The oral cavity is separated from the sinonasal region by the hard and soft palate and from the oropharynx by the circumvallate papilla of the tongue, soft palate, and anterior tonsillar pillars (Fig. 4.1). The maxillary tuberosity serves as the radiographic border between the oral cavity and oropharynx.

From an imaging standpoint, the oral cavity can be subdivided into four distinct regions: (1) the oral mucosal surface, including the floor of mouth; (2) the root of the tongue; (3) the sublingual space; and (4) the submandibular space (Fig. 4.2). All the surfaces of the oral cavity structures are covered with keratinizing stratified squamous epithelium along with minor salivary gland rests.

The tongue is divided into an oral part (anterior two-thirds within the oral cavity), the base (posterior one-third within the oropharynx), and the root. The circumvallate papilla of the tongue separates the oral tongue from the base of tongue, which also includes the lingual tonsils. The tongue consists of extrinsic muscles (genioglossus, hyoglossus, styloglossus, and palatoglossus), which predominantly move the tongue body, and the intrinsic muscles (superior and inferior longitudinal, transverse, and vertical muscles), which alter tongue shape during swallowing and speech. The root of the tongue consists of the genioglossus-geniohyoid complex and the lingual septum; it extends from the extrinsic tongue muscles superiorly to the mylohyoid sling inferiorly (Fig. 4.3). The vasculature of the tongue consists of the lingual artery (second branch of external carotid) and corresponding lingual vein within the sublingual space. The hypoglossal nerve provides the motor supply and enters the sublingual space just lateral to the posterior hyoglossus. Sensory supply for the anterior two-thirds is by the lingual nerve, while the posterior one-thirds is supplied by branches from glossopharyngeal nerve (CN IX). The chorda tympani nerve, a branch from the facial nerve (CN VII), carries the taste fibers from the anterior two-thirds within the lingual nerve. All these run in the sublingual space (Fig. 4.2).

The floor of the mouth includes the mylohyoid muscle, the hyoglossus muscle, and the sublingual space, which is located between these two muscles (Fig. 4.2). The sublingual space is a horizontal horseshoe-shaped space deep to the oral tongue that communicates posteriorly within the submandibular space and the inferior parapharyngeal space. It lies in between the mylohyoid muscle inferolaterally and the genioglossus muscle medially. The contents of the sublingual space include the lingual artery and vein, distal segments of CNs IX and X, the lingual nerve containing sensory fibers from CN V3 (mandibular branch of trigeminal) as well as taste fibers through the chorda tympani from CN VII (facial), the sublingual gland and ducts, the deep portion of the submandibular gland, and the submandibular duct.

The submandibular space is a vertical horseshoe-shaped space, inferolateral to the mylohyoid muscle and superior to the hyoid bone. It communicates both with the inferior parapharyngeal space and the posterior aspect of the sublingual space. The submandibular space contains the larger superficial portion of the submandibular gland, anterior belly of the digastric, the facial vein and artery, the inferior loop of the hypoglossal nerve (CN XII), and the submental and submandibular lymph node groups (Fig. 4.2).

The oropharynx includes the posterior third of tongue (base of tongue), the vallecula, the palatine tonsils and tonsillar fossa, the soft palate, the uvula, and the posterior and lateral pharyngeal walls from the soft palate superiorly to the pharyngo-epiglottic folds inferiorly (Fig. 4.1).

Regional nodal metastasis from oral cavity and base of tongue cancers commonly occurs to the submental, submandibular, and upper and mid-cervical lymph chains (levels I–III); for tonsil and pharyngeal wall cancers, regional metastases rarely involve level I.

The most commonly recognized risk factors for SCCs in the head and neck include long-term overuse of alcohol and tobacco. Recently, human papillomavirus (HPV) has been shown to be responsible for the majority of new oropharyngeal SCCs, with HPV-16 accounting for 90–95 % of such cases. Radiologists should be aware of a unique presentation of HPV-associated oropharyngeal cancers, which have an increased tendency to present with large cystic/necrotic level IIa nodal metastases. Given that they have a tendency to occur in younger nonsmokers, these can easily be misinterpreted as second branchial cleft cysts, resulting in missed diagnosis and/or delayed treatment (Fig. 4.4). While second branchial cleft cysts also present as level IIa cystic lesions, they typically present in the first two decades and, unless infected, appear smooth, well circumscribed, and unilocular without any stranding of surrounding structures and without thick or nodular walls. Thus, the presence of an underlying head and neck primary cancer should always be excluded in a young adult with a newly diagnosed level II neck mass.