute to the development of infections affecting the submandibular, sublingual, and parotid glands. The minor salivary glands rarely show inflammatory change other than mucus retention cysts from local obstruction. Vallecular (minor salivary gland) cysts can get huge and obstruct the airway.

Sialodochitis

Sialodochitis refers to inflammation of the main salivary ductal system. A number of autoimmune conditions may cause sialadenitis and sialodochitis. When the process is limited to the salivary glands without other associated findings, the disease is termed Mikulicz disease or, in the most recent classification, Sjögren type 1 disease. This is an autoimmune disorder that causes chronic sialadenitis and sialodochitis and leads to fibrous salivary gland tissue (primarily of the minor glands) with resultant dry mouth. This disorder usually affects middle-aged women. When the disease is associated with a collagen vascular disease (most commonly rheumatoid arthritis more so than systemic lupus erythematosus) and involvement of the lacrimal glands, the disorder is classified as Sjögren syndrome (Sjögren type 2). Sjögren syndrome is an autoimmune disorder that causes dry eyes, dry mouth, and arthritis. Patients with Sjögren syndrome have tenfold increased risk of lymphoma, which may have its first manifestations in the parotid glands. The lymphoma is usually of the non-Hodgkin variety and may affect any other area of the head and neck as well (Fig. 15-8). Often these patients are scanned to survey for the possibility of lymphoma.

Figure 15-8 Sjögren syndrome. A, The coronal T2-weighted image (T2WI) reveals many tiny benign lymphoepithelial lesions as well as nodes inferiorly and bilaterally. B, Dominant masses are seen on the T2WI, but which do you biopsy? Fine-needle aspiration revealed lymphoid aggregates. C, Lymphoma of the parotid gland and Sjögren syndrome in another case. The axial computed tomography scan shows a mass (M) in the left parotid gland diffusely infiltrating its superficial portion. The right gland is not normal, assuming an acinar pattern of glandular density and fatty replacement. Did you notice the separation of C1–C2 due to atlantoaxial subluxation from the patient’s collagen vascular disease associated with Sjögren syndrome?

Sjögren syndrome is characterized on conventional or MR sialography by punctate, globular, cavitary, or destructive appearance of the ducts of the parotid glands. Tiny pools of contrast may be seen in the gland. With increasing severity of disease there is greater and greater replacement of glandular tissue with fat. Thus, some have suggested that the severity of fat deposition correlates well with the impairment of salivary flow in Sjögren patients. Sjögren syndrome is also associated with the presence of lymphoepithelial cysts and nodules akin to those seen in patients with HIV-associated parotid lesions (see following discussion).

The cross-sectional imaging appearance of parotid glands in patients who have Sjögren disease may range from normal to a dried-up, scarred-down, atrophic gland, to one with lots of large or tiny cysts and nodules within it, to one with a dominant mass within it. This looks very much like HIV-related parotid disease (Fig. 15-9).

Figure 15-9 Autoimmune sialadenitis. Axial unenhanced computed tomography shows bilateral enlargement and increased density to the parotid glands. This patient had Mikulicz disease with superimposed small calculi.

Lymphoepithelial Lesions

Since the ascent of acquired immunodeficiency syndrome (AIDS) in the young population, lymphoepithelial lesions of the parotid gland have become much more common. These may include purely cystic lesions or solid lesions of lymphoid aggregates (Fig. 15-10). Therefore, in a younger patient with multiple lesions in the parotid gland you should consider lymphoepithelial lesions, as opposed to multiple Warthin’s tumors. The differential diagnosis also includes multiple intraparotid lymph nodes and lymphoma. The lymphoepithelial lesions of the parotid have been associated with HIV seropositivity, and the presence of these abnormalities may predate the infection that classifies the patient as having AIDS. Associated findings with HIV-related parotid disease include diffuse generalized lymphadenopathy in the neck and prominence of adenoidal and tonsillar tissue. Bone marrow signal intensity on T1-weighted imaging (T1WI) may be lower than normal. When the lymphoepithelial lesion is cystic, it has low density on CT and signal intensity characteristics of CSF on T1WI and T2WI. However, the lymphoepithelial solid nodules may have a more variable density and signal intensity on cross-sectional imaging.

Figure 15-10 Lymphoepithelial lesions associated with human immunodeficiency virus (HIV). Parotid glands have multiple high-intensity masses (m) in this fat-suppressed fast spin-echo T2-weighted image, typical of lymphoepithelial cysts, in this HIV-positive man.

