What are the contouring guidelines for Gross Tumor Volume (GTV), Clinical Target Volume (CTV), and nodal Clinical Target Volume (nCTV) in definitive chemoradiation therapy (CTRT) for a patient with carcinoma (Ca) of the soft palate, clinical stage T3N0?

Contouring Guidelines for Soft Palate cT3N0 Definitive Chemoradiation

GTV Delineation

For soft palate carcinoma cT3N0, the GTV should include all visible and palpable tumor on clinical examination and imaging, with MRI-CT fusion mandatory for accurate target delineation, particularly to assess skull base extension and perineural involvement. 1

• Contour the GTV on the planning CT scan with MRI fusion to define the primary tumor extent 1
• Use standardized CT window settings for optimal visualization: window width 400 and level 20 for soft tissue structures 1
• Include any visible tumor extension into adjacent structures (hard palate, tonsillar pillars, base of tongue) 1
• Review imaging with a head and neck radiologist when uncertainty exists regarding disease extent 1
• PET-CT should be obtained for staging and can aid in target delineation, with FDG-positive disease included in the GTV 1

CTV Expansion from GTV

The CTV should encompass the GTV with a 5-10 mm geometric expansion, modified by anatomical boundaries, to account for microscopic disease extension. 2, 3

• Apply a 5 mm margin from GTV to CTV as recommended by EORTC guidelines for head and neck sites 2
• Geometric expansion is more reproducible than purely anatomical margins and results in more uniform treatment plans 3
• Trim the CTV at natural anatomical barriers (bone without invasion, air spaces) 1
• The CTV should include the entire soft palate and adjacent high-risk mucosal sites 1
• Do not use manual PTV adjustments; maintain systematic margin expansions 1, 4

Nodal CTV (nCTV) Inclusion

Bilateral neck irradiation from retropharyngeal nodes through level IV is mandatory for soft palate cT3N0, with level V included bilaterally and level IB omitted unless there is anterior nasal cavity involvement or high-risk nodal features. 1

Nodal Levels to Include:

• Retropharyngeal nodes bilaterally - high risk for microscopic involvement in oropharyngeal primaries 1
• Levels II, III, and IV bilaterally - standard elective coverage for soft palate tumors 1
• Level V bilaterally - included in standard elective volumes 1
• Level IB may be omitted unless anterior nasal cavity is involved or level II nodes show extranodal extension, size >2 cm, or bilateral involvement 1

Nodal CTV Delineation:

• Contour nodal CTV based on vascular anatomy with 7 mm expansion around vessels, excluding bone and muscle 1
• For retropharyngeal nodes, include the space between the pharyngeal constrictors and prevertebral fascia 1
• No elective lower neck (below level IV) irradiation is needed for N0 disease in the contralateral uninvolved neck 1
• Trim nodal CTV 3 mm from skin surface in absence of skin involvement 1

PTV Expansion

The PTV should be created by expanding the CTV by 3-5 mm for IMRT with daily image guidance, or 5-10 mm for conventional setup verification. 2, 4

• Use 3-5 mm CTV-to-PTV expansion when daily cone-beam CT or kV imaging is performed 2, 4
• Increase to 5-10 mm if using less frequent image guidance 4
• Account for setup uncertainties and patient positioning variability in the PTV margin 5
• A margin of 1.65 standard deviations provides 95% confidence that any point on the CTV surface is within the PTV 5

Critical Technical Considerations

• Obtain planning CT with 2-3 mm slice thickness for accurate delineation 4
• Use intravenous contrast to improve visualization of tumor and nodal structures 1, 4
• Immobilize patient with thermoplastic mask in treatment position 1
• Place radio-opaque markers on palpable tumor borders and anatomical landmarks during simulation 1
• Verify doses to target areas with in vivo dosimetry at first treatment 1

Common Pitfalls to Avoid

• Do not reduce GTV to post-chemotherapy volumes if induction chemotherapy was given; the preinduction tumor extent within bony anatomy should receive full dose 1
• Avoid symmetric geometric expansions for nodal volumes without considering anatomical boundaries 1
• Do not omit retropharyngeal nodes in oropharyngeal primaries, even with N0 neck 1
• Ensure adequate coverage of the entire soft palate as microscopic disease extends beyond visible tumor 3