Trigeminal nerve (V) contouring guidelines in head and neck cancers
TL;DRPractical CTV guidance for PNI along V1/V2/V3: CTV-P1 = GTV + 5 mm along nerve; CTV-P2 = nerve path ≥30 mm, skull base often recommended.
EORTC 22931
The ≥30 mm CTV-P2 extension exists because MRI underreads PNI extent: extent misdefined in 17% and 11% of surgical-correlation series, and radiologic skip lesions sit on pathologically continuous disease. That gap is the argument for elective retrograde nerve-path coverage toward skull base in ACC, extensive-PNI SCC, and PPF or masticator invasion.
In H&N SCC with extensive PNI, ACC, or PPF or masticator space invasion, this supports retrograde nerve-path CTV-P2 coverage toward skull base; it does not extend to focal microscopic PNI without other high-risk features, which the authors leave case-by-case.
The ≥30 mm CTV-P2 extension exists because MRI underreads PNI extent (17% and 11% misdefined in surgical-correlation series) and skip lesions sit on continuous disease. That gap justifies retrograde nerve-path coverage toward skull base in ACC, extensive-PNI SCC, and PPF or masticator invasion.
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Practical contouring guideline from a multi-institution author group, built on V1/V2/V3 radio-anatomy, PNI spread routes, and published imaging series. No outcome endpoint.
Primary RT: GTV = post-contrast enhancement along the nerve; CTV-P1 = GTV + 5 mm along the nerve path; CTV-P2 ≥30 mm along the nerve, skull base often recommended. Post-op R1: CTV-P1 = margin + 5 mm along the nerve path plus PNI in the whole tumor bed. Skull base involvement extends CTV-P2 to the brainstem.
High-risk spread is defined by histology (ACC; cutaneous or non-cutaneous SCC with extensive PNI) or site (pterygopalatine fossa or masticator space invasion, nasopharyngeal or maxillary sinus primaries). Non-lateralized tumors get bilateral nerve-path consideration.
Imaging guidance: MRI is the modality of choice, CT complements for foraminal bony change, and 18FDG-PET-CT is often inferior to MRI for macroscopic PNI. MRI co-registration in treatment position is recommended whenever possible.
| Scenario | High-risk CTV (CTV-P1) | Low-risk CTV (CTV-P2) |
|---|---|---|
| Primary RT, unresected macroscopic PNI | GTV (post-contrast enhancement along nerve) + 5 mm along nerve path | Nerve path ≥30 mm; extension to skull base discussed, often recommended |
| Post-op RT, resected macroscopic PNI with R1 margin | Positive margin + 5 mm along nerve path; PNI in entire tumor bed | Per histology/site risk (row below) |
| High-risk histology or site (ACC, SCC with extensive PNI, PPF or masticator invasion, NPC, maxillary sinus) | PNI in tumor bed | Nerve path ≥30 mm per primary site; skull base discussed, often recommended |
| Skull base or intracranial involvement | As above | Extend CTV-P2 to the brainstem |
EORTC 22931 included pts with PNI and showed concurrent cisplatin improved locoregional control over post-op RT alone. The Zukauskaite primary-RT series placed most recurrences in the GTV but excluded the PNI-prone sites this guideline targets.
No dosimetric or failure-pattern validation of the margins is reported. Large CTV-P2 extensions near brainstem and optic pathway force OAR compromises the guideline leaves to case-by-case judgment.
The document standardizes nerve-path delineation where IMRT planning on axial CT routinely misses retrograde spread. It does not settle how far elective coverage must go, or whether skull base extension improves control enough to justify the added OAR dose.
- Does elective skull base nerve coverage reduce marginal failures in ACC?
- Adjuvant RT benefit when focal PNI is the only risk feature
- Role of particle therapy for inoperable or residual ACC n=250 · primary completion 2026-12 · randomised carbon ion vs photon/proton, unresectable or R2n=314 · primary completion 2027-12 · carbon ion only vs bimodal, inoperable or R1/R2 HN ACC
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The longer read
The practical value of this guideline is that it turns a qualitative principle, cover the nerve, into geometric instructions a dosimetrist can draw. Its two numbers, a 5 mm high-risk expansion along the nerve and at least 30 mm of low-risk nerve path, are not derived from failure-pattern data. They are reasoned from anatomy and from the known weakness of imaging. That weakness is the strongest part of the argument: in the surgical-correlation series the authors cite, MRI misdefined extent in 5 of 30 cases in one and in 4 of 36 detected cases in another, and radiologic skip lesions sit over disease that pathology usually shows to be continuous. If imaging systematically underreads, a margin along the nerve rather than an isotropic expansion is the logical response, and the guideline is internally consistent on that point.
Where confidence should drop is the low-risk volume. The 30 mm figure and the recommendation to discuss coverage to skull base are expert positions, and the authors are candid that randomized evidence is absent, both for particle therapy in ACC and for adjuvant RT when PNI is the only indication. The one large outcome dataset invoked, the Zukauskaite primary-RT series of 1576 pts, located most recurrences in the GTV, which on its face argues against generous elective volumes. The authors' rebuttal is reasonable: that series excluded ACC, nasopharynx, paranasal sinus and skin primaries, precisely the populations at highest risk of neural spread. So the dataset neither supports nor refutes the guideline for the cases it matters most in, and a reader should treat the extension length as a convention rather than a validated threshold.
The guideline's anatomic framing is also its most transferable contribution. Mapping skin of the upper, middle and lower face to V1, V2 and V3, and treating the pterygopalatine fossa as the junction connecting all three divisions and the facial nerve via the Vidian nerve, gives a reproducible logic for choosing which path to cover. The parotid example, where the auriculotemporal branch of V3 communicates with VII, is a reminder that a parotid ACC recurrence along V3 is a dual-nerve problem.
What would make the recommendations wrong is evidence that marginal and out-of-field failures along the nerve are rare in ACC and extensive-PNI SCC treated to GTV-plus-tumor-bed volumes alone. Until patterns-of-failure data in those histologies exist, the trade-off the authors explicitly leave open, undercoverage risk versus brainstem, brain and optic pathway dose near the skull base, remains a case-by-case judgment rather than something this document resolves.