Som and colleagues have coined the term “acquired immunodeficiency syndrome-related parotid cysts” to describe the cysts associated with HIV infection and note that they are hard to distinguish from Sjögren-related benign lymphoepithelial lesions.

Sialocele

A sialocele refers to a collection of saliva that communicates with the parent duct in a manner similar to that of a pharyngocele or a laryngocele filled with fluid (Fig. 15-11). The most common cause of sialoceles is penetrating trauma, although blunt trauma may also cause disruption of the duct and leakage of salivary contents into the parenchyma and outside the gland. This most commonly occurs in the parotid gland, either from a punch to the side of the face or from a stab wound. The entity is distinguished from a pseudocyst because it communicates with the parent duct and is not lined by fibrous tissue.

Figure 15-11 Sialocele. Axial computed tomography scan after left parotid sialography demonstrates opacification of a sialocele (s). One can see the normal parotid duct (arrows) coursing to and communicating with the sialocele on the left side. This patient has been punched in the left side of the face.

Ranula

Another entity, more fully described in Chapter 14 in the discussion of oral cavity lesions, is the ranula. This is a postinflammatory cystic lesion that results from obstruction of either the sublingual or submandibular duct and that produces a cystic mass either confined by the mylohyoid muscle (simple ranula, epithelial lined) or extending to the submandibular region (a plunging ranula, not epithelial lined) (see Fig. 14-22). A ranula has also been termed a “mucus escape cyst,” a mucus retention cyst, and a mucocele of the sublingual gland or neighboring minor salivary glandular tissue. The simple ranula is usually addressed transorally but may be treated with resection or, in some cases, marsupialization. The lingual and hypoglossal nerves must be carefully identified during the operation. A plunging ranula may be excised through a transcervical submandibular incision with a neck dissection. This allows complete resection of the cyst and will help spare the lingual and hypoglossal nerve. Alternatively, the surgeon may excise the sublingual gland transorally and pack the cyst or place a drain in it. By treating the gland, some believe the plunging cyst will resolve on its own.

Retention Cysts

Retention cysts are very common benign “masses” that result from inflammation and obstruction of minor salivary gland ducts and therefore may be seen throughout the aerodigestive system’s mucosal surface.

Miscellaneous Inflammatory Disorders

Sarcoidosis may also affect the parotid gland, usually manifesting as bilaterally enlarged glands with multifocal nodules. Gallium uptake on nuclear medicine scans may be striking.

A mucus plug in the duct may also cause a painful swollen gland (Kussmaul disease). One pseudomass associated with calcifications in the gland is termed the Kuttner tumor, a focal masslike firmness of the submandibular gland due to chronic sialadenitis from sialolithiasis.

Pleomorphic Adenoma

Nearly 80% of benign parotid neoplasms are pleomorphic adenomas (Box 15-3). Pleomorphic adenomas, also known as benign mixed tumors, occur most commonly in middle aged women. Most pleomorphic adenomas are well-defined lesions that commonly appear solid and round. Pleomorphic adenomas are well identified on both CT and MR against the fatty background of the normal adult’s parotid gland (Fig. 15-12). On CT, the lesions have density similar to that of muscle and demonstrate mild to moderate enhancement. With a delay, one may see an increase in the degree and homogeneity of enhancement in parotid pleomorphic adenomas. On MR, the lesions are best identified on T1WI amid the bright signal of the parotid fat; however, they are usually seen on T2WI as very bright lesions (add that to your 80% rule—80% of bright lesions in the parotid are pleomorphic adenomas). Additional MR findings include a complete capsule (often low intensity on T2WI) and a lobulated contour. Pleomorphic adenomas inconstantly have cystic degeneration or calcification within them. Because the incidence of calcification is so much lower in other types of parotid tumors, the presence of calcification nonetheless suggests pleomorphic adenoma.

Figure 15-12 Pleomorphic adenoma. A, The well-defined mass (asterisk) in the superficial portion of the right parotid gland turned out to be a pleomorphic adenoma, following the 80% rules of the parotid gland (see Box 15-3). B, A different patient had a mass (asterisk) identified incidentally on this T1-weighted image (T1WI) in the left parotid gland. C, The brilliant bright signal on T2WI, in the face of a that does NOT l ook like a cyst, suggests a pleomorphic adenoma (asterisk)—80% of the time. D, Avid enhancement (asterisk) on postcontrast T1WI is characteristic as well. E, Monomorphic adenoma of the right submandibular gland (arrow), low in density, well defined, easily removed.

