Advances in radiation oncology · 2026
Impact of Updated Level IVa Nodal Delineation on Thyroid Dose and Hypothyroidism Risk in Nasopharyngeal Carcinoma.
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Lui Jeffrey C F, Lee Victor H F, Lee Francis K H, Au Kwok Hung, Wong Kenneth C W, Cai Jing, Chow James C H
Аннотация
PURPOSE: The 2024 and 2025 international consensus guidelines redefined the anterior border of the level IVa elective nodal volume for nasopharyngeal carcinoma (NPC). This study aimed to provide the first dosimetric validation of this anatomic refinement by quantifying its impact on thyroid and larynx doses and translating these reductions into clinically interpretable normal tissue complication probability (NTCP) estimates.
METHODS AND MATERIALS: Thirty patients with NPC treated with helical tomotherapy were selected to represent a full spectrum of lower cervical nodal involvement. For each patient, 2 comparative treatment plans for different nodal clinical target volume (CTVn) definitions were generated: (1) CTVn based on 2018 guidelines, defining the level IVa anterior border at the anterior edge of the sternocleidomastoid muscle; and (2) CTVn based on 2024/2025 guidelines, shifting this border to the posterior edge of the infrahyoid ribbon muscles. Plans were optimized to minimize thyroid and larynx doses while maintaining equivalent target coverage. Dosimetric parameters and NTCP for hypothyroidism and laryngeal edema were compared using paired analyses.
RESULTS: The updated CTVn delineation significantly reduced thyroid mean dose from 48.5 Gy to 44.8 Gy (P < .001) and thyroid volume receiving ≥50 Gy from 59.9% to 46.3% (P < .001). Larynx mean dose decreased from 25.9 Gy to 21.7 Gy (P < .001). The NTCP for hypothyroidism was reduced from 43.9% to 34.9% (P < .001), corresponding to a 20.5% relative risk reduction. Target coverage and doses to other organs at risk remained comparable between plans.
CONCLUSIONS: Adopting the updated 2024/2025 international consensus guidelines for level IVa delineation significantly reduces radiation dose to the thyroid and larynx without compromising target coverage. This translates into a clinically meaningful reduction in the risk of primary hypothyroidism, supporting the widespread implementation of these guidelines to improve survivorship outcomes in patients with NPC.
METHODS AND MATERIALS: Thirty patients with NPC treated with helical tomotherapy were selected to represent a full spectrum of lower cervical nodal involvement. For each patient, 2 comparative treatment plans for different nodal clinical target volume (CTVn) definitions were generated: (1) CTVn based on 2018 guidelines, defining the level IVa anterior border at the anterior edge of the sternocleidomastoid muscle; and (2) CTVn based on 2024/2025 guidelines, shifting this border to the posterior edge of the infrahyoid ribbon muscles. Plans were optimized to minimize thyroid and larynx doses while maintaining equivalent target coverage. Dosimetric parameters and NTCP for hypothyroidism and laryngeal edema were compared using paired analyses.
RESULTS: The updated CTVn delineation significantly reduced thyroid mean dose from 48.5 Gy to 44.8 Gy (P < .001) and thyroid volume receiving ≥50 Gy from 59.9% to 46.3% (P < .001). Larynx mean dose decreased from 25.9 Gy to 21.7 Gy (P < .001). The NTCP for hypothyroidism was reduced from 43.9% to 34.9% (P < .001), corresponding to a 20.5% relative risk reduction. Target coverage and doses to other organs at risk remained comparable between plans.
CONCLUSIONS: Adopting the updated 2024/2025 international consensus guidelines for level IVa delineation significantly reduces radiation dose to the thyroid and larynx without compromising target coverage. This translates into a clinically meaningful reduction in the risk of primary hypothyroidism, supporting the widespread implementation of these guidelines to improve survivorship outcomes in patients with NPC.