There is a broad spectrum of pathology that occurs in the oral cavity. Knowledge of the different anatomic subsites and contents of each is important for accurate diagnosis and treatment. Oral cavity tumors are predominantly malignant in nature, but there are many nonmalignant lesions of which the practicing clinician should be aware. This article will discuss the anatomy, imaging approaches, and imaging characteristics of nonmalignant and malignant pathology in the oral cavity
Key points
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The oral cavity is a complex anatomic space bordered by but separate from the oropharynx.
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Knowledge of oral cavity subsites is important to form an accurate differential diagnosis of the many pathologies, both benign and malignant that may occur.
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There is a wide spectrum of benign pathologies including congenital and vascular lesions with characteristic imaging appearances.
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The most common oral cavity malignancy is squamous cell carcinoma.
Introduction
The oral cavity is the entry way to the aerodigestive tract and is a complex anatomic site serving multiple essential functions. The components of the oral cavity, bounded anteriorly by the lips, posteriorly by the oropharynx, laterally by the buccoalveolar regions, superiorly by the hard palate, and inferiorly by the floor of mouth, can give rise to a multitude of lesions, spanning benign and malignant etiologies. Although the oral cavity is readily accessible for physical examination, imaging plays a critical role in delineating oral cavity lesions. Here we will highlight a practical approach to the imaging assessment of oral cavity lesions and review the major differential diagnoses of both benign and malignant lesions.
Anatomy
Gingivobuccal Region
The oral vestibule lies between the cheeks, lips, and teeth ( Fig. 1 ). The mucosa of the lips and cheeks serves as the external boundary of the oral vestibule whereas the teeth and gingiva serve as the internal boundary. The buccal mucosa covers the inner cheeks and reflects superiorly over the maxilla and inferiorly over the mandible. The buccal and gingival mucosae meet at the GBS.
Deep to the mucosal covering of the cheek lies the buccinator muscle covered by the buccal fat pad. The parotid gland (Stensen’s) duct courses through the buccal fat pad and pierces the buccinator muscle exiting at the level of the second maxillary molar.
The retromolar trigone is a triangular region just posterior to the mandibular third molar and is close to the pterygomandibular raphe. This fibrous band extends from the hamulus of the medial pterygoid to the mandibular mylohyoid ridge and forms a boundary between the oral cavity and oropharynx, attaching to the buccinator muscle anteriorly and superior pharyngeal constrictor muscle posteriorly. Although not well seen on imaging, the pterygomandibular raphe can serve as a conduit for the spread of tumor between the oral cavity and oropharynx.
Oral Tongue
The oral cavity proper is bounded superiorly by the hard palate, inferiorly by the floor of mouth, and anteriorly by the teeth with the oral tongue as its largest anatomic structure (see Fig. 1 ; Figs. 2 and 3 ). The anterior two-thirds of the tongue is considered the oral tongue and is delineated from the base of the tongue by the circumvallate papillae. The posterior one-third of the tongue is the tongue base, which is a subsite of the oropharynx. The intrinsic muscles of the oral tongue include the inferior and superior longitudinal lingual muscles, which curl the tongue upward and downward, respectively, and the transverse and vertical lingual muscles, responsible for elongating and flattening the tongue, respectively. The extrinsic muscles of the tongue include the genioglossus, styloglossus, hyoglossus, and palatoglossus muscles. The extrinsic muscles have bony attachments whereas the intrinsic muscles do not. The genioglossus, styloglossus, and hyoglossus muscles are innervated by the hypoglossal nerve; the palatoglossus is innervated by the pharyngeal branch of the vagus nerve. The genioglossus divided by the lingual septum is responsible for protruding the tongue, whereas the hyoglossus muscle retracts the tongue. The styloglossus controls the rising of the sides of the tongue to form a channel for swallowing. The palatoglossus is responsible for elevating the posterior tongue and contributes to the initiation of swallowing.