A small percentage of pleomorphic adenomas degenerate into carcinomas. It is not known whether a carcinoma is present within the pleomorphic adenoma from its outset or whether this is a manifestation of malignant transformation, but most pathologists favor the latter theory. In any case it is because of this carcinomatous association that pleomorphic adenomas are removed in their entirety with a cuff of normal salivary tissue in most instances. Pleomorphic adenomas are the most common tumor of the parotid gland in both its superficial and deep portions. In fact, pleomorphic adenomas are the most common benign tumor of the submandibular, sublingual, and minor salivary glands. They are multicentric in 0.5% but as they account for 80% of benign tumors and benign tumors are 80% of all tumors, this means that approximately 3% of multiple parotid neoplasms are pleomorphic adenomas.

Monomorphic adenomas and myoepitheliomas are the other common benign tumors and may arise in both parotid and submandibular glands. Monomorphic adenomas (myoepitheliomas, oncocytic adenomas, canalicular adenomas) look very much like pleomorphic adenomas but are more commonly seen in submandibular glands. They are well-defined and enhance.

Warthin’s Tumor

Warthin’s tumors are also known as cystadenoma lymphomatosum. These tumors are nearly exclusive to the parotid gland and are the most common multiple and bilateral tumors in the parotid (Box 15-4). As opposed to pleomorphic adenomas, which are generally seen in middle-aged women, Warthin’s tumors are most commonly seen in elderly men. Warthin’s tumors may have a tumoral cyst and favor the parotid’s tail. These lesions are entirely benign and show no evidence of malignant transformation. Therefore if a fine-needle aspiration identifies a lesion as Warthin’s tumor, surgeons may conservatively watch the tumor rather than remove it. On MR, the lesions are well seen on T1WI opposite the high signal intensity of the parotid gland (Fig. 15-13). However, the signal intensity on T2WI is often heterogeneous and variable and may overlap that of the bright signal of pleomorphic adenomas or the darker intensity of malignancies of the parotid gland. Warthin’s tumors, like oncocytomas, have increased uptake on technetium-99m pertechnetate nuclear medicine scans. Therefore, if fine-needle aspiration is equivocal or nondiagnostic, recommend a technetium nuclear medicine scan to make the diagnosis of Warthin’s tumors.

Figure 15-13 Warthin’s tumor. A, Axial T1-weighted image (T1WI) demonstrates bilateral Warthin’s tumor (w). Note heterogeneity to the signal intensity in the posterior tumor on the right side, which is typical of Warthin’s tumors. B, In a different patient the T2WI shows bilateral masses with heterogeneity most marked on the left side.

Oncocytoma

The oncocytoma is a relatively rare benign tumor almost exclusively seen in the parotid gland. These lesions have an MR appearance similar to that of pleomorphic adenomas, being generally bright on T2WI. However, because they are sufficiently rare, the signal intensity characteristics of these tumors have not been well delineated. The tumors may also take up technetium on nuclear medicine scans.

Other Benign Tumors

Lipomas, schwannomas, and neurofibromas may also be seen in association with the salivary glands, most commonly the parotid. Lipomas have fat density and present in children. The neurogenic tumors generally follow cranial nerves V (submandibular, sublingual, and parotid glands) or VII (parotid glands). Be careful to trace these tumors along the expected course of the nerves, even to their foramina.

Mucoepidermoid Carcinoma

The most common malignant lesion of the parotid gland is mucoepidermoid carcinoma; in the submandibular, sublingual, and minor salivary glands it is adenoid cystic carcinoma. Mucoepidermoid carcinomas, like squamous cell carcinoma, can be graded from low to high, and the prognosis varies with the grade. Mucoepidermoid carcinomas account for 30% of all salivary gland malignancies, and 60% of them occur in the parotid gland (Fig. 15-14). Mucoepidermoid carcinoma is the most common pediatric salivary gland malignancy. Thirty-five percent of salivary gland neoplasms in children are malignant; of these, 60% are mucoepidermoid carcinomas.

Figure 15-14 Mucoepidermoid carcinoma. A, Coronal enhanced T1- weighted image (T1WI) delineates a mass (m) emanating from the hard palate minor salivary gland tissue, which protrudes into the maxillary sinus (s). Mass has moderate enhancement. B, On T2WI, note that the mass (m) has intermediate signal intensity. Higher signal intensity anteriorly is caused by inflammatory change associated with the lesion.

