What are the guidelines for oral cavity contouring in radiation therapy?

Oral Cavity Contouring Guidelines in Radiation Therapy

Oral cavity contouring for radiation therapy should follow the Radiation Therapy Oncology Group (RTOG) atlas guidelines, which provide standardized anatomical definitions for target volumes and organs at risk. 1

Standard Contouring Approach

Target Volume Definitions

The fundamental target volumes must be clearly delineated according to established principles 1:

• Gross Tumor Volume (GTV): Encompasses all visible/palpable gross disease 1
• Clinical Target Volume (CTV): Includes the GTV plus margins for microscopic disease spread, or the postoperative tumor bed with microscopic areas at risk 1
• Planning Target Volume (PTV): Adds margins to the CTV to account for setup uncertainties and patient positioning variations 1

Organs at Risk Contouring

Normal tissue structures must be contoured according to RTOG atlas specifications, with particular attention to dose-sensitive structures. 1

Critical structures requiring careful delineation include 1:

• Spinal cord: A planning-at-risk volume of 3-5 mm should be added to account for positioning uncertainties 1
• Parotid glands: Both glands should be contoured separately to enable dose-sparing strategies 2
• Mandible: Complete bony structure contouring is essential to predict osteoradionecrosis risk 2
• Submandibular glands: Should be delineated when feasible for xerostomia risk assessment 3
• Oral mucosa: Can be contoured using either the entire oral cavity volume or the mucosal surface technique 4

Oral Cavity-Specific Considerations

Mucosal Surface vs. Oral Cavity Volume

Two validated approaches exist for oral mucosa contouring 4:

• Oral Cavity Contours (OCC): The entire oral cavity volume is delineated, with V30Gy <73% predicting lower risk of grade ≥3 acute mucositis (sensitivity 84%, specificity 67%) 4
• Mucosal Surface Contours (MSC): Only the mucosal surface is contoured, with V50Gy <14% as the predictive threshold (sensitivity 84%, specificity 58%) 4

Both methods accurately predict acute toxicity, but the oral cavity volume approach (OCC) appears more practical for routine clinical use. 4

Dose-Volume Constraints for Oral Structures

To minimize xerostomia and mandibular complications 2, 3:

• Mean parotid dose: Should be maintained <25-26 Gy when possible (IMRT achieves this vs. 42 Gy with 3D conformal) 2
• Parotid V30: Keep <45% of volume receiving 30 Gy 2
• Mandible V50, V55, V60: Minimize volumes receiving these dose levels to reduce osteoradionecrosis risk 2
• Oral cavity mean dose: Reduction through volume-reduced contouring approaches can decrease toxicity 3

Clinical Target Volume Margins

Definitive Treatment

For primary oral cavity tumors treated with definitive radiotherapy 1:

• Gross disease: 70 Gy in 2.0 Gy fractions to primary and gross adenopathy 1
• Elective nodal regions: 44-64 Gy (1.6-2.0 Gy/fraction) to uninvolved nodal stations at risk 1
• Microscopic disease: Approximately 50 Gy in 2-Gy fractions (or biologically equivalent) to clinically negative regions at risk 1

Postoperative Setting

Adjuvant radiation volumes depend on pathologic risk factors 1, 5:

• High-risk areas (positive margins, extracapsular extension): 60-66 Gy at 2 Gy/fraction 1
• Tumor bed and dissected nodal regions: 56-60 Gy in standard fractionation without adverse features 1
• Treatment must begin within 6-7 weeks of surgery, with the entire sequence completed within 11 weeks 5

Volume Reduction Strategies

Modern approaches favor reducing elective treatment volumes rather than dose to minimize long-term toxicity while maintaining tumor control. 3

Volume-reduced contouring achieves 3:

• Significantly smaller elective PTV volumes (56 Gy level)
• Reduced high-risk PTV volumes (63 Gy shrinking field)
• Lower mean doses to oral cavity structures, larynx, swallowing muscles, and submandibular glands
• Potential reduction in dysphagia, mucositis, and xerostomia rates

Technical Considerations

IMRT vs. 3D Conformal Planning

Intensity-modulated radiation therapy (IMRT) provides superior target coverage and normal tissue sparing compared to 3D conformal techniques for oral cavity cancer. 2

IMRT advantages include 2:

• Improved dose homogeneity within target volumes (lower dose range: 15.3 Gy vs. 24.7 Gy for 3DCRT)
• Significantly reduced mean parotid doses (25.6 Gy vs. 42.0 Gy)
• Lower mandibular doses at all threshold levels (V50, V55, V60)
• Maintained spinal cord and brainstem sparing

Common Pitfalls to Avoid

Critical errors in oral cavity contouring include:

• Inadequate margin expansion: Failing to account for microscopic spread patterns specific to oral cavity subsites 1
• Inconsistent atlas use: Not adhering to standardized RTOG definitions leads to inter-observer variability 1
• Neglecting dental evaluation: Patients receiving significant intraoral radiation require pre-treatment dental assessment and ongoing monitoring 1
• Omitting planning-at-risk volumes: Particularly for spinal cord, where 3-5 mm expansion is standard 1

Follow-Up Imaging Requirements

Post-treatment surveillance necessitates 1, 5:

• Baseline imaging within 3-6 months of treatment completion (Category 2B recommendation) 1, 5
• Dental evaluation every 6 months for patients who received radiotherapy 5
• TSH monitoring every 6-12 months if neck was irradiated 1, 5