Floor of Mouth
The floor of the mouth is bounded inferiorly by the sling-shaped mylohyoid muscle (see Figs. 2 and 3 ). The mylohyoid muscles originate along the length of the mylohyoid ridge of the mandible and extend to the last molar teeth posteriorly. A gap exists between the posterior border of the mylohyoid muscle and the hyoglossus, where the submandibular gland extends around the mylohyoid muscle. The anterior bellies of the digastric muscle run inferiorly along the mylohyoid muscle in an anterior-posterior direction providing additional support for the floor of the mouth.
Sublingual and Submandibular Space
The sublingual space, containing the sublingual glands, lies beneath the tongue and is bounded inferiorly by the mylohyoid muscle, anteriorly by the mandible, and medially by the genioglossus geniohyoid muscle complex (see Figs. 2 and 3 ). The hyoglossus, styloglossus, and palatoglossus muscles run along the sublingual space. Posteriorly, the sublingual space communicates with the submandibular space with the deep lobe of the submandibular gland wrapping around the posterior border of the mylohyoid muscle and extending into the sublingual space. The submandibular gland (Wharton’s) duct, lingual nerve, artery, and vein course through the sublingual space as do the glossopharyngeal nerve and hypoglossal nerve.
The sublingual and submandibular spaces are separated by the mylohyoid muscle. The submandibular space is bordered inferiorly by the hyoid bone, superiorly by the mylohyoid muscle, anteriorly and laterally by the mandible, and medially by the anterior bellies of the digastric muscles. Within the submandibular space lies the superficial portion of the mandibular gland, facial artery, and vein.
Imaging Modalities
Panorex radiographs image the entire complement of teeth and allow evaluation of the alveolar bone around the teeth. X-ray imaging of the mandible via panorex is helpful in assessing any bony asymmetry and secondary bony effects of soft tissue lesions.
Computed tomography (CT) and MRI are the mainstays of imaging the oral cavity. CT offers precise evaluation of cortical bone which is essential in staging oral cavity cancers and is readily available. Contrast-enhanced CT (CECT) has been shown to have the highest specificity in detecting bone erosion, an upstaging feature in oral cavity cancer. Invasion of extrinsic tongue musculature has been replaced as a T4 upstaging feature in the American Joint Committee on Cancer (AJCC) 8th edition by depth of invasion (DOI). DOI is not synonymous with the measurement of tumor thickness on cross-sectional imaging and is determined histologically.
Techniques to improve CT image quality in the face of dental amalgam artifacts include a second pass CT with angled gantry and metal reduction reconstruction algorithms. A puffed cheek technique can be employed to improve the detection of mucosal lesions along the buccal and gingival surfaces.
MRI offers superior soft tissue resolution compared with CT. Delineation of typical signal characteristics such as fat signal within benign dermoid cysts and the depiction of tumor infiltration, perineural spread, and marrow involvement are some of the advantages of MRI. MRI is also better able to delineate anatomic structures such as the extrinsic muscles of the tongue when compared with CT. Disadvantages of MRI include longer imaging time with increased motion artifact and pulsation artifact from vascular structures in the head and neck. The lack of beam-hardening artifact due to metal is an advantage of MRI over CT, and metal suppression MRI techniques may aid in further decreasing metallic susceptibility artifact.
Dynamic contrast-enhanced (DCE)-MRI is a technique that evaluates the microvascular features of tumors. As in perfusion imaging of the brain, DCE-MRI of head and neck tumors, including oral cavity tumors, can provide information about the presence of abnormal leaky vessels and resulting permeability between the intravascular and extravascular environments. There is also evidence DCE-MRI may be a useful noninvasive method of helping to predict human papilloma virus (HPV) and epidermal growth factor receptor (EGFR) status before treatment in oropharyngeal and oral cavity tumors with implications for prognosis and expected response to therapy.
Ultrasound is useful in evaluating nodal disease in the neck and for guidance for fine needle aspiration. Intraoral ultrasound can provide accurate estimates of tumor thickness in oral cavity cancers pre-operatively and intraoperatively and can aid in improving margin resection.