In the parotid gland, a lesion’s morphology may be misleading as far as predicting benignity versus malignancy. Some pleomorphic adenomas have tentacles with an irregular margin. By the same token, some mucoepidermoid carcinomas are well defined by a pseudocapsule and do not appear to be invasive. Therefore, you cannot rely on shape to distinguish cancer from benign tumors.

Unfortunately, density and intensity provide paltry clues to a lesion’s identity. On CT, most tumors of the parotid gland have a density equal to muscle—no help there. Low-grade mucoepidermoid carcinomas may have high signal intensity, and high-grade, poorly differentiated mucoepidermoid carcinomas are usually low in intensity on T2WI. Therefore, a low-grade mucoepidermoid carcinoma may have intensity characteristics that are identical to those of a pleomorphic adenoma, and a Warthin’s tumor may simulate a high-grade mucoepidermoid carcinoma. Nonetheless the presence of a lesion that is intermediate to dark on a T2-weighted scan should be considered malignant until proven otherwise. Recommend aspiration.

Adenoid Cystic Carcinoma

Adenoid cystic carcinoma, the second most common primary malignancy of the parotid gland and the most common tumor of the submandibular, sublingual, and minor salivary glands, is notorious for its propensity for perineural spread (50% to 60%) and persistence despite “complete surgical removal.” Similar to the mucoepidermoid carcinoma, variable intensity occurs with the T2WI, which allows a weak guess at histology (Fig. 15-15). Noncystic masses in the parotid gland have muscular CT density, low intensity on T1WI, and mild to moderate enhancement. An adenoid cystic carcinoma of the parotid gland may spread via the ramifications of cranial nerve VII retrograde into the temporal bone or may spread via the auriculotemporal branches of cranial nerve V to the Meckel’s cave region through the foramen ovale. One would think that spread via cranial nerve IX because of the innervation could occur, but that has not been seen by the authors … yet. Again, adenoid cystic carcinomas may be very well defined within the parotid gland, and the diagnosis of a malignancy may not be suspected before biopsy. In the other salivary glands adenoid cystic carcinoma generally demonstrates perineural extension along the branches of the second and third divisions of cranial nerve V. This cancer is a relentless, slow-growing tumor whose prognosis is generally measured in terms of decades rather than 5-year survival rate because of its prolonged course.

Figure 15-15 Adenoid cystic carcinoma of the parotid gland. T1- weighted image of the left parotid shows infiltrative mass (a) extending into both superficial and deep portions of the gland. Note how well the mass is identified by the replacement of the normal high intensity parotid tissue. Better watch cranial nerve VII on this lesion. Mind your facials!

Squamous Cell Carcinoma

Squamous cell carcinoma may be seen within the parotid gland. Sometimes it is difficult to tell whether the squamous cell carcinoma is present secondary to invasion of lymph nodes from a primary site outside the parotid or is intrinsic to the parotid gland (Fig. 15-16). How it gets there is mysterious; it is presumably caused by metaplasia of the ductal columnar epithelium into squamous cells. This same difficulty lies with lymphoma of the parotid—is it a primary parotid tumor or secondary spread? When multifocal in the parotid, it is generally accepted that the squamous cell carcinoma is probably within lymph nodes in the parotid gland. A search for a primary tumor should be undertaken. The overlying skin and ear are the primary sites that drain to the parotid gland; however, the parotid lymph nodes may be involved with diffuse lesions such as lymphoma. Squamous cell carcinoma does not generally occur in submandibular, sublingual, or minor salivary glands as a primary site, although it certainly spreads from adjacent mucosal surfaces or lymph nodes. As mentioned in Chapter 14, obstruction of submandibular or sublingual gland ducts may be a presenting symptom of floor of mouth cancers.

Figure 15-16 Squamous cell carcinoma of the parotid gland. A, Axial T1-weighted image (T1WI) demonstrates an ill-defined mass (m) in the right parotid gland invading the deep portion (arrow) and extending posteriorly to the stylomastoid foramen (open arrow). B, There is perineural spread up the intramastoid cranial nerve VII (below the arrows) on this sagittal T1WI. C, As is typical of squamous cell carcinoma in the parotid gland, the lesion has low intensity on T2WI.

Squamous cell carcinomas are virtually always hypointense on T2WI unless necrosis coexists.

