Reducing radiation-induced hypothyroidism by modified delineation of cervical lymphatic drainage area for nasopharyngeal carcinoma treated by intensity-modulated radiation Therapy: 3 years' experience.

Background and purpose: Radiation-induced hypothyroidism (RIHT) is a late complication of intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC). We evaluated thyroid protection in NPC patients receiving IMRT using modified delineation (MD) of cervical lymphatic drainage areas, sparing the common carotid artery within the clinical target volume (CTV), to assess its impact on thyroid function and survival outcomes.

Materials and methods: This retrospective cohort study included patients without metastatic lymph nodes at levels III and IV who received neck irradiation. Patients with normal thyroid function before radiotherapy and regular thyroid monitoring thereafter were included in the regular thyroid-function monitoring cohort. MD was used to adjust the medial edge of level III and IVa lymphatic areas from the medial (standard delineation [SD]) to the lateral edge of the common carotid artery.

Results: Among 374 patients (SD: 223; MD: 151), the median Dmean, V45, and V50 were significantly lower in the MD group than in the SD group. In the regular monitoring cohort (SD: 113; MD: 108), the 3-year RIHT incidence was lower in the MD group (23.5 % vs 40.0 %; P = 0.005). MD was associated with a lower risk of RIHT (HR: 0.49; P = 0.004). The 3-year locoregional recurrence-free survival (97.2 % vs. 97.3 %, P = 0.962) and overall survival (96.2 % vs. 92.2 %, P = 0.221) rates were comparable between MD and SD groups.

Conclusions: Sparing the common carotid artery region in the CTV is associated with reduced thyroid radiation dose and a lower RIHT incidence without increasing regional failure risk or affecting overall survival.