Which anatomical structures should be contoured when delineating radiotherapy volumes for the buccal mucosa?

Anatomical Structures for Buccal Mucosa Radiotherapy Delineation

When delineating radiotherapy volumes for buccal mucosa cancer, you must contour the primary tumor bed/gross tumor volume, the buccal mucosa itself as a pseudo-organ-at-risk, the mandible and maxilla (divided into sextants for dose tracking), ipsilateral inguinofemoral and iliac lymph nodes, and critical organs at risk including parotid glands, submandibular glands, spinal cord, brainstem, optic structures, and contralateral oral structures.

Primary Target Structures

Gross Tumor Volume (GTV) and Clinical Target Volume (CTV)

• Contour the pre-surgical gross tumor extent with 5-10 mm isotropic margin for the primary tumor bed 1
• Include the flap-body tissue junction with 5-10 mm margin if reconstruction was performed 1
• The entire flap does NOT need intentional coverage unless it traverses involved nodal regions 1
• Use MRI-CT image fusion for accurate delineation, particularly for bone involvement 2
• Place radio-opaque markers at key landmarks during simulation to define the primary target volume 3

Buccal Mucosa as Pseudo-OAR

Critical innovation: Delineate the anterior oral mucosa and contralateral buccal mucosa as a pseudo-organ-at-risk with dose constraint of Dmean ≤30 Gy 4. This reduces mean dose by approximately 75% (from 31.28 Gy to 7.87 Gy, p=0.001) without compromising target coverage, potentially decreasing oral mucositis incidence 4.

Use the puffed-cheek technique during CT simulation—have the patient inflate their cheeks with pursed lips to separate tumor from normal mucosa and provide clearer delineation of buccal mucosa tumors 5.

Mandible and Maxilla Delineation

Divide both jaws into sextants (3 segments each in mandible and maxilla) using bony landmarks 6. This enables:

• Accurate dose recording to specific jaw regions
• Meaningful communication with dental colleagues for post-treatment care
• Assessment of osteoradionecrosis risk (NTCP modeling shows 5.2% vs 8.7% risk with optimized planning, p<0.001) 7

For bone flap reconstruction cases, separately contour 8:

• Native mandible
• Bone flap (distinct from native bone)
• Osteotomy sites
• Fixation hardware/plates
• Use adapted bone and metal windowing for accurate cortical interface identification

Nodal Volumes

Ipsilateral Nodes

Include inguinofemoral and iliac lymph nodes in the clinical target volume 3:

• Superior border: bottom of sacroiliac joints (or L4/L5 junction if pelvic nodes uninvolved)
• Lateral border: vertical line from anterior-inferior iliac spine
• Inferior border: 2 cm below most distal vulvar extent, covering inguinofemoral bed to intertrochanteric line

Contralateral Elective Nodal Irradiation

Only perform contralateral ENI when 1:

• Tumor crosses midline AND heavy nodal burden at ipsilateral level Ia/Ib
• Tumor has high propensity for contralateral involvement
• Otherwise, contralateral ENI is NOT routinely indicated (only 2.1% contralateral failures without it)

Critical Organs at Risk

Salivary Glands

• Parotid glands: Mean dose significantly impacts xerostomia (NTCP 27.3% vs 38.4% with optimized planning, p<0.001) 7
• Submandibular glands: Contralateral gland sparing is crucial—mean dose 18.6 Gy with IMRT vs 3.2 Gy with 3DCRT 9
• Contralateral structures receive lower priority but should be contoured for dose assessment

Neural and Vascular Structures

Contour with maximum dose constraints 9:

• Spinal cord: Max 1cc dose 31.8 Gy (IMRT) vs 20.9 Gy (3DCRT)
• Brainstem: Max 1cc dose 29.7 Gy (IMRT) vs 14.1 Gy (3DCRT)
• Optic chiasm: Max 1cc dose 29.2 Gy (IMRT) vs 12 Gy (3DCRT)
• Optic nerves: Ipsilateral 22 Gy vs 11.7 Gy

Dental Structures

Separately contour teeth (individual or grouped) with dose constraint to reduce long-term complications 10, 7:

• Mean dose to dental structures: 30.4 Gy vs 36.1 Gy with optimized planning (p=0.002)
• Volume receiving 30 Gy: 40.8% vs 50.4% (p=0.001)

Technical Planning Considerations

For early-stage buccal mucosa with lateralized target, 3D-CRT may be sufficient and actually superior to IMRT for contralateral structure sparing 9. IMRT shows higher doses to brainstem, optic chiasm, spinal cord, and contralateral salivary glands when the target is well-lateralized.

Use CT-based planning with contouring of all target structures 3. IMRT or volumetric modulated arc therapy is recommended when target geometry requires it, with strict dose constraints to normal organs 11.

Apply bolus to ensure adequate dosing to superficial target volumes 3.