OROPHARYNX-TARGET VOLUME DELINEATION

Overview

• Oropharyngeal carcinoma: primary tumors of tonsils, base of tongue, soft palate, and posterior pharyngeal wall.
• Majority are squamous cell carcinomas.
• Human papillomavirus (HPV)-associated cancers form a large and clinically important subgroup; HPV-positive tumours have a superior prognosis compared with HPV-negative tumours.
• HPV-unrelated cancers commonly associated with tobacco and alcohol exposure.
• Staging systems: AJCC has separate staging systems for HPV-positive and HPV-negative oropharyngeal cancers — hence routine viral testing (p16/HPV) is recommended for all oropharyngeal carcinoma patients because of prognostic and treatment implications.
• De-escalation of therapy based solely on HPV status should not be performed outside clinical trials at present.
• When planning radiation, consider microscopic mucosal spread beyond gross tumor — careful endoscopic documentation at consult/simulation is useful.

Anatomy & patterns of spread — key points

• The oropharynx is a roughly cuboidal space bounded by: oral cavity (anterior), nasopharynx (superior), and larynx/hypopharynx (inferior).
• Four major subsites: tonsils (palatine tonsil), base of tongue, soft palate, and pharyngeal wall. Most cases arise in the tonsils and base of tongue.
• Rich lymphatic drainage with frequent involvement of cervical nodes — nodal mapping is critical when deciding elective nodal volumes.
• Patterns of spread include direct mucosal extension, submucosal spread along lymphoid tissue and infiltration into adjacent spaces (e.g. parapharyngeal space, pterygopalatine fossa) in advanced disease.
• Consider risk of contralateral spread for midline or crossing lesions (e.g. soft palate, extensive base-of-tongue tumours) — influences decision for bilateral neck treatment.

Imaging & clinical exam — detailed checklist

• Combine imaging (CT, MRI, PET/CT) with careful physical and endoscopic examination for accurate GTV delineation.
• Photographic documentation of the mucosal surface at consultation/simulation helps define mucosal extent that may be poorly visible on cross-sectional imaging.
• Contrast-enhanced CT: Mainstay of diagnostic imaging for initial staging and planning. Obtain thin slices ≤3 mm for simulation and contouring.
• MRI:
  • T1-weighted pre-contrast sequences — excellent for fat planes and bone marrow signals.
  • T1-weighted contrast-enhanced sequences — critical for evaluating anterior extension of base-of-tongue tumors and assessing perineural invasion.
  • T2-weighted fat-suppressed sequences — useful for assessing retropharyngeal nodes and soft tissue extent in parapharyngeal and pre-epiglottic spaces.
• FDG-PET/CT:
  • Provides metabolic information complementary to CT/MRI and can identify additional sites of disease not obvious on CT/MRI.
  • Limitations: poorer spatial resolution than CT/MRI and lower sensitivity for small-volume lymph node metastases — absence of FDG uptake does not exclude microscopic nodal disease.
  • Using the immobilization mask during PET improves registration accuracy to the CT simulation dataset.
• Clinical exam and endoscopy: Essential for mucosal and superficial extent — palpation, visual inspection, and fiberoptic endoscopy guide the GTV and mucosal margins.
• When fusing MRI/PET to CT, ensure consistent patient position and immobilization to minimize registration errors — note that MRI mask use may preclude a dedicated head and neck coil.

Simulation setup — protocolized points

• Position: Supine with headrest; neck slightly extended to reproduce daily treatment position.
• Immobilization: Use a customized 5-point Aquaplast mask (head, neck and shoulder immobilization). Consider a bite-block or mouth guard when needed to stabilize oral structures or improve reproducibility.
• Patient instructions: Ask patient not to swallow during scans and, if possible, during image acquisition to reduce motion-related uncertainties.
• CT simulation: With IV contrast, slice thickness ≤3 mm; scan from skull vertex through carina to include potential nodal regions and adjacent critical structures.
• Isocenter placement: Typically at the level of the arytenoid cartilages. If using a low anterior AP neck field, plan the matching to IMRT fields accordingly.
• Image fusion: Register MRI and PET to CT simulation dataset for contouring. Use the immobilization mask during PET for better fusion accuracy; recognize MRI mask may limit coil use.
• Daily image guidance (examples from MSKCC):
  • Daily linear accelerator–mounted 2D kV imaging and daily kV and cone-beam CT for setup verification.
  • Cone-beam CT can be used weekly or daily; daily KV imaging is used as an alternative/augmentation.
  • Other image-guidance methods: orthogonal kV imaging systems (e.g., ExacTrac) or linac-mounted MV CT (e.g., TomoTherapy) depending on equipment availability.

