5.2 Head & Neck Sites: Anatomy, Lymphatics & Patterns of Spread

Anatomy and Subsites

The head and neck region is subdivided into several anatomically and functionally distinct subsites, each with characteristic patterns of disease and treatment planning implications. The oral cavity includes the lips, buccal mucosa, floor of mouth, oral tongue (anterior two-thirds), hard palate, and retromolar trigone. The oropharynx includes the base of tongue (posterior third), tonsils, soft palate, and posterior pharyngeal wall, and is the subsite most associated with human papillomavirus (HPV)-driven squamous cell carcinoma in recent decades. The nasopharynx sits superior to the oropharynx behind the nasal cavity and is strongly associated with Epstein-Barr virus (EBV) in endemic populations. The hypopharynx (including the piriform sinuses and postcricoid area) lies inferior to the oropharynx and communicates with the larynx, which is divided into the supraglottis, glottis (true vocal cords), and subglottis. Major salivary glands (parotid, submandibular, sublingual) and paranasal sinuses round out the major subsites.

Most head and neck malignancies are squamous cell carcinomas arising from the mucosal surface, though salivary gland tumors are typically adenocarcinomas, adenoid cystic carcinomas, or mucoepidermoid carcinomas with distinct behavior, including a strong tendency toward perineural spread along cranial nerves.

Lymphatic Drainage and Nodal Levels

Cervical lymph nodes are classified into standardized levels that guide both diagnostic staging and elective nodal irradiation:

Predictable, orderly progression of nodal spread (level I to II to III to IV) is typical of oral cavity primaries, which is why elective neck irradiation fields are designed around these levels even when nodes are not grossly enlarged on imaging. Nasopharyngeal carcinoma, by contrast, drains early and bilaterally to level II and the retropharyngeal nodes because of the midline location and rich lymphatic network of the nasopharynx.

Patterns of Spread and Special Considerations

HPV-related oropharyngeal squamous cell carcinoma behaves differently from traditional tobacco/alcohol-driven disease: it tends to present with a small or occult primary tumor alongside bulky cystic neck adenopathy, carries a substantially better prognosis, and is now staged separately under AJCC 8th edition criteria. Oral tongue carcinoma is notable for skip metastases, in which level II nodes are bypassed and disease appears directly in level III or IV, a pattern that influences the design of elective neck fields. Perineural invasion, in which tumor tracks along nerve sheaths (classically along the facial nerve for parotid tumors or the trigeminal nerve for cutaneous and sinonasal tumors), can extend disease well beyond the visible primary tumor and often requires irradiation of the nerve course back to the skull base. Nasopharyngeal carcinoma has a propensity for skull-base invasion and cranial nerve involvement given its proximity to the foramen lacerum and cavernous sinus.

Distant metastatic spread in head and neck cancer most commonly targets the lungs, followed by bone and liver, and is more frequent with advanced nodal disease, lower neck (level IV) involvement, and extranodal extension.

Critical Structures and Therapist Setup Considerations

Modern head and neck radiotherapy relies almost exclusively on intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) to shape dose around numerous adjacent critical structures:

• Parotid glands: sparing at least one parotid below a mean dose threshold (commonly cited around 26 Gy) substantially reduces the incidence of permanent xerostomia, one of the most quality-of-life-limiting long-term toxicities.
• Spinal cord and brainstem: hard dose constraints avoid myelopathy or brainstem injury, particularly relevant given the cord's proximity to posterior neck nodal levels.
• Mandible: excessive dose, especially combined with dental extractions, raises the risk of osteoradionecrosis; dental evaluation and extractions are typically completed before simulation.
• Larynx and pharyngeal constrictors: dose to these structures correlates with long-term dysphagia and aspiration risk, so swallowing-sparing IMRT techniques are increasingly emphasized.
• Esophagus: contributes to acute esophagitis and odynophagia during treatment, especially with lower neck and supraclavicular fields.
• Brachial plexus: a constraint of growing importance, especially for lower neck/supraclavicular fields, to avoid plexopathy.
• Optic structures, cochlea, and pituitary: relevant for nasopharyngeal and paranasal sinus primaries due to skull-base proximity.

Setup is highly standardized: patients are immobilized supine in a thermoplastic mask that typically extends from the vertex through the shoulders, often with a bite block or tongue depressor to displace the oral tongue or hard palate out of the high-dose region and improve reproducibility. Shoulders are pulled down and secured with straps for lower neck/supraclavicular coverage. Daily image guidance (CBCT) is standard given the tight margins around the spinal cord and parotids, and frequent physician review of setup, weight loss, and mask fit is important because significant weight loss during treatment can change contour and dosimetry, sometimes prompting a replan.

Concurrent chemoradiation or bio-radiation is standard for locally advanced disease, and altered fractionation schedules (hyperfractionation or concomitant boost) are sometimes used to improve local control in bulky tumors. Because acute mucositis, dysphagia, and dermatitis accumulate over the multi-week course, many patients require a prophylactic feeding tube and close weekly monitoring of oral intake, hydration, and skin reaction, all of which the treating therapist helps track alongside the physician and dietitian as part of the treatment team.