Fluorodeoxyglucose (FDG) PET-CT plays an important role in the initial staging of oral cavity cancers. Oral cavity cancers are currently staged using the AJCC 8th edition. FDG PET-CT plays an important role in the delineation of the extent of primary tumor which can sometimes be obscured by dental amalgam artifacts on CT and motion artifacts on MRI. Compared with CT or MRI alone, FDG PET-CT has reported sensitivities for detection of the primary oral cavity tumor of up to 98%. Nodal involvement is one of the most important prognostic indicators at initial diagnosis, and FDG PET-CT has demonstrated a sensitivity of 83%, specificity of 85%, and superior negative predictive value of 93% compared with CT and MRI alone. In addition, FDG PET-CT images the entire body, detecting distant metastatic disease and possible additional primary tumors, both of which alter prognosis and management.
Nonmalignant pathology
Cystic Lesions
Of the spectrum of nonmalignant lesions that develop in the oral cavity, congenital lesions make up a major category. The most commonly encountered lesions include epidermoid and dermoid cysts, vascular malformations, and anomalies.
Epidermoid cysts result from inclusion of ectodermal elements during neural tube closure. Acquired epidermoid cysts are rare and may result from surgery or penetrating trauma. On CT, epidermoid cysts demonstrate low attenuation similar to cerebrospinal fluid (CSF) and may contain internal calcifications. Lesions are typically low signal on T1-weighted images but rare “white” epidermoid cysts demonstrate T1 hyperintense signal due to increased proteinaceous content ( Fig. 4 ). MRI demonstrates T2 hyperintense signal within the lesion but incomplete nulling of signal on fluid-attenuated inversion recovery (FLAIR) resulting in increased signal within the lesion as compared with CSF. Epidermoid cysts classically demonstrate internal restricted diffusion and show no enhancement.
Dermoid cysts are derived from totipotent stem cells that form the ectodermal germ layer. Dermoid cysts may contain any ectodermal elements including cutaneous appendages such as hair, sebaceous glands, and teeth. This is in contrast to epidermoid cysts, which do not contain sebaceous glands, sweat glands, and hair follicles. On imaging, the different ectodermal elements within dermoid cysts help make the diagnosis. CT may show hyperdense teeth and fat attenuation within the lesion, and MRI will show fat signal intensity within the lesions or a characteristic sack of marbles appearance of fat globules with no enhancement (see Fig. 4 ).
Ranulas represent mucous retention cysts of the sublingual glands or minor salivary glands in the floor of mouth and may result from trauma to the gland or from obstruction. Ranulas confined to the floor of mouth are termed simple ranulas whereas those extending inferior to the floor of mouth are termed plunging or diving ( Fig. 5 ).
On MRI, ranulas will demonstrate fluid signal and may show thin peripheral enhancement. If infected, they may show a thickened peripherally enhancing wall. A tapered narrowed “tail sign” has been described to denote the collapsed sublingual space component of the lesion. Differentiating ranula from dermoid cysts can be made by assessing for the absence of fat within the lesion on CT or MRI, and differentiation from epidermoid cysts can be made by the absence of restricted diffusion within the lesion on MRI.
Vascular anomalies
When designating vascular and lymphatic anomalies, there has been extensive controversy in nomenclature that has caused marked complexity in their classification. Previously, many vascular lesions have been referred to as hemangiomas and angiomas despite many of these lesions representing vascular malformations with no neoplastic potential. Creating consistency and clarity in nomenclature is increasingly important in clinical care as many vascular anomalies require multidisciplinary care, necessitating mutual understanding. The initial classification system proposed in 1982 by Mulliken and Glowacki has been formally refined into the International Society for the Study of Vascular Anomalies (ISSVA) classifications, which was most recently revised in 2018. The ISSVA classification system separates vascular anomalies into two primary groups: vascular malformations and vascular tumors ( Fig. 6 ). This diagram illustrates the ISSVA designations for vascular anomalies. Vascular anomalies are first separated into vascular tumors and vascular malformations. Thereafter, vascular tumors are divided into benign, locally aggressive/borderline, and malignant groups. Vascular malformations are separated into simple, combined, of major vessels, and associated with other anomaly groups.
