Carcinoma of the Posterior One-Third of the Tongue (Base of Tongue)

Anatomy

• Anteriorly: anterior palatoglossal folds (vertical plane)
• Superiorly: horizontal line at the level of the hard palate
• Inferiorly: floor of the valleculae

• Sensation (general and taste): Glossopharyngeal nerve (CN IX) - via lingual branches providing common sensation, taste, and secretomotor supply
• Near the vallecula/epiglottis: Internal laryngeal nerve (branch of vagus)

Classification (TNM Staging - UICC/AJCC)

p16-Negative (non-HPV-associated) OPC

p16-Positive (HPV-associated) OPC

• No Tis category (due to non-aggressive pattern of invasion and lack of distinct basement membrane in Waldeyer's ring epithelium)
• No T4b category (survival curves of T4a and T4b are indistinguishable in HPV-positive disease)

Epidemiology and Aetiology

• Base of tongue carcinomas are among the most common oropharyngeal subsites for malignancy
• The palatine tonsils and base of tongue are specifically described as common sites for HPV-associated squamous cell carcinoma
• HPV-16 has the strongest association with oropharyngeal cancer, especially tonsillar and base-of-tongue cancer
• HPV-positive head and neck SCC is associated with improved prognosis compared to HPV-negative disease

Clinical Features (Presentation)

• Lesions of the base of tongue are less symptomatic than oral tongue lesions
• The majority present at Stage III/IV - this contrasts sharply with oral tongue cancers which usually present at Stage I/II
• May spread along the palatoglossal muscle to involve the anterior tonsillar pillar
• May spread along the hyoglossal and mylohyoid muscles to the floor of mouth
• Differentiation from normal lingual lymphoid tissue may be difficult on all cross-sectional imaging
• Can present as a neck mass (unknown primary) - the tongue base is a common occult primary site (PET-CT shows increased activity in the tongue base)

Workup and Investigations

1. Clinical examination - difficult due to location; line-of-sight constraints
2. CT and MRI - standard cross-sectional imaging; MRI preferred for soft tissue detail (e.g. T2N2b base of tongue carcinoma)
3. PET-CT - particularly useful when:
   • Presenting with neck nodes and no apparent primary on clinical examination or conventional CT
   • Response assessment after chemoradiotherapy (performed at 12-16 weeks post-treatment for highest diagnostic accuracy)
   • Note: physiological FDG activity in normal lingual lymphoid tissue can complicate interpretation
4. Biopsy - tongue base biopsies may be needed; distinguishing SCC from normal lymphoid tissue can be difficult
5. Pre-treatment FDG-tumour uptake - independent prognostic factor; high uptake may indicate better treatment with surgery + post-operative radiotherapy ± chemotherapy

Spread of Tumour

• Local spread: Along palatoglossal muscle to anterior tonsillar pillar; along hyoglossal/mylohyoid muscles to floor of mouth
• Nodal spread: Bilateral cervical lymphadenopathy (midline structure - drains bilaterally)
• Occult cervical metastasis: present in up to 53% of patients with tongue cancer
• Up to 4.5% may have occult disease in the contralateral neck
• Clinical N0 neck management in T1/T2 disease remains a dilemma as CT, MRI, and PET-CT all fail to detect micrometastases reliably

Treatment

Modalities

1. Surgery
2. Radiotherapy with or without chemotherapy
3. Combined surgery + radiotherapy

Surgical Access

• Usually achieved via a bivalved distending oropharyngoscope
• Even then, complete tumour exposure is not guaranteed due to line-of-sight constraints
• Resection proceeds in a superficial to deep direction, appreciating the 3D orientation of the tongue base and vallecula
• Frozen section may be used to distinguish tumour from normal lingual lymphoid tissue

Transoral Laser Microsurgery (TLM) and Transoral Robotic Surgery (TORS)

• The da Vinci robot provides excellent 3D visualization, immaculate precision, and absence of tremor
• Allows tongue base reduction and management of malignancy with improved access
• Marginal biopsies taken from mucosal edges and deep tissues after resection; zero tolerance for positive margins
• Similar resection principles to tonsillar tumours but marginal biopsy positions are less proscriptive

Surgical Margin

• Resection with 1 cm margin in three dimensions
• Even with apparently adequate margins, 10% of resections may demonstrate histologically positive margins
• For resections of posterior tongue: preserve the contralateral vascular pedicle if tongue tip is to be maintained

Neck Management

• T1/T2 N0: elective neck treatment dilemma (micrometastases unreliably detected by imaging)
• The contralateral N0 neck: staging selective neck dissection may be performed routinely given bilateral lymphatic drainage
• Post-treatment: PET-CT used to guide neck dissection - only performed if residual nodal disease is present

Chemoradiotherapy

• Response to definitive radiotherapy ± chemotherapy is a powerful predictor of outcome
• Perfusion CT can predict response to induction chemotherapy
• Post-treatment PET-CT (at 12-16 weeks) guides decision making on salvage surgery

Prognosis

• Base of tongue tumours have worse prognosis than oral tongue tumours - primarily because they present at advanced stage (Stage III/IV)
• HPV-positive OPC has significantly improved prognosis compared to HPV-negative disease
• This has driven trials of treatment de-escalation in HPV-positive patients to reduce morbidity while maintaining oncological outcomes (e.g. ECOG 3311 trial - transoral surgery followed by low-dose or standard-dose radiotherapy ± chemotherapy in HPV-positive Stage III/IVA OPC)
• Extracapsular nodal spread (identified on imaging by poorly defined nodal margins and surrounding fat stranding) is associated with a 3-5-fold increase in local recurrence
• High pre-treatment FDG uptake on PET-CT is an independent adverse prognostic factor