Selected IMRT dose & fractionation schemes

• Multiple approaches exist; institutional practice varies. MSKCC commonly uses a sequential boost technique with total dose to gross disease of 70 Gy.
• Example for HPV-related tumours:
  • Subclinical regions may receive ~30 Gy in 2 Gy fractions (ie 15 fractions) followed by a cone-down; the gross disease then receives additional dose (described as receiving 40 Gy in 2 Gy fractions to the gross disease in that particular schema — totalizing to the institutional recommended gross dose; verify locally).
• Simultaneous integrated boost (SIB) technique (commonly used with concurrent chemotherapy):
  • Gross disease: ~70 Gy (≈2 Gy/fx).
  • High-risk subclinical: ~56 Gy (eg 1.6 Gy/fx).
  • Low-risk subclinical: ~50–52.5 Gy (eg 1.43–1.5 Gy/fx).
• Other fractionation schemes referenced in the literature include RTOG protocols (eg RTOG 0022, RTOG 1016); always cross-check protocol-specific dose constraints and fractionation before implementation.
• Reduced elective doses should be considered only when treating with concurrent cisplatin-based chemotherapy and within protocol guidance.

Suggested target volumes — gross disease

Target volume	Definition & description
GTV70	All gross disease identified by clinical exam and imaging (CT, MRI, PET). Include the primary tumour and all radiologically or clinically suspicious lymph nodes (eg >1 cm, necrotic, showing irregular enhancement, or FDG-avid on PET). Borderline suspicious nodes may be considered for lower boost dose (eg 60–66 Gy) depending on multidisciplinary discussion.
CTV70	Area considered to contain microscopic extension around the GTV. In regions with excellent visualization, CTV70 may equal GTV70 (no added margin). Where uncertainty exists, add a margin (commonly 3–5 mm) to account for microscopic spread while respecting anatomic barriers (bone, air, skin).
PTV70	Planning target volume to account for setup variation and motion: commonly CTV + 3–5 mm depending on institutional daily setup accuracy and image guidance availability. If critical organ constraints or image guidance permit, consider smaller or zero PTV margins at specific interfaces.

Suggested target volumes — subclinical disease

Target volume	Definition & recommended considerations
General guideline (CTVsubclinical)	As a useful rule, the primary-site CTVsubclinical should encompass the GTV plus approximately 1 cm of margin, shaved off anatomic barriers to spread (bone, air, and skin). Adjust margins for mucosal surfaces and known pathways of spread.
Tonsil primary	Ensure ~1 cm mucosal and submucosal margin around the primary. In advanced tumours include pterygoid plates and consider inclusion of the ipsilateral retromolar trigone if anterior spread is suspected. Consider parapharyngeal space/pterygopalatine fossa coverage for advanced disease.
Base of tongue primary	Include glossotonsillar sulcus, valleculae and pre-epiglottic space. Ensure a mucosal margin of at least 1.0 cm around the base-of-tongue primary tumour; anterior extension into the oral tongue should be accounted for when present. MRI is particularly helpful to define the anterior extent.
Soft palate primary	Cover the entire soft palate, superior aspect of the tonsillar pillars and fossa. Include adjacent nasopharynx superiorly and the pterygopalatine fossa when advanced; if pterygopalatine fossa is involved, assessment of skull base with MRI is required. Ensure anterior mucosal coverage which may include the hard palate portion depending on spread.
Pharyngeal wall primary	Provide generous superior and inferior margins because of the possibility of skip lesions. In advanced tumours consider cranial extension of CTV to cover nasopharynx and caudally to include the hypopharynx where appropriate.
Elective (nodal) regions	Elective nodal regions may be treated to microscopic doses typically around 54 Gy in 1.8 Gy fractions (multiple acceptable fractionation patterns exist including 54.12 Gy in 1.64 Gy fractions, 56 Gy in 1.6 Gy fractions, 59.4 Gy in 1.8 Gy fractions depending on the chosen regimen). At-risk nodal regions usually include bilateral levels II–IV and the lateral retropharyngeal nodes when indicated; in node-negative patients at risk areas are determined by primary site and laterality. MSKCC practice note: They do not routinely include levels IB or V electively unless gross involvement or specific patterns of spread are present (eg oral cavity extension affecting level IB). IB nodes may be considered when gross oral cavity extension is present. In node-positive patients, ensure coverage of retropharyngeal nodes and retrostyloid nodes with cranial extent to the skull base as required. If gross involvement of low-lying nodes is present, consider inclusion of the supraclavicular fossa. T1–2, N0–N1 well-lateralized tonsil cancers (≥1 cm lateral from midline) with no extension to base of tongue or soft palate: ipsilateral neck treatment may be acceptable in selected cases.

Practical notes on margins and OAR considerations

• CTV expansions should respect anatomic barriers. Where tumor abuts critical organs (eg brainstem, spinal cord, parotid glands), contour margins may be reduced, and individualized PTV/CTV decisions made in MDT.
• PTV margins: common practice is 3–5 mm — choose based on immobilization reproducibility and image-guidance frequency. With daily CBCT and robust immobilization, smaller PTV margins may be justified selectively.
• When using cone-down boosts, clearly document the criteria for cone-down (eg radiologic resolution, planned dose distribution) and ensure cumulative dose to OARs remains within constraints.