KEY POINTS
- This retrospective planning study included 30 patients with nasopharyngeal carcinoma treated with definitive helical tomotherapy and deliberately sampled across five patterns of lower-neck nodal involvement. Each patient was replanned using the 2018 level IVa definition and the updated 2024/2025 international consensus definition.
- The key anatomical change was moving the anterior level IVa boundary from the anterior edge of the sternocleidomastoid muscle to the posterior edge of the infrahyoid ribbon muscles. Both plans used identical target prescriptions—70 Gy and 56 Gy in 35 fractions—and were optimized to preserve equivalent target coverage and other organ-at-risk doses.
- Mean thyroid dose fell from 48.5 to 44.8 Gy, an absolute reduction of 3.7 Gy (95% CI 3.2–4.3; p<0.001). Thyroid V50 also decreased from 59.9% to 46.3%, a 13.6-percentage-point reduction (p<0.001).
- Translating those dose changes through a validated NTCP model, predicted primary hypothyroidism fell from 43.9% to 34.9%, an absolute reduction of 9.0 percentage points and a relative reduction of 20.5% (p<0.001).
- Laryngeal sparing improved as well: mean dose decreased from 25.9 to 21.7 Gy, a 4.3-Gy reduction (p<0.001). Predicted grade ≥2 laryngeal edema declined only from 0.7% to 0.2%, reflecting the low baseline risk at these dose levels.
- Target coverage was essentially unchanged: PTV70 V100 remained 98.7% versus 98.7%, and PTV56 V100 99.2% versus 99.2%. Spinal cord, esophageal and post-cricoid doses were also comparable, supporting that the thyroid benefit came from the contour refinement rather than global plan degradation. The dose distributions shown in Figure 1 visually demonstrate the reduction around the thyroid and larynx while preserving the elective target.
- Benefit depended on nodal anatomy. Patients with at least one elective lower-neck target without level IV metastasis achieved greater median thyroid-dose reduction than those with level IV disease in all elective targets (4.2 vs 2.6 Gy; p=0.006) and greater hypothyroidism-risk reduction (10.4% vs 5.9%; p=0.005).
CLINICAL TAKEAWAY
The updated 2024/2025 level IVa boundary appears to provide a meaningful thyroid-sparing advantage at essentially no cost to target coverage, supporting adoption of the new NPC contouring recommendations. The estimated 9% absolute reduction in hypothyroidism is clinically attractive, but it comes from NTCP modeling in a 30-patient planning study rather than observed long-term endocrine outcomes.