Nasopharyngeal Carcinoma Contouring Guidelines
Imaging Requirements
MRI-CT fusion is mandatory for all nasopharyngeal carcinoma target delineation—this is not optional. 1, 2 MRI provides superior detection of skull base extension, perineural disease without bone involvement, marrow infiltration, paranasal sinus and orbital extension, and retropharyngeal lymph node involvement, while CT remains superior for detecting neck nodal disease and cortical bone invasion. 1, 2 The radiation oncologist should review imaging with a head and neck radiologist when uncertainty exists regarding disease extent. 1
For optimal MRI evaluation, use gadolinium-enhanced axial T1-weighted sequences, axial T2-weighted sequences, and both coronal and sagittal T1 planes—all registered to the planning CT. 3 Using only axial T1 sequences underestimates tumor volume by approximately 30%, with critical missed extensions at the skull base both cranially and caudally. 3
Gross Tumor Volume (GTV) Delineation
Primary Tumor GTV (GTVp)
Delineate GTVp using all available MRI sequences and planes fused with planning CT, including the entire visible tumor on imaging and clinical examination. 1, 2, 3 The GTV should encompass all FDG-avid disease if PET-CT is available. 4
Nodal GTV (GTVn)
Include all pathologically enlarged lymph nodes with a 5-10 mm margin, considering 10 mm expansion if extracapsular extension is present. 1
Clinical Target Volume (CTV) Delineation
High-Risk Primary CTV (CTVp1)
CTVp1 = GTVp + 5 mm margin (1 mm if tumor is in close proximity to brainstem or spinal cord) + entire nasopharynx. 1, 2 This volume receives the full therapeutic dose of 70 Gy. 1
The CTVp1 must include: 1
- Entire nasopharynx
- Posterior nasal cavity (at least 5 mm from choana)
- Posterior maxillary sinuses (at least 5 mm from posterior wall to ensure pterygopalatine fossa coverage)
- Posterior ethmoid sinus (include vomer)
- Base of skull (cover foramina ovale, rotundum, lacerum, and petrous tip)
- Cavernous sinus (if T3/T4, only involved side)
- Full parapharyngeal spaces bilaterally
- Sphenoid sinus (inferior 1/2 if T1-2; entire sinus if T3-4)
- Clivus (anterior 1/3 if no invasion; entire clivus if invaded)
Intermediate-Risk Primary CTV (CTVp2)
CTVp2 = GTVp + 10 mm margin (2 mm if tumor is in close proximity to brainstem or spinal cord) + entire nasopharynx. 1 This volume receives 60-63 Gy. 1
High-Risk Nodal CTV (CTVn1)
CTVn1 = GTVn + 5 mm margin (consider 10 mm if nodal extracapsular extension present). 1 This volume receives 60-66 Gy. 1
Intermediate-Risk Nodal CTV (CTVn2)
Include bilateral retropharyngeal nodes and levels II, III, and VA in all patients, plus at least one level below the lowest involved nodal level. 1 This volume receives 54-60 Gy. 1
Include ipsilateral level IB only if: 1
- Level IB nodes are positive
- Tumor involves structures draining to level IB as first echelon (anterior half of nasal cavity)
- Submandibular gland involvement present
- Ipsilateral level IIA nodes show extracapsular extension
- Ipsilateral level IIA nodes exceed 2 cm maximum diameter
- Bilateral level II involvement present
Low-Risk Nodal CTV (CTVn3) - Optional
Consider omitting levels IV and VB to the clavicle if nodal involvement is confined to level II nodes only, or if the patient is N0 or N1 based solely on retropharyngeal node positivity. 1 This approach reduces toxicity without compromising regional control in node-negative disease. 1
Special Considerations After Induction Chemotherapy
For patients receiving induction chemotherapy, fuse the pre-induction scan with post-induction CT simulation to illustrate initial disease extent. 1
The GTV should generally follow the pre-induction tumor extent, especially within bony anatomy at the skull base. 1, 2 This is critical because of difficulty appreciating disease extent within bone, lack of salvage options at the skull base, and uncertain benefit from post-induction volume reduction. 1
However, carefully tailoring around residual GTV after induction chemotherapy is acceptable if the resolved pre-induction GTV receives at least an intermediate dose of 64 Gy. 1 A phase III RCT demonstrated that this approach (64 Gy to pre-induction volume with 70 Gy to post-induction volume) achieved equivalent disease control with improved xerostomia scores and cognitive function compared to 70 Gy to the entire pre-induction volume. 1
Planning Target Volume (PTV)
Use 3-5 mm CTV-to-PTV expansion when daily cone-beam CT or kV imaging is performed. 4 If using less frequent image guidance, increase the expansion to 5-10 mm. 4
Dose Prescriptions
Standard Fractionation (Recommended)
Deliver 70 Gy in 35 fractions (2.0 Gy per fraction) to high-risk volumes (GTVp, GTVn, CTVp1, CTVn1) using simultaneous integrated boost technique. 1, 2 Alternative acceptable regimens include 69.96 Gy in 33 fractions (2.12 Gy per fraction). 1
Deliver 60-63 Gy in 35 fractions (1.71-1.8 Gy per fraction) to intermediate-risk volumes (CTVp2, CTVn2). 1
Deliver 54-56 Gy in 35 fractions (1.54-1.6 Gy per fraction) to low-risk volumes (CTVn3) if included. 1
Technical Delivery
Use simultaneous integrated boost (SIB) as the technique of choice due to convenience and equivalent outcomes compared to sequential boost. 1 A randomized trial found no difference in clinical outcomes or toxicities between SIB and sequential boost approaches. 1
Critical Pitfalls to Avoid
Never contour without MRI-CT fusion—using CT alone misses critical disease extent at the skull base and perineural involvement. 1, 2, 3 Using only axial T1 MRI sequences underestimates tumor volume by 30% with missed extensions at the skull base. 3
Never reduce GTV at the skull base after induction chemotherapy unless the pre-induction extent receives at least 64 Gy intermediate dose. 1, 2 The skull base is a critical site with no salvage options and uncertain benefit from volume reduction. 1
Never use fraction sizes exceeding 2.12 Gy with concurrent chemotherapy—dose escalation beyond 70-76 Gy carries high risk of severe late toxicities including osteoradionecrosis, carotid pseudoaneurysm, and neurological complications without proven survival benefit. 1
Never omit bilateral neck coverage (levels II-V and retropharyngeal nodes)—nasopharyngeal carcinoma has high incidence of occult bilateral nodal involvement regardless of clinical N-stage. 1, 2
Never use symmetric geometric expansions for nodal volumes without considering anatomical boundaries—trim CTV at natural barriers such as uninvolved bone and air spaces. 4