Adenocarcinoma

Adenocarcinomas may also arise within the parotid gland, sublingual gland, submandibular gland, or minor salivary glands. This lesion generally has a worse prognosis than that of mucoepidermoid carcinoma and adenoid cystic carcinoma (Box 15-6). This tumor is derived from the glandular tissue itself as opposed to ductal tissue. Signal intensity is variable, depending on mucinous, cystic, or solid contributions. Some adenocarcinomas occur from malignant degeneration of pleomorphic adenomas.

The polymorphous low-grade adenocarcinoma of the salivary gland is a low-grade neoplasm that predominantly occurs in the minor salivary glands of the oral cavity (mucosa of the soft and hard palates, in the buccal mucosa, and in the upper lip) but can be seen in the parotid gland. It has a much more benign prognosis and course than the traditional adenocarcinoma.

Acinic Cell Carcinoma

Acinic cell carcinoma may present as a primary multifocal malignancy to affect the parotid gland. Its prognosis is intermediate between that of mucoepidermoid carcinoma and adenocarcinoma of the parotid gland. This tumor is seen exclusively in the parotid gland, and the incidence of bilateral acinic cell carcinomas is approximately 3%. No specific imaging features other than its multifocality suggest this diagnosis. Other multifocal malignancies to consider would include secondary processes such as primary parotid lymphoma, nodal spread of squamous cell carcinoma, systemic lymphoma, or leukemia.

Undifferentiated Carcinoma

Undifferentiated carcinoma is also seen as a salivary gland tumor. This lesion has a very poor prognosis but fortunately is rarely seen (<10% of cases).

Carcinoma Ex Pleomorphic Adenoma

As mentioned previously, if left alone to grow, a pleomorphic adenoma will degenerate into or coexist with a carcinoma in a significant (1.5% in 5 years, but 9.5% >15 years) percentage of cases. It accounts for 11.7% of all salivary malignancies. Cut it out! Usually the histologic subtype is an adenocarcinoma, and the lesion may be seen with rapid growth or even distant metastases (lung, bone, and nodes). The apparent diffusion coefficient values of carcinoma ex pleomorphic adenoma may be low due to hypercellularity.

The incidence of nodal metastases in untreated parotid cancers is low except in T3 or T4 lesions. Although mucoepidermoid carcinoma and squamous cell carcinoma metastasize to nodes in 37% to 44% of cases, the other histologic types are only associated with lymphadenopathy in 5% to 21%, with adenoid cystic carcinoma being the lowest.

Odontogenic Lesions

Most of the inflammatory lesions of the masticator space relate to infections of odontogenic origin. Therefore, abscesses around the teeth, osteomyelitis of the mandible, and cellulitis associated with carious teeth are the prime offenders in this category (Fig. 15-19). Occasionally, cellulitis and myositis of the masticator space develop as a result of penetrating injuries, superficial facial infections, or adjacent parotitis. Most inflammatory lesions of the masticator space can be readily identified on CT or MR. On CT, look for thickening of the adjacent muscle, infiltration of the nearby fat, subcutaneous tissue, or skin with a strand pattern to it. If an abscess has developed, it will appear in a fashion similar to abscesses elsewhere, with a low-density center and a peripheral enhancing rim. On MR, inflammatory lesions, because of their high water content from edema, are very bright on T2WI. There are some exceptions to this rule, namely actinomycosis (because of sulfur granule deposition) and fungal infections (because of paramagnetic iron and manganese accumulation).

Figure 15-19 Abscess in masticator space. Axial computed tomography scan shows an abscess (a) centered on the left mandible. Note that the parapharyngeal fat (asterisks) is displaced posteromedially by this masticator space infection.

Bruxism and Atrophy

Bruxism may develop in patients who constantly gnash their teeth. This is a fancy word for (usually) bilateral enlargement of the muscles of mastication. It occurs most commonly idiopathically but may develop as a result of malocclusion, excessive chewing, or clenching the teeth. Rarely, this may be a unilateral phenomenon. Alternatively, one may see atrophy of the muscles of mastication when one has a cranial nerve V abnormality (Fig. 15-20). This may be caused by lesions in the peripheral branches of the third division of cranial nerve V or by loss of central input to the motor portion of the trigeminal nerve from lesions in the frontal cortex. Perioperative injury may cause denervation atrophy, as can trauma. With denervation atrophy, one may see enhancement after gadolinium administration or high signal intensity on T2WI in the muscles of mastication. Neurogenic tumors may also induce these imaging characteristics.

Figure 15-20 Atrophy of muscles of mastication. This enhanced T1-weighted image demonstrates the differential size of the right masticator musculature from that of the left masticator musculature (m). This finding should prompt a search for cranial nerve V lesions. Voilà! One can find a trigeminal schwannoma (s) in the cavernous sinus on the right side. ?, small right enhancing lateral pterygoid.

Fibrous Dysplasia

Fibrous dysplasia causes enlargement of the mandible with expansion of the outer cortex of the bone, generally without erosion through the bone. This may affect the entire portion of the mandible and is more commonly seen there than in the maxilla. McCune-Albright syndrome refers to precocious puberty, café-au-lait spots, and polyostotic fibrous dysplasia (which may affect the mandible unilaterally) in girls. When a patient has bilateral bubbly bone lesions with a ground-glass appearance, consider either fibrous dysplasia or cherubism, which is a related disorder. Both of these disorders tend to expand during adolescence, are possibly related to hormonal influences, and either regress or stabilize in young adulthood.

The skull and facial bones are involved in 10% to 25% of cases of monostotic fibrous dysplasia and in 50% of polyostotic fibrous dysplasia. The mandible, maxilla, and calvarium are frequently involved.

Most of the lesions of the mandible are odontogenic in origin. As explained in the discussion of the oral cavity (see Tables 14-1 and 14-2), radicular cysts from carious teeth and dentigerous cysts associated with unerupted teeth are the most common causes of cystic lesions of the mandible.

Temporomandibular Joint Syndrome

The temporomandibular joint (TMJ) falls under the rubric of the masticator space. The TMJ may be the source of chronic facial or head pain. TMJ (or chronic maxillofacial pain) syndrome is seen in women nine times more frequently than men, is often precipitated by a traumatic event, and is very poorly understood. Presently, most imaging is performed with MR where the meniscus, condyle, glenoid fossa, and surrounding soft tissues may be evaluated. Rarely, arthrography may be performed through injection of the joint under fluoroscopy. The meniscus has an anterior triangular band and a larger posterior band, which are joined in the middle by an intermediate zone (Fig. 15-21). The posterior band is attached to the posterior joint by the retrodiskal tissue, or bilaminar zone. The meniscus should be centered over the condylar head in open- and closed-mouth positions with the posterior margin of the posterior band between the 11 and 12 o’clock point of the condyle in the closed-mouth position. The joint itself has an anterosuperior compartment and an inferior compartment, which usually do not communicate.

Figure 15-21 Normal temporomandibular joint (TMJ) anatomy. A, The meniscus in the closed-mouth view has its posterior edge (arrow) between 11 and 12 o’clock in relationship to the condylar head (c). The cortex of the glenoid fossa (open arrows) and the articular eminence (a) make up the superior landmarks of the TMJ. Note the meniscus has an anterior and posterior triangular band. B, In the open-mouth view, the condyle translates forward to a position under the eminence. The meniscus maintains its “bow-tie” configuration with its intermediate zone (+) centered over the condyle.

Anterior meniscal dislocations are the most common type in patients with TMJ complaints. In this setting the meniscus’ posterior band is dislocated anteriorly from directly over the condyle. This displacement is more than 10 degrees anterior to the 12 o’clock position, more like the 9 or 10 o’clock position. It may be far in front of the condyle in the closed-mouth position on sagittal T1WI (Fig. 15-22). The dislocation may reduce on opening (often with a clicking sound—the “opening click”) and may redislocate on closing (a closing click). The timing of the opening click may correlate with the degree of anterior dislocation of the meniscus. Alternatively, the meniscus may remain anteriorly dislocated even on opening. The location of the disk in front of the condyle may restrict the joint’s motion (a closed-lock situation).

Figure 15-22 Anterior dislocation of the meniscus. A, Meniscus (arrow) is far anterior and inferior on the closed-mouth view. There is decreased distance between the condyle and glenoid fossa. The next issue is whether the meniscus returns to a normal appearance on the open-mouth view (relocates or recaptures) or remains anteriorly dislocated. B, Bad news. This meniscus (arrow) remains anteriorly located and misshapen on the open mouth view. Note good range of motion (sometimes mobility is restricted by the meniscus—”closed locked position”), but you can imagine the friction between the condyle and eminence